Osmotic 2.pdf

J. Fd Techno/. (1984) 19,6549

Osmotic concentration of potato. 11. Spatial distribution of the osmotic effect A . LENART’

AND

J . M . FLINKT

Summary

The spatial distribution of solids and moisture depends on a number of factors, such as solute type, solution concentration, osmosis time, osmosis temperature and solution agitation. The spatial distribution was characterized by two terms: (i) the depth of penetration of the osmosis effect (e.g. depth to which solids concentration is higher than that found in the original tissue), and (ii) the moisture content of the surface relative to the initial moisture content. It was shown that to obtain usable distribution information, shrinkage of the sample which is occurring in the osmosis process must be considered in the calculations. Osmosis distribution curves were very different for osmosis in sucrose or in salt solutions. A model based on the development of a compacted surface tissue layer and on the presence of a liquid surface film for non-agitated systems is used to explain observed behaviour for osmosis in concentrated sucrose solutions. Osmosis in salt solutions is shown to follow a much different model, in which a significant salt transport occurs. Sucrose/salt mixtures show behaviour characteristics typical for osmosis in each of the pure solutes. On the basis of these models, the improved osmosis behaviour and improved product quality reported in the literature for products osmosed in mixed sucrose-salt solutions having sucrose concentrations above 40% is discussed. Introduction Osmotic concentration is a water removal process in which cellular materials (such as fruits or vegetables) are placed in a concentrated solution of soluble solutes. The water and solute activity gradients results in a flow of water across the cell’s semi-permeable membrane. In addition, the non-ideal nature of the membrane results in some transport of solute, primarily from the osmosis solution to the material. The course of the various transports can be followed by determining the degree of water loss ( W L ) and solids gained (SG) in the process. Simple means for conducting this analysis have been presented by Hawkes & Hink (1978). Their method presents the uptake and loss values on a n average basis, and thus, neglects the spatial distribution of the osmosis effect (uneven loss of water or uptake of solids). It is well known that when a solid material not having ‘infinitely small’ dimensions is placed in a solute-containing environment, the solute will be transported through the Authors’ address: Department for the Technology of Plant Food Products, Royal Veterinary and Agricultural University, Copenhagen, Denmark. *Permanent address: Department of Food Technology, Warsaw Agricultural University, Warsaw, Poland. ?To whom correspondence should be addressed. 5

66

Osmosis ofpotato. II

solid by a diffusion process. Crank (1975) has made a detailed theoretical description of the diffusion process. A particular property of non-steady state diffusion processes is the presence of non-linear concentration gradients for the diffusing species. A gradient for a given component is in effect measured by the spatial distribution of a component’s concentration in the solid material. In the area of osmotic concentration, very little information has been published regarding the spatial distribution of the sample’s components. Hughes, Chichester & Sterling (1958) published distribution curves for sugars inside peach tissue following long term contact with a canning syrup. The heat treatment used as a part of the process, which will change tissue properties, limits the applicability of these results to osmosis processes where heating is not used. Lee & Salunke (1968) noted that following osmotic concentration of apple slices, sucrose is found to the largest extent in the surface layer, and that sucrose concentration seems unchanged from the level in the initial apple for all tissue depths below that surface layer. Hawkes & Flink (1978) noted that sucrose uptake kinetics measured during osmosis indicated that the sucrose was likely to be located as a thin surface layer, but they had not measured spatial distribution of the sucrose. In a number of studies on diffusion of small molecules in foods or model systems, spatial distribution of the diffusing solutes have been presented. It is necessary to note that these conditions do not necessarily simulate an osmosis process, as in osmosis there is bidirectional flow while in the following references, mono-directional flow of the diffusing species is predominant. Stahl & Loncin (1979) investigated the diffusion of cyclohexane in potato tissue and presented a measure of its spatial distribution at given times. They indicated that cell walls and membranes can influence the rate of transport, and that cell membrane non-polarity could influence diffusion behaviour, especially for this non-polar diffusant. Further, they noted that extension of the analysis permits evaluation of the effect of skin on mass transport behaviour. Naesens, Bresseleers & Tobback (1982) investigated the effect of water activity on the spatial distribution of tripalmatin following diffusion into a food model system. They showed that tripalmatin was more rapidly transported, the higher the sample water activity. Belton & Wilson (1982) investigated the diffusion of colouring materials from aqueus solution to a model food gel. They also presented the spatial distribution of the solute concentration at given diffusion times. Beyond transport related to diffusion, it is possible for other mechanisms to be of importance with respect to overall mass transport. In osmosis processes, it is possible for the combined effect of water loss and solids gain to result in high solute concentrations in the solid material. From some literature, it is known that high solute concentrations can result in cell structure breakdown. which could then result in marked changes in transport properties and altered solids gain and water loss behaviour. High solute concentrations external to the cell, not leading to cell breakdown, will give an extensive dehydration of the cell, with an associated shrinkage. This shrinkage could then result in a cell structure having reduced transport properties. In some studies, cell breakdown due to high solute concentrations has been related to cell injury occurring during freezing (Meryman, Williams & Douglas, 1977; Wiest & Steponkus, 1978). For example, Meryman et al. (1977) postulated that osmotic dehydration of the cell, which occurs during freezing, results eventually in physical damage of the plasma membrane. An important part of their theory relates to volume decrease of the cell (i.e. shrinkage), behaviour which is also observed in osmotic concentration. Wiest & Steponkus (1978) simulated cellular dehydration in freezing by exposure of cells to solutions of high solute concentrations (i.e. equivalent to osmosis).

A . Lenart and J. M . Flink

67

They noted that osmotic dehydration, occurring due to either freezing or osmotic concentration gave similar cell survival behaviour when based on observed changes in cell volume. Sterling (1968) reports on the difference of texture of carrot tissue following immersion in either sugar or salt solutions. Sugar gives a firming of the tissue, while immersion in monovalent ions, such as from NaCl, gives a softening. It can be expected that these observed textural differences would be reflected in the tissue’s mass transport characteristics. After osmosis in sucrose-a material which is noted to act as a cryoprotective agent when freezing cells (Weist & Steponkus, 1978)-it can be expected that cellular dehydration results in compressed cells with little cellular membrane breakdown. Immersion in salt, on the other hand, seems to act to destroy membrane function, this then changing solute permeability of the cells (Towill & Mazur, 1976). For salt osmosis, this would result in decreased cellular dehydration, thus giving less shrinkage, increased solute uptake and higher transport properties. The eventual effect of cellular dehydration on transport properties will depend, in part, on tissue properties, especially the intercellular space present in the tissue and the amount of insoluble solids present, expressed as a percent of total solids. Transport properties of potato, which has a high starch content (insoluble solids) and low intercellular space will be more sensitive to tissue dehydration than, for example, apple tissue, which has a higher percentage of total solids as soluble solids and a larger proportion of intercellular space. The current study was designed to investigate the spatial distribution of moisture and solids in potato tissue following osmotic concentration with various solution compositions and at various process conditions. From these distributions, it would be possible to comment on the mechanisms governing solids and water transport in the osmotic concentration process. Materials and methods

