Research Paper Thesis

Study of the changes in Mechanical Properties (Texture) of Biscuit by the sorption of Moisture at different ERH Kshitij Shrestha1*, Prof. Dr. Dilip Subba2 Food Research Officer, Department of Food Technology and Quality Control, HMG/Nepal 2 Assistant Dean, Central Campus of Technology, Dharan, Nepal

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Abstract The study was carried out to study the changes in Mechanical properties (texture) of biscuit by the sorption of moisture at different ERH(Equilibrium Relative Humidity). The biscuit samples were kept at the ERH of 10, 20, 25, 30, 35, 40, 45, 50, 60 and 75% and stored at room temperature (Avg. 25 oC) until equilibrium is achieved. Then, the samples were analysed for the Mechanical properties (Breaking force, Elasticity, and Maximum deformation) using Forcedeformation measuring Instrument. The break and shift was observed in sorption curve in between 35% and 40% ERH and found to be correlated with the phase transition of sucrose from amorphous (glassy) to crystalline state. The BET monolayer value was calculated to be 4.77 %. The Mechanical properties (texture) of biscuit were found to be affected significantly by the phase transition of sucrose. It was observed that crispy and brittle characteristics of the biscuit retain only if sucrose remains in glassy state but if phase transition of sucrose from glassy to viscous liquid (i.e. rubber) and to crystalline phase occurs then, there will be corresponding significant change in the mechanical properties of biscuit, which contribute to the kinaesthetic sensation perceived during eating. It was also concluded that 6 % moisture level was adequate to control the changes in texture of biscuit. Key words: - ERH (Equilibrium Relative Humidity), Breaking force, Elasticity, Deformation, EMC (Equilibrium Moisture Content), Glass transition

Introduction Water is the most abundant individual constituent in the majority of food. The amount of water present in many foods can vary over a limited range without causing much apparent alteration in the product itself. For example, some biscuit can absorb 2 % more moisture than that presents when they are freshly baked, and consumer would not be able to detect a difference. However, a distinct lowering of quality would have been noticed above this level (Paine & Paine, 1983). Therefore, it requires defining critical moisture content for a product to be satisfactory to the consumer. The importance of study of sorption characteristics of biscuit can be seen from the fact that the most important deterioration index for biscuit is “Moisture changes” and only then “Oxidative changes” and “Physical damage” comes (Paine & Paine, 1983). The reason for the moisture change to be most important deterioration index is its direct influence on the texture of biscuit. Texture is an important aspect of food quality, sometime even more important than flavour and colour, for example, in the case of biscuit, a crisp and dry food. Texture is most important in bland foods and foods that are crunchy and crispy (Rangana, 2000). Subjective evaluation of texture could also be made. The technique, however, is time consuming, requires considerable trained personnel and may not always be useful in the standardization of texture of foods. Consequently, objective methods have been made use, to provide efficient and precise quantitative prescription (Rangana, 2000). The mechanical parameters that correlate with sensory crispiness are resistance of a food to deformation measured as the slope of force deformation curve, (Bourne et al., 1966; Iles and Elson, 1972; Brennan et al., 1974) and also with deformation to fracture (Iles and Elson, 1972; Brennan et al., 1974).Bend deformation to fracture, a measure of brittleness has also been shown to correlate with sensory crispiness (Iles and Elson; 1972). It also is an integrated parameter. This Rheological parameter provides excellent indicators of Crispiness and Crunchiness for a single product type (Vickers, 1979). Consumer assessment of texture involves large deformation, so obviously measurements of this type are more likely to correlate with perceived texture than the results of experiments involving small deformations. As an example, consumer assessment of the hardness of gels correlated with the rupture strength rather than the elastic modulus. Plastic materials don not have a well-defined rupture point but brittle material have well-defined rupture points (Mitchell, 1984). The work was carried out to study the sorption characteristics of the biscuit so as to study the effect of moisture and ERH on the different mechanical properties (texture) of biscuit.

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Materials and Methods Sample The biscuit chosen for the study was the Glucose Biscuit (soft dough) obtained from the local market. Study of sorption behaviour of biscuit For the determination of Equilibrium moisture content, Wink's weight Equilibrium method, as described by Rangana (2000) was followed. Study of mechanical properties of biscuits For the study of mechanical properties, a simple instrument was designed. The important part of the arrangement was the measurement of force and corresponding measurement of deformation of the material. Mitchell (1984) observed that both the stress and strain at rupture depend on the rate of deformation. So, to keep the constant rate of stress, constant water flow rate was maintained and stored in water holding bin as source of force. The balance tank was used to keep the constant level of water by over-flowing the excess water. The height of water level to the level of output nozzle (h) was adjusted and kept constant so as to keep constant water output rate (5ml/sec). Timer was used to measure the time as an index of water deposited in the water holding bin i.e. force being directly applied to the biscuit. For the measurement of deformation, a needle was connected to the moving prove. The adjustment was done in such a way that small deformation was magnified linearly, as shown in figure 1. Force due to weight of water