Materials Reagent grade sucrose and sodium chloride were used to prepare osmosis solutions. Hansa potato was obtained at a local supermarket. Medium size tubers ( 9 5 2 5 ~ 55?5 X45+5 mm) were selected for use in these tests. Preparation methods The osmosis solutions were prepared by blending the desired solute(s) on a w/w basis with deionized water. Osmotic concentration was conducted for a period of 1-20 hr in a glass vessel containing 500 g of osmosis solution, with or without gentle agitation from a magnetic stirrer. The glass vessel was sealed with aluminium foil to prevent evaporation. An osmosis experiment would be conducted as follows (Fig. 1). Potato tuber was cut cross-sectionally into the stem and bud halves. Slices (see below) from the stem half closest to the cut surface were used for determination of moisture contents of fresh potato. The apex of the bud half was removed so that a slab of about 40 mm thickness remained. The average diameter (+1 mm) and slab thickness (t0.05mm) was then accurately measured. The slab was then enveloped in a thin skin, tight elastic material, such that the main surface was free. The slab was then placed into the osmosis vessel such that the free surface was 5 mm below the solution surface. The sample was removed at the end of the osmosis period and gently blotted with paper to remove surface solution. After the average diameter and thickness of the slab was again

68

Osmosis of potato. II

measured, a sharp tubular cork borer (23 mm i.d.) was used to cut a right cylindrical plug from the centre of the slab, taking as much of the inner pholem region of the tuber as possible. Aluminium fo,il Holder

Cylindrical

Potato

3

Y

I

I

~,

Slob

I

Slices

Free surface

solution

Figure 1. Summary of experimental procedure for osmosis of potato slab and preparation of slices.

The tubular cork borer, which now contained the cylindrical plug, was placed into a modified microtome and 1 mm slices were cut from the plug, starting at the surface exposed during osmosis. After each slice was cut, the new exposed surface was covered (with the tared piece of weighing paper to be used with the next slice) to prevent evaporation. The cut slice was immediately weighed ( 2 1 mg) and the thickness then accurately measured at the slice centre (?0.05 mm) using a penetrometer (PNR 6, SUR, Berlin). A number of preliminary experiments were conducted for 20 hr with high effectivity osmosis solutions to determine how many slices need be cut so that slices which were not influenced by the osmosis treatment (i.e. having the same moisture content as unosmosed tissue) would also be sampled. Moisture content in potato slices was determined gravimetrically by air drying at 90°C to a constant weight (generally about 20-24 hr). As the air oven was initially at room temperature, the initial phase of drying (about 0.5-1 hr) was conducted at a temperature lower than 90°C. In some experiments, slices were cut into two halves-one half being used for determining moisture content while the other half was used to determine NaCl content. A modified Mohr titration method suitable for the 200 mg sample size was used. Salt content was expressed as percent of the final mass of the slice. Moisture content was expressed both as a fraction of the final mass of the slice, and as a relative moisture content with respect to the initial moisture content. Calculation methods Means for calculation of mass transport data for osmosis have been described by Hawkes & Flink (1978) and Lenart & Flink (1984). In this study, mass transport data (solids gain and water loss) were calculated for the individual slices and for the osmosed cylinder as a whole. During osmosis of potato, shrinkage of the tissue is observed. This shrinkage can influence the basis for the mass transport data calculations, and thus it was necessary to ddermine if the analysis need take shrinkage into consideration. However, as the spatial distribution of shrinkage in slices is not easily measured, the overall shrinkage of the slab was determined and then on the basis of the shrinkage measured in the slab, the analysis of mass transport data for slices was conducted with and without accounting for this shrinkage. For the analysis of individual slices, water

A . Lenart and J . M . Flink

69

loss (with shrinkage) was calculated from:

WLS= ( l / A ) ( ( m h / t ’-(moho/to)), ) while solid gain (with shrinkage) is obtained from:

SGS = ( 1 / A ) ( ( m / t ’ )l-h)-(mo/to)( ( l-ho)). When shrinkage was not to be included in the analysis, the value o f t is used instead oft’. The mass transport data for the cylinder was calculated by three methods, two for the individual slice (one with and one without shrinkage) and the third for the whole slab. When based on the individual slices, weights and moisture contents for all slices from the free surface to the first slice whose moisture content was the same as the moisture content of fresh potato were included in the calculations. Water loss for the cylinder (with shrinkage) is given by:

[

WLc = (l/A) i(m-rnd)-(mo/to)ho

(3)

and solids gain (with shrinkage) by:

SGc = (l/A)’[ $rnd-(mo/to)(l-ho) i ‘ t ’ ] .

(4)

To calculate equation 3 and 4 without shrinkage, t’ is replaced by t . By summing the above information from the individual slices, the mass loss under osmosis for the whole cylinder could then be calculated with or without accounting for shrinkage. Another, independent way to determine the mass loss of the cylinder is to evaluate the mass loss of the whole slab and then determine the fraction (area based) that the cylinder is of the slab using:

(5) In the following Results section, two parameters of the distribution of the osmotic effect are used to characterize osmosis behaviour and compare the effect of various osmosis treatments. These are the osmosis effect penetration depth (call PenDep) and the relative moisture content of the first (surface) slice (called SurRMC). PenDep is the distance from the free surface to the point where tissue moisture content is equal to the moisture content of unosmosed tissue. It therefore reflects the loss of water €rom the tissue and uptake of solids by the tissue. It must be remembered in the following sections that PenDep can refer to the distance over which water loss alone has resulted in a change in tissue moisture content. SurRMC is defined as the ratio of average moisture content of the first 1mm thickness of the sample to the moisture content of the unosmosed tissue. AMo = (l/A)(Mi-Mf)(d/D).

Results

In presenting the results of this study, each major osmosis solute system is considered individually. Under each osmosis solute system, the results of studies of the individual osmosis process parameters are presented. Osmosis with sucrose Effect of solute concentration. The effect of solute concentration on osmosis distribution is given in Table 1 and Fig. 2. After 2 hr of osmotic concentration, the

Osmosis of potato. II

70

Table 1. Influence of sucrose concentration on osmosis penetration depth and relative moisture content of the sample surface (20"C, without agitation) Sucrose solution conc. (%)

Osmosis penetration depth PenDep (mm) Surface relative moisture content

Time (hr)

0

20

40

60

2 8

I .o 8.5

3.2 7.0

4.7 8.5

5.8 5.6

2

0.99

0.92

0.85

0.71

8

1.03

0.91

0.75

0.62

SurRMC

osmosis penetration depth (PenDep) increases from 1to 6 mm as sucrose concentration increases from 0 to 60%. The relative moisture content of the first slice (SurRMC) decreases from 0.98 to 0.71 over the same range of sucrose concentration. In general. it can be seen in the osmosis distribution curves (Fig. 2) that for 2 hr osmosis, as sucrose concentration increases, the relative moisture content falls at any given distance from the free surface. It is noted (Table 1) that after 8 hr osmosis, sucrose concentrations of 0-40% d o not appear to influence PenDep, while for 60% sucrose PenDep is lower than that found for 40% sucrose. In analysing the behaviour at concentrations below 40%, it is necessary to consider that potato tissue is isotonic around 10% sucrose (Lenart & Flink, 1984) and that at this concentration PenDep can be expected to be 0. A t concentrations less than 10% PenDep will register the penetration of water into the tissue, while at concentrations above 10% PenDep will register the flow of water out of the tissue. The decrease in PenDep at 60% (compared with 40%) would appear to be due to the formation of mass transport resistant surface layer (due to tissue shrinkage, increased tissue fluid viscosity and low SurRMC resulting from the high sucrose concentration at the potato surface) which hinders the transport of water and solutes. This surface resistance would give a quite steep relative moisture content distribution with a sizeable change in relative moisture content for the first few slices. Results not presented here showed this behaviour to be correct. Thus, it is more likely that the PenDep dependence on sucrose concentration (rather than being independent at concentrations below 40%), actually shows an S-shape curve with a minimum at the isotonic concentration and a maximum at a concentration between isotonic and 60%, after which it decreases again. After 8 hr of osmotic concentration. SurRMC shows the same behaviour as after 2 hr, in that SurRMC decreases as sucrose concentration increases over the range tested.

Effect of osmosis time. To evaluate the influence of osmosis time (with or without agitation), osmosis studies were conducted for 1-20 hr. The results are shown in Table 2 and Figs 3 and 4. In Fig. 3, it can be seen that for 60% sucrose without mixing, PenDep after 1 hr has reached about 6 mm. For times of 2-8 hr, it is not possible to find a trend for PenDep versus time, though the values lie between those found for 1 hr (6 mm) and 20 h (10 mm). Generally, SurRMC shows a sizeable decrease with increasing osmosis

71

A. Leaart and J . M.FIink I

.oo

S u c r o s e conc

0 o/'

..-.

x

20%

0

40%

A

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<

-

\

.$

0.80

+

a c 0

u a L

3 &

Lo

E a

.-z

; 0.70 a

L X

0.60

0.5C

1

I

1

10

5 D i s t a n c e , t (rnrn)

Figure 2. Influence of sucrose concentration on distribution of osmosis effect after 2 hr without agitation.

time, though for 2 and 4 hr the SurRMC levels are not distinguishable. Thus, it can be noted with osmosis in 60% sucrose solution without mixing, that while time has a sizeable influence on SurRMC, time only effects PenDep after a long time. It can be seen in Table 2 that agitation of the 60% sucrose solution gives SurRMC values which are lower by about 0.07 units for all times evaluated, while agitation only influences PenDep after long-time osmosis (time > 4 hr). In addition, the similarity of the 1-8 hr PenDep values, found for osmosis without agitation, vanishes when the osmosis solution was agitated. In general, the appearance (but not absolute values) of the osmosis distribution curve for 60%sucrose with agitation is similar to that obtained with 40% sucrose without agitation (see Fig. 4). For osmosis in 40% sucrose solution without

Osmosis of potato. II

72

Table 2. Jnfluence of osmosis time on osmosis penetration depth and relative moisture content of the sample surface (20°C. without agitation) Osmosis time (hr) Sucrose conc. (%) 1 2 4 Osmosis penetration depth PenDep (mm)

40

3.4

4.7

6.0

60 60*

5.7 5.6

5.8 5.2

6.2 7.1

Surface relative moisture content SurRMC

40 60 60*

0.85 0.85 0.80 0.86 0.71 0.70 0.79 0.70 0.61

8

20 8.7 5.6

>13

0.76 0.62 0.56

>12

10.2 0.70 0.54 -

*With agitation

agitation, PenDep increases with increasing osmosis time, while SurRMC decreases from 0.85 to 0.70 for 1-20 hr. The fact that agitation of a high concentration sucrose solution gives similar osmosis distribution curves to those found with a medium concentration sucrose solution without mixing, could be related to the effect of solution viscosity and to the extent to which a solution film which reduces sucrose concentration could develop at the tissue surface in non-agitated solutions of high sucrose concentration. This is discussed more fully in the Discussion section.

Effect of temperafure. The general shape of the osmosis distribution curves for osmosis in sucrose solutions at elevated temperatures (up to 50OC) are similar to those already shown for room temperature (Fig. 2). From Table 3 , it is seen that PenDep increases and SurRMC decreases with increasing temperature. The effect is much greater for an increase from 30to 50°C than for an increase from 20 to 30°C. This effect is undoubtedly related to the change of diffusion coefficients with temperature, as well as changes in tissue properties (entrapped air, cell fluid viscosity, membrane permeability), though cell structure should not be destroyed on the basis of this temperature alone.

Osmosis with salt (NaCI)

The osmosis distribution curves for osmosis in salt are very different from those found for sucrose (compare Figs 2 and 5). In general, the distribution curves for salt are relatively flat, with the SurRMC values being higher than 0.90 in all cases studied. PenDep is generally higher than for sucrose. The results obtained for all the osmosis studies with salt are given in Table 4.

Effect of solute concentration. Osmosis with salt was conducted over the range 6 5 1 5 % . After either 2 or 8 hr, both PenDep and SurRMC are not dependent on the salt concentration for concentrations below 10%. At the higher salt concentration of

A . Lenart and J . M . Flink

73

1.00

0.90

: 0.80 -c.

0

I hr

x

2 hr

o

4 hr

b

8 hr

0

20 hr

0

-" c

aJ c

c 0 m

L

3

c VI .-

: a,

i?

a,

0.70

(r

0.60

0.50 I

I

1

5

10

Distance, I (mm)

Figure 3. Influence of osmosis time on distribution of osmosis effect for osmosis in 60% sucrose without agitation.

15%, short time osmosis (2 hr) gives a relatively small increase in PenDep, while for the longer osmosis time (8 hr), PenDep increase is much larger. For both times SurRMC shows a slight decrease when salt concentration was increased to 15%. Effect of osmosis time. Increasing the osmosis time from 2 to 8 hr results in an approximately 65% increase in PenDep for the 6.5 and 9.8% salt solutions, while the increase in PenDep was somewhat larger (90%) for the 15% salt solution (Table 4). The SurRMC was not influenced by increase in osmosis time or by agitation. It would seem that already after 2 hr osmosis in salt solution, the surface layer of the potato had attained equilibrium. This rapid attainment of equilibrium for osmosis in salt solutions

Osmosis of potato. II

74 1.00

0.90 I hr X

2 hr

o 4hr

-r.

b

8hr

Q

-r.. 0.8C

0 20 hr

c

m c +

c

0

w L

7

+

._

Y)

0

E > or

ar

0.70

a

0.6C

0.5( I

I

I

5

10

Distance, t i m m )

Figure 4. Influence of osmosis time on distribution of osmosis effect for osmosis in 40% sucrose without agitation.

has been reported by Lenart & Flink (1984). Agitation of the osmosis solution only influenced PenDep, with this effect being greater at the lower salt concentrations.

Effect of temperature. The effect of temperature on osmosis distribution for salt solutions is shown in Table 3. Osmosis distribution behaviour was similar to that noted with sucrose. with PenDep increasing and SurRMC decreasing with increasing temperature. The relative increase in PenDep (approximately 100% for 20-50°C) is much greater than the relative decrease in SurRMC (about 5 % ~ ) .

75

A. Lenart and J . M . Flink Table 3. Influence of temperature on osmosis penetration depth and relative moisture content of the sample surface (8 hr: without agitation)

Osmosis penetration depth PenDep (mm) Surface relative moisture content SurRMC

Osmosis solution

Temperature ("C)

(w/w %)

20

sucrose 60

5.6 11

15

>I8

salt 9.8

10.8

13.5

>I8

sucrose 60

0.62 0.60

0.50

sucroselsalt 45/15

0.62 0.58

0.44

salt 9.8

0.95

0.92

0.90

6.5%

'

13

7.6

sucrose/salt 45/15

0

0.90

50

30

salt

15.0% s a l t

5

I

10

15

Distance, t (mm)

Figure 5 . Influence of salt concentration on distribution of osmosis effect after 8 h r without agitation. Table 4. Influence of salt concentration on osmosis penetration dcpth and relative moisture content of the sample surface (20°C; without agitation) ~

~~

Salt (NaCI) conc. (%) Time

Osmosis penetration depth PenDep (mm)

Surface relative moisture content SurRMC

(hr)

6.5

9.8

15.0

2

6.5

6.3

7.3

8

10.8

10.8

14.0

2*

8.5

8.5

8.5

2

0.95

0.96

0.92

8

0.96

0.95

0.93

2*

0.94

0.96

0.93

*With agitation.

76

Osmosis of potato. I1

Osmosis with sucrose/salt mixtures Osmosis of potato was conducted using mixtures of sucrose and salt in various proportions. The proportions were chosen such that two test series were obtained, one in which the total weight concentration was the same for all mixtures, while the second had all total mole concentrations equal. (As all osmosis experiments were conducted with 500 g of osmosis solution, equal mole concentration was based on total moles of solute per 1000g solution.) In the section below, in which results of the effect of solution concentration are presented, reference is made to tests with the same weight concentration or same mole concentration.

Effect of solute concentration. Results for the test series conducted at equal weight concentration are given in Table 5 and Fig. 6. At 2 hr, increase of the salt proportion of the mixture over the range of 5-15% gives an increase in PenDep, while after 8 hr osmosis there is no longer any influence of salt proportion on PenDep. In general, it can be noted that addition of salt to the osmosis system (with corresponding decrease of the sugar) results in an increase in PenDep and a significantly changed curvature of the osmosis distribution curves (Fig. 6). Table 5. Influence of osmosis solution composition (equal weight concentration) on osmosis penetration depth and relative moisture content of the sample surface (20°C. without agitation) Sucroselsalt conc. (% wlw) Osmosis parameters

Time (hr)

Osmosis penetration depth PenDep (mm) Surface relativc moisture content SurRMC

2

0.71

0.71

0.74

0.71

8

0.62

0.61

0.69

0.62

45/15

50/10

2

9.0

8.4

5.4

5.8

8

11.0

1I.S

10.3

5.6

SS/S

60/0

SurRMC values were essentially equal for sucrose alone and mixes with 10 or 15% salt. The SurRMC value for the 5% salt mix was higher than expected (attributed to the use of a new batch of potatoes), but it can be seen in Fig. 6 that the osmosis distribution curve is otherwise normal. In general, it can be mentioned that the SurRMC behaviour is dominated by the sucrose effect, while PenDep is dominated by the salt effect. This agrees well with the PenDep and SurRMC behaviour noted above for sucrose alone or salt alone. Results for the test series conducted at equal mole concentration are given in Table 6 and Fig. 7. In general, an increase in the salt proportion of the sucrosefsalt mixture at equal total mole concentration results in increases of PenDep and SurRh4C. For example, after 8 hr osmotic concentration in 60% sucrose solution, SurRMC is 0.62, while the sucrose/salt mixture (40/3.3) gives a 15% increase to0.715. A 2W6.53 mixture shows a 35% increase, up to 0.835, while with salt alone (9.8%) the increase relative to

77

A. Lenart and J . M . Flink 1.00

S o l u t e conc.

0.90

(O/O)

sucrose /sa I t

45/15

c -z

0

55/5

A

601'0

0

\

0.8C

&

c

W

+

"

c 0 W L

3 L

Y)

0

E m >

-

: 0.7c n t

0.6(

0.51

I

I

5

10 Distance,

t

(mm)

Figure 6 . Influence of sucrose/salt proportions at equal total weight concentration on distribution of osmosis effect after 8 hr without agitation.

(a%wlw)

sucrose is about 50% to 0.945. From Fig. 7, it can be seen that salt proportion of the mix has an effect on the shape of the osmosis distribution curve, with the curves being flatter as the salt concentration increases. When the data on penetration depth in the process is combined with information from the osmosis distribution curve, the solution 40/3.3 would seem to be optimum as this gives deep penetration and reasonable lowering of the SurRMC.

Effect of osmosis time.Osmosis experiments were conducted for 2 or 8 hr. In both the constant total weight concentration and constant total mole concentration test

Osmosis of potato. I1

78

Table 6. Influence of osmosis solution composition (equal niole concentration) on osmosis penetration depth and relative moisture content of the sample surface (20°C. without agitation)

Sucrose/salt conc. (% w/w) Osmosis parameters

Time (hr)

Osmosis penetration depth P e n D e p (mm)

2 8

Surface relative moisture content SurRMC

2

8

20/6.5

40/3.3

60/0

W9.8

7.6

6.9

5.8

7.5

11.4

8.7

5.5

10.8

0.84

0.82

0.71

0.95

0.84

0.72

0.62

0.95

series (Tables 5 and 6), increase of osmosis time gives a significant increase of PenDep for all cases except sucrose alone. SurRMC values are also noted to be lower with longer osmosis time for mixtures with high sucrose concentrations. There was no influence of time on the shape of the osmosis distribution curves.

Effect o f temperature. The effect of temperature (20-50°C) for sucrose/salt (45/15) osmosis is given in Table 3. It is noted that the effect on PenDep (increase with increased temperature) is similar to that found for sucrose or salt alone. For SurRh4C it is noted that there is a slight decrease in going from 20 to 30"C, but that there is a sharp decrease when the temperature rises to 50°C. The large increase of PenDep and decrease of SurRMC at 50°C presumably results from the effect of temperature on diffusion constants, temperature on the membrane properties and the effect of salt at higher temperature on potato tissue and cellular properties. The behaviour noted here indicates that in sucrose/salt mixtures, the osmosis response is an additive effect from both the sucrose and the salt in the mixture. Mass transport data for osmosis in sucrose solutions In this section, the character of the osmosis distridution curves is analysed to eventually determine the spatial distribution of solids gain and water loss in the osmosed tissue. As a first step, it is necessary to determine whether shrinkage must be used as a factor influencing mass transport data. To accomplish this, mass transport data were calculated for the cylindrical plug by three methods, from the slice data with or without accounting for shrinkage and from whole slab data. (Details are given in materials and methods-Calculation methods.) Results of these calculations are shown in Table 7. Total shrinkage of the slab after osmotic concentration is significant and increases with duration of osmosis. When osmosis time increases from 1 to 20 hr, total shrinkage increases from 0.70 to 2.10 mm. To determine the mass loss (AMc) values for an osmosis, data obtained from the whole slab are easiest to obtain, and thus serve as the basis for comparing calculations made with or without shrinkage. When the mass loss ( A M c ) values calculated by the three methods are compared, it is noted that the mass loss calculated from slices without shrinkage is significantly smaller than the mass loss

A. Lenart and J . M . Flink 1.00

79

O / O

I 0.90

-r. 0

-. <

-

I

0.80

-

a,

" c

0

m

L

3

Ln

0

E m > L

-O (L

0.70

II I

0.60

0.50

I

0

I

I

I

I

5

10

5

1

15

D i s t a n c e , I (mrn)

Figure 7. Influence of sucrosehlt proportion at equal total mole concentration ( I .7S rno1/500g solution) o n distribution of osmosis effect after 8 hr without agitation.

calculated from slices with shrinkage, and that the slices with shrinkage values are comparable in size with the whole slab values. (The range of difference for these two sets of values for osmosis times of 1-8 hr is about 10%. After 20 hr osmosis, there is a larger difference, but observed textural changes in the whole slab raises the question as to whether the whole slab still reflects the properties of the central cylinder.) On the basis of the above analysis, it was determined that shrinkage must be included in the analysis of osmosis mass transport data. To calculate solids gain ( S C ) and water loss ( W L ) of slices, it is necessary to distribute the measured total shrinkage of the slab to the various slices. While a number

80

Osmosis of potato. II

Table 7. Mass transport data for potato cylinder following osmosis agitation)

in

60% sucrose for 1-20 hr (20°C: n o

Osmosis time (hr) 1

2

4

8

20

I M n (mglrnm')

Calculated from whole sample data 0.804 0.992 1,697

2.072

3.942

I T * (mm) SGc (mg/mm') WLc(mg/mm') I M c (mg/mm3

Calculated from slice data with shrinkage 0.70 0.85 1.30 0.065 0.157 0.111 0.818 1.230 1.709 0.753 1.073 1.598

I.80 0.123 1.945 1.x22

2.10 0.014 2.582 2.568

SGc(mg/mm2) WLc(mg/mm2) IMc(mg/mm2)

Calculated from slice data without shrinkage 0.181 0.337 0.359 0.241 0.570 0.623 0.060 0.233 0.264

0.486 0.544 0.058

0.433 0.513 0.080

~~

~

~~

'Total shrinkage used in calculation

of shrinkage distribution methods were evaluated, shrinkage distribution based on the distribution of moisture content for the slices as used by Suzuki et al. (1976) was chosen for use in the following calculations. Figure 8 shows the dependence of slice shrinkage on slice relative moisture content for osmosis in 60% sucrose. It was found that

0.50 c

E E

\

E

E

v

k

0)

0.40

I

I

2 hr

Time:

A 8 hr

\ X

0.30II. ._ L

r v) 0)

.u-

0.20-

v)

0.10-

X

A

i

0 0.2

0.6

0.8

0.90

0.95

0.98

Relative moisture content ( h / h o )

Figure 8. Relationship of slice shrinkage to slice relative moisture content after osmosisin 60% sucrose.

0.99

81

A . LenartandJ. M . Flink ( a ) 20% sucrose ; 50-

( b ) 40% sucrose

( c l 60% sucrose

50-

50. 7

E E

\

E

-

' '

\ \

Y

N I

?! ?!

0 x

-

2

x

water loss

- without

\

\

Cn

solids gain (SG)

\ \

--

shrinkage

w ith ith shrinkage shrinkage -- w

i

\

\ \

7

N-

0

\

significant shrinkage occurs for relative moisture contents below 0.94-0.96. After osmosis in 60% sucrose, relative moisture contents will be below 0.94-0.96 for a depth of up to 4-5 mm from the free surface. Thus, it can be expected that essentially all of the sample shrinkage will occur in the 4-5 mm at the surface. The distribution of solids gain (SG) and water loss ( W L ) of slices after osmotic concentration in sucrose solution is shown in Figs 9 and 10. The results given in these

solids gain X

water loss

-

without shrinkage

-__

w i t h shrinkage

I

P x

Distance, t

(rnrn)

Distance, t ( r n m )

Figure 10. Influence of osmosis time o n spatial distribution of mass transport data after osmosis in 60% sucrose with agitation. (a) 2 hr: (h) 8 hr. 6

Osmosis ofpotato. II

82

figures show distributions calculated with and without shrinkage. It can be seen that when shrinkage is included in the calculations, SC levels fall and W L levels rise significantly. (It can also be noted that shrinkage does not appear to have an effect on the osmosis penetration depth for W L and relative moisture content.) While the figures were prepared to show the influence of including shrinkage on mass transport data, it must be noted that from the above analysis, the more correct mass transport data are obtained only when shrinkage is included. The calculated values for W L appear to penetrate the sample to the same depth as the measured relative moisture content changes, while the calculated SG values are distributed such that the major amount is located in the first 2-3 mm of the sample. With increase in sucrose solution concentration (from 20 to 60%, Fig. 9) and increase in osmosis time (from 2 to 8 hr, Fig. lo), depth of W L increases significantly, depth of SG remains essentially unchanged, while the amount of SG and WL both increase. It can be noted that the mass transport data increase in going from 2 to 8 hr osmosis (60% sucrose solution with agitation) is much greater than when going from 20 to 60% sucrose (2 hr without agitation). An increase of sucrose concentration from 20 to 40% gives an increase in W L and SG and in their distribution depth. But, when going to high concentration of sucrose (60%), it can be seen that only the amount of W L and SG increases, the distribution depth remaining unchanged. This is the same behaviour as has been noted for PenDep. Mass transport data for osmosis in salt solutions

Using the same methods for distribution of shrinkage as have been described above for osmosis in sucrose, the distribution of solids gain (SG)and water loss ( W L )of slices was calculated. In Fig. l l a it can be seen that the distribution of SG and W L is to the same depth as relative moisture content, but that the major amount of SG (for 9.8% salt) is up to a depth of 2-3 mm. These tendencies are the same as noted for sucrose. When shrinkage was not included in the calculation of mass transport data, it was calculated ( b ) 4 5 s u c r o s e / l 5 salt

( a ) 9.8% s a l t

E

\

-F

"

Distance, I (rnrn)

-

1

\

0

s o l i d s gain

X

water loss

-

without shrinkage

-

\

__

w i t h shrinkage

\

\*

Distance, f ( m m )

Figure 11. Spatial distribution o f mass transport data after 2 hr osmotic conccntration without agitation. (a) 9.8% salt. (b) 45% sucrose/lS%l salt.

83

A . Lenart and J . M . Flink

hr

*

6 . 8 % salt

o 15.0% salt b 45/15 sucrose/salt

Time 8 hr x 15.0% Salt

Figure 12. Influence of salt concentration and time on the spatial distribution of salt in osmosed potato tissue.

that the first slices gave water uptake instead of water loss. This behaviour is not possible under the osmosis conditions used, which further supports the necessity of including shrinkage in the calculations. Calculations of mass transport data with shrinkage gave the expected behaviour for a salt osmosis process, with W L being higher than SG and the difference of W L and SG being much less than for the sucrose osmosis. These results for distribution of SG and W L for osmosis in salt indicates that there is an equality of sorts between the exchange of water and salt between potato tissue and the osmosis solution. This is very different from the behaviour of osmosis in sucrose solution (Fig. 10) and in sucrose/salt mixtures (Fig. l l b ) where the water exchange is much higher than the solids exchange. In order to investigate the solids gain distribution for salt osmosis, the individual slices were analysed for their salt content. Results of these analyses are shown in Fig. 12. It is obvious that salt can easily penetrate into potato tissue. After 2 hr osmosis, the depth of salt penetration is from 4.5 to 6.5 mm, and depends on the salt concentration. After 8 hr osmosis, the penetration depth has increased up to 12-13 mm. It was further noted that at 8 hr, this penetration depth was the same for all salt concentrations tested. These salt penetration depths are about 1-2 mm less than PenDep, which is determined from the relative moisture content measurements. It is further noted that the concentration of salt in the osmosis solution has an influence on the overall salt content of the sample. Salt penetration depth for osmotic concentration after 2 hr in sucrose/salt mixtures (45% sucrose/l5% salt) is the same as when osmosis is conducted in 15% salt alone, and this depth is about 1 mrn less than PenDep, based on relative moisture content change. In Fig. 12, it can also be noted that the salt content at each location in the sample is lower for the 45/15% sucrose/salt mixture than for the 15% salt alone. This behaviour is related to the fact that with osmotic concentration in the sucroselsalt mixture, there is a resistance to diffusion of the salt due to the presence of the sugar and the higher solids concentration in the potato tissue than for osmosis in salt alone.

Osmosis of potato. II

84

2 hr

0

10

1

-

x 8 h r 0 20hr

. I

.

.

.

;

.

.

.

-

. 10

Distance,

.

-

.

-

. 15

t (mm)

Figure 13. influence of osmosis time on the spatial distribution of salt after osmosis in a 4S% sucrose/l5% salt mixture without agitation.

T h e influence of osmosis time on the distribution of salt is shown in Fig. 13 for the 45/15% sucrose/salt mixture. As noted above, it can be seen that osmosis time has a significant effect on the salt penetration depth. After 2 hr osmosis, this depth is about 7 mm, after 8 hr about 12 mm and after 20 hr more than 15 mm (probably about 20 mm). While depth of salt penetration was noted to be 2 mm less than PenDep after 2 hr osmosis, at longer osmosis times salt penetration and PenDep were the same. It can also be noted that with increasing osmosis time, the amount of salt taken up by the piece has increased at all locations. At 20 hr, it appears that the salt concentration in the first 2 slices (about 2 mm depth) is that which is in equilibrium with the salt concentration in the osmosis solution. Discussion

It has been shown that in an osmosis process, there is a significant spatial distribution of solids and moisture in the potato tissue and that this distribution depends on a number of factors in the osmosis process, such as solute type, solution concentration, osmosis time, osmosis temperature and the use of agitation of the solution. In particular, it was noted that the osmosis distribution curves differ greatly for sucrose osmosis solutions (high curvature with low PenDep and SurRMC) and salt osmosis solutions (only little curvature with high PenDep and SurRMC), while sucrose/salt mixtures showed characteristics of both solutes (intermediate curvature with high PenDep and low SurRMC). In discussing the influence of solution concentration on distribution of the osmotic effect for various osmosis systems, it is necessary also to consider the interactive

A . Lenart and J. M . Flink

85

influence of osmosis time. With few exceptions, it can be stated as a general rule that: increasing the concentration of solute in an osmosis solution or increasing the osmosis time gives increasing osmosis penetration depth and a decrease of the relative moisture content at all locations in the sample. Three specific observations from experiments on osmosis of potato in concentrated sucrose solution, which are exceptions to the above rule, are particularly useful in developing a model by which the osmosis behaviour of potato tissue in sucrose solution can be described. One observation notes that for long-time osmosis (8 hr) at high sucrose concentration (60%) without agitation, the major decrease of relative moisture content is limited to about a 2 mm depth in the sample. At this depth, there is a sharp rise of relative moisture content to the value found in unosmosed tissue, such that the osmosis penetration depth (about 6 mm) does not significantly change from the value observed after 2 hr of osmosis. The second observation notes that with agitation of a 60% sucrose solution, penetration depth and relative moisture content behaviour again follow the general rule. The third observation needed for discussing osmosis behaviour in sucrose notes that when comparing osmosis in 40% and 60% sucrose solutions, at short times PenDep is higher for the 60% solution, while at longer times it is the 40% solution which has higher PenDep. From these observations on the effect of agitation at 60% concentration and the changing relative effectiveness with time for 40 and 60% solutions, a model can be developed describing osmosis behaviour of potato in sucrose solution. The model involves two major influencing factors, (i) the formation of a compacted surface layer in the potato at high sucrose concentration, and (ii) that in the absence of agitation, there exists a solution film at the potato surface having a nonequilibrium solids concentration lower than the bulk osmosis solution. In particular, it would appear that in concentrated sucrose solution, which has a high osmotic pressure, there will occur very rapid water loss (plus some sucrose uptake) at the surface which results in changes to the tissue structure in the surface, this giving a compacted surface layer having a higher resistance to transport of solutes and water. As the osmosis process continues, this surface layer will hinder both water loss from the interior of the sample as well as further uptake of sucrose from the osmosis solution. This behaviour can be seen by comparing the penetration depths and surface relative moisture contents for 40 and 60% sucrose solutions. At all times the SurRMC for the 60% solution is lower than for the 40% solution, indicating that the driving force for osmosis is higher for the 60% solution, as would be expected. At short time ( 2 hr), this higher driving force expresses itself in the higher PenDep for the 60% solution. At longer time ( > 4 hr), however, the higher driving force does not result in higher PenDep, this presumably being due to increased resistances to mass transport in the samples osmosed in the 60% solution. This would relate to the above mentioned compacted surface layer, which would form at high sucrose concentrations both with and without solution agitation. A second mass transport reduction would develop in concentrated solution in the absence of agitation due to reduction of driving force of osmosis associated with a diluted surface solution film (diluted due to uptake of water from potato and loss of solids to potato at the sample surface). Osmosis penetration depth and relative moisture content after longer times in concentrated sucrose solution will thus depend on agitation of the osmosis solution, as this will give regeneration of the diluted osmosis media, which will give a higher average concentration gradient between the potato surface and the osmosis solution. This behaviour can be seen by comparing the results for osmosis in 60% sucrose with and without agitation. The sizeable decrease in SurRMC with the agitated system would indicate that the solution film at the tissue surface has a higher

86

Osmosis of potato. I1

solids concentration for the agitated system. The converse of this is that in the nonagitated system, the solution film at the surface has a lower solids concentration than the bulk osmosis solution, this resulting in reduced driving force for osmosis, giving reduced water loss and solids gain which is expressed as a lower PenDep. It has been noted earlier that the osmosis distribution curves for salt are quite different from those of sucrose. The behaviour of salt in general does not follow the rules noted above for sucrose. For example, increasing salt concentration from 6.5 to 9.8% showed no influence on PenDep or SurRMC, and the decrease in SurRMC when salt concentration increases to 15% is quite small. With salt osmosis, a number of observations are of importance for the development of a model describing osmosis behaviour of potato in salt solution. It can first be noted that with increasing osmosis time, PenDep increases while SurRMC remains unchanged, and that agitation of the osmosis solution, while having some effect on PenDep, has no effect on SurRMC. The fact that agitation has no effect on SurRMC would indicate that with salt there is little, if any, solution film at the potato surface, this presumably resulting from the lower viscosity of the salt solutions used and the limited water transport from the potato to the solution. T h e constant value for SurRMC with time would indicate that the surface quickly equilibrates with the solution and that the driving force for osmosis is constant. Analysis of the water loss and solids gain distribution, together with the relative moisture content and measured salt distribution lead to the conclusion that in salt osmosis, there is bidirectional transport of mass for the entire measured penetration depth, such that water flows from the tissue to the solution while salt is transported into the tissue. The differences of this model and that for sucrose are described in more detail later. As could be expected, temperature shows an important impact on osmosis distribution behaviour. Elevated osmosis temperature in all cases resulted in a significant increase in the osmosis penetration depth and a decrease of the relative moisture content distribution, both of which would be related to faster and/or more complete moisture and solids transport. As diffusion coefficients are temperature dependent, it is not surprising that higher transport rates will occur at the higher temperatures. It is also possible that higher temperature (50°C) will affect various factors of tissue structure (e.g. entrapped air, cell wall and membrane permeability, etc.) which will also give increased rates of mass transport. For osmosis in salt solutions. temperature showed less effect on the relative moisture content at the surface than with sucrose, since already at 20°C the sample surface was at equilibrium with the solution. Thus. temperature increases to 30 or 50°C give only little added effect at the surface. Penetration depth will, however, continue to be affected for the same reasons as noted above for sucrose solution (change of diffusion coefficients, tissue structure. etc.). Considered in terms of the above models, the effect of temperature on salt osmosis relates to the rates of water loss and salt penetration, while for sucrose osmosis, the effect of temperature appears to be related primarily to the rate of water loss. There are a number of major differences in the models which have been proposed above for sucrose and salt osmosis. Salt was noted to be able to easily penetrate into potato tissue, while sucrose penetrates to much shorter distances. Further, from measurements of solids gain distribution and salt distribution, it is noted that increasing osmosis time greatly increases the penetration depth for salt, while time has much less effect on sucrose penetration. The relative moisture contents at the surface for salt osmosis fell only to about 0.90, while sucrose samples could achieve surface relative moisture contents as low as 0.55. These observations would indicate that there are very different ‘mechanisms’ for osmosis with salt or sucrose.

A . Lenart and J . M . Flink

87

It is interesting to note that the behaviour of sucrose/salt mixtures can be described on the basis of combining the models for sucrose and for salt. Osmosis distribution curves for sucrose/salt mixtures show characteristics of the osmosis distribution curves noted above for sucrose or salt alone, namely increased penetration of salt (related to the salt model) and lowered relative moisture content at the surface (related to the sucrose model). As the shape of the sucrose/salt osmosis distribution curves for constant total weight concentration are similar to that obtained with sucrose alone, it appears that at relatively high sucrose concentrations (> 40%), it is the sucrose in the mixture which is the more critical component in determining the osmosis behaviour. The addition of salt manifests itself primarily by giving an increased osmosis penetration depth through combined penetration of salt and removal of water. With constant total mole concentration, the change in osmosis distribution is very marked as sucrose is replaced by salt. There is a definitive trade-off between surface relative moisture content and osmosis penetration depth, such that solutions which have high PenDep do not give extensive reduction in SurRMC, and vice versa. It appears that again it is first when the sucrose concentration is above 40% that a significant reduction in relative moisture content occurs. It was observed that the improvement of osmosis behaviour with addition of salt to sucrose (45% sucrose/l5% salt) was similar to that obtained by increasing the osmosis temperature to 50°C for a 60% sucrose solution. This would indicate that the effect of salt addition on the potato properties can be compared with the effect of temperature increase. It could be possible that the addition of salt gives an osmosis solution of lower viscosity for the same osmotic potential, and that salt can have a significant effect on the cell wall and/or membrane properties, both these effects also being achievable by elevated osmosis temperature. In further analysing these models, it is important to be aware that it has recently been noted that equality of water activity and soluble solids concentration between the osmosed tissue and the osmosis solution is the equilibrium condition for osmosis (Lenart & Flink, 1984). Thus the driving forces for osmosis are related to the differences of water activity or soluble solids concentration between the solution and tissue. There are a number of ways in which the eventual water activity lowering and corresponding soluble solids concentration increase can be achieved. For example, to achieve a given level of soluble solids concentration, the following are possible: 1 tissue water could be removed at constant tissue solids content; 2 tissue water could be removed and soluble solids could be taken up from solution by the tissue; or 3 soluble solids from solution could be taken up by the tissue and no tissue water removed. To achieve a given equilibrium water activity, it is obvious that for the three cases listed, there would have to be a balance of water removed and solids taken up such that the final soluble solids concentration in the tissue is the same for all three cases. It would seem that for osmosis with salt, the approach to equilibrium occurs with significant uptake of solids, which means that the amount of water to be removed to achieve equilibrium is reduced from that which would be required if no solids were taken up. Sucrose osmosis, on the other hand, appears to approach equilibrium primarily by removal of water from the tissue, and thus to achieve equilibrium will require the removal of a significant amount of water, as there is little solids uptake (relative to the water loss). Thus, a major difference between salt and sucrose osmosis is the extent of water removal at the surface of the sample, and the effect that this water removal has on

88

Osmosis ofpotato.

II

the properties of the potato tissue. With salt osmosis, the tissue at the surface is only slightly dehydrated (90% of the water found in the fresh tissue is still at the surface), while in sucrose osmosis, with its large water losses, the surface tissue can be dehydrated to such an extent that 50% of the original tissue water has been lost. This major loss of water and uptake of solids at the tissue surface can result in major changes in the cell/tissue structure, such as shrinkage and collapse of the surface cells, which would then influence the mass transport properties of these regions of the sample. This would be the compacted tissue surface layer noted earlier. To conclude this discussion, we can note that the above results can also explain why binary sucrose/salt osmosis solutions have been found to be so effective (Islam & Flink, 1982; Flink, 1980). With sucrose/salt mixtures at sucrose concentrations above 40%, the water removal effect is as high as with 60% sucrose, since the high sucrose concentration gives formation of the compacted surface layer. With sucrose alone, the osmosis penetration depth is limited, presumably due to this surface layer, and with the resultant limited water removal, the interior water activity will remain high. It has been shown by Lenart & Flink (1984) that for sucrose/salt mixtures at 60% total weight concentration, increasing the salt proportion gives significant decreases in water activity, and thus there is a much higher driving force for osmosis in a 45% sucrose/l5% salt solution (a,+.= 0.691) than with a 60% sucrose osmosis solution (% = 0.899). Further, it appears that the presence of salt in the osmosis solution can hinder the formation of the compacted surface layer, thus allowing improved rates of water loss and solids gain. With the addition of salt to the osmosis media, there also occurs uptake of salt, which unlike sucrose, can penetrate to the interior of the sample and act to effectively lower the interior water activity. All these factors apparently contribute to an improved osmosis treatment which yields products of improved stability. List of symbols

c

-salt content (% of slice mass-osmosed or unosmosed) - d i a m e t e r of cylindrical plug (d = 23 mm) -moisture content of osmosed slice (fraction of mass of slice) -moisture content of slice from unosmosed region (= initial fresh moisture content) (fraction of mass of slice) h/ho -relative moisture content (fraction of initial moisture content) rn -mass of osmosed slice (mg) rnd --dry matter of slice (osmosed or unosmosed) (mg) rno -mass of slice from unosmosed region (mg) n -number of slices t -actual thickness of osmosed slice (mm) to -actual thickness of slice from unosmosed region (= slice with no shrinkage) (mm) t' -effective thickness of osmosed slice after correcting for shrinkage (t' = t/ (1 - At)) (mm) At -shrinkage of slice (mmlmm initial thickness) A -base area of cylindrical plug (415.5mm2) D - d i a m e t e r of potato 'slab' (mm) Mi -mass of slab before osmosis (mg) Mf -mass of slab after osmosis (mg) SGc -solids gain for cylinder (mg/mm2)

d h ho

A. Lenart and J . M . Flink

89

SGs --solids gain for slice (mg/mm2) AT -shrinkage of slab (mm) WLc -water loss for cylinder (mgimm') WLs -water loss for slice (rng/mm') A M c -mass loss for cylinder as calculated from slice data (mg/mm') A M 0 -mass loss for cylinder from whole slab data (mg/mm2) Acknowledgment

W e would like to thank the Governments of Poland and Denmark for their support of the study visit of A.L. through the Polish-Danish Cultural Exchange Programme. We would also like to thank the Danish Veterinary and Agricultural Research Council for their support and lektor H. Buus Johansen for his rapid production of the slicing apparatus. References Belton, P.S. & Wilson, R.H. (1982). An experimentally simple method for measuring diffusion in food gels. Journal of Food Technology, 17, 531539. Crank, J . (1975). The Mathematics of Diffusion. Oxford: Clarendon Press. Flink, J.M. (1980). Dehydrated carrot slices: Influence of osmotic -concentration on drying behaviour and product quality. In Food Process Engineering. Vol. 1. (Ed. by P. Linko, Y.Malkki, J . Olkku & J. Larinkari). Pp. 412-418. London: Academic Press. Hawkes, J. & Flink, J.M. (1978). Osmotic concentration of fruit slices prior to freeze dehydration. Journal of Food Processing and Preservation, 2, 265-284. Hughes, R.E., Chichester, C.O. & Sterling, C. (1958). Penetration of maltosacchandes in processed Clingstone peaches. Food Technology, 12, 111 - 115. Islam, M.N. & Flink, J.M. (1982). Dehydration of potato. 11. Osmotic concentration and its effect on air drying behaviour. Journal of Food Technology, 17,387-403. Lee, C.Y. & Salunke, D.K. (1968). Sucrose penetration in osmo-freeze dehydrated apple slices. Current Science, 37, (lo), 297.

Lenart, A. & Flink, J.M. (1984). Osmotic concentration of potato. I. Criteria for the end-point of the osmosis process. Journal of Food Technology, 19, ooO-ooO. Meryman, H.T., Williams, R.J. & Douglas, M.St. (1977). Freezing injury from 'solution effects' and its prevention by natural or artificial cryoprotection. Cryobiology, 14,287-302. Naesens, W . , Bresseleers, G. &Tobback, P. (1982). Diffusional behaviour of tripalmitin in a freezedried model system at different water activities. Journal of Food Science, 47, 1245- 1249. Stahl, R. & Loncin, M. (1979). Prediction of diffusion in solid foodstuffs. Journal of Food Processing and Preservation, 3,213-223. Sterling, C . (1968). Effect of solutes and pH on the structure and firmness of cooked carrot. Journal of Food Technology, 3,367-371. Suzuki, K., Kubata, K.,Hasegawa, T. & Hasaka, H. (1976). Shrinkage in dehydration of root vegetables. Journal of Food Science, 41,1189-1193. Towill, L.E. & Maxur, P. (1976). Osmoticshrinkage as a factor in freezing injury in plant tissue cultures. Plant Physiology. 57,290-296. Wiest, S.C. & Steponkus, P.L. (1978). Freeze-thaw injury to isolated spinach protoplasts and its simulation at above freezing temperatures. Plant PhySiOlogy, 62,699-670.

(Received 24 January 1983)