C Actual deformation

O

B

A Biscuit

Observed Deformation

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Fig. 1. Experimental arrangement for measurement of actual deformation From figure 1 we can write AC = AO = K (Constant) BD BO So, AC = K × BD i.e. Actual deformation = K × Observed deformation The value for the designed instrument was calculated to be 0.097. The Force acting on biscuit at any instant is given by Force (N) = Time (t) × Rate of water flow (R) × acceleration due to gravity (g) For each sample of different humidity, at least 10 biscuits were tested for mechanical properties. The graph was drawn between force and deformation for each reading and the slope of the first straight portion was taken by linear regression on computer for the computation of elasticity. Chemical Analysis Analytical procedures were carried out as per Rangana (2000) and AOAC (1975). All the moisture content data are expressed in dry basis. Statistical analysis Statistical analyses were carried out using Genstat 5 Release 3.2 software, Lawes Agricultural Trust (Rothamsted Experimental Station).

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Results and discussion The sorption curve of Biscuit After the attainment of equilibrium (wt. variation of <1mg), Moisture content of each sample was determined. The observed results are tabulated below: Table.2. Equilibrium moisture content of Biscuit at room temperature (avg. 25oC) Relative Humidity Equilibrium Moisture Content aw (%) (%d.b.) X (1 - aw) 10 3.27 0.034 20 4.52 0.055 25 5.13 0.065 30 5.88 0.073 35 6.86 0.079 40 6.99 0.095 45 8.06 0.102 50 8.48 0.118 60 10.87 0.138 75 17.34 0.170 The observed sorption curve is shown in figure 2. 20

Equilibrium Moisture Content (%)

18 16 14

Sucrose Crystallization

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Fig.2 Sorption curve of Biscuit The graph shows linear increment in the moisture content with the RH in the region of 10 to 30%. The moisture content of 5.88% was found to be critical moisture content, the ERH of which corresponds to 30%. This is because, above this level, a higher rate of uptake of moisture with slight increment of RH occurs and enters into a very significant portion of graph, which is break and shift in the sorption curve in between 35 and 40% ERH. At this point, moisture content has decreased slightly and again started to increase with increasing ERH. The break and shift obtained was compared with the sorption behaviour of sucrose. This observation was related to the phase change in the sucrose molecule from amorphous to crystalline structure i.e. crystallization of sucrose (Karel, 1993). During baking, the drying rate is such a high that sugar can’t crystallize and remain as amorphous structure (Peleg & Bagley, 1999). Karel (1993) also observed similar results. He observed that sucrose crystallization occurs in 2 days at 35% RH, whereas it takes 100 days to crystallize at 30 % RH, during which it releases moisture. The moisture content of crystalline sugar is below 0.5%. 3

Fitting of BET equation 0.18 0.16

y = 0.2097x + 0.0111 R2 = 0.9959

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aw/x(1-aw)

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Fig.3. Fitting of BET Equation for Biscuit The observed data were analysed for fitting of BET equation. A very good fitting of equation over wide range of ERH was found (Corr. Coeff. of 0.9976). aw = 0.0111+ 0.2095 aw X (1 - aw) From calculation, the BET monolayer value was observed to be 4.77%. This is the most stable moisture content. Similar monolayer values for biscuit were also observed by Rahman and Labuza (1999). The observed BET equation is

Chemical composétion The biscuit under observation was the soft dough glucose biscuit i.e. high sugar high fat biscuit. The chemical composition of Biscuit was carried out. The chemical composition was within the range of soft dough glucose biscuit. The results obtained are tabulated below: Table.1. Chemical composition of biscuit (Glucose) under study Parameters Observed Value Moisture Content 4.7 % Fat Content 12.1 % Sucrose Content 23.3 % Protein Content 8.2 % Ash Content 0.9 %

Effect of ERH (and EMC) on Mechanical Properties of Biscuit Kase (1953) observed that even when measurements are made at a constant rate of strain, rupture strength are much less reproducible than small deformation parameters because failure takes place as a defect in the sample and the number and extent of such defect varies considerably from sample to sample. The distribution of rupture strength is represented by a double exponential frequency distribution function. Its integral form is following

σ b (n) = σ b + S ln –ln( n – 0.5 ) N Where, σ b (n) = Stress or strain at rupture of the nth sample. When N samples are ranked in order of decreasing magnitude. 4

σ b = Mode (Most Probable Value) of the distribution S = Standard Deviation of the distribution. Both these parameters can be obtained from a plot of ln –ln ( n – 0.5 ) against σ b (n). (Kase, 1953). N Hence, using the Kase’s equation, the modal values of breaking force and maximum deformation was calculated and used for the comparison purposes. The modal values as given by Kase’s equation from the ten observations from each sample of different ERH were calculated. The observed result is presented below: Table. Effect of ERH and EMC on different Mechanical Parameters ERH Moisture Breaking force Mean value (%) content (gm) of Elasticity (%) (dyne cm-2) Modal Mean value 10 3.27 1540 1492.5±102.1a (13.0±0.75)×107a b 20 4.52 1327 1298.0±66.7 (11.3±0.36)×107b c 25 5.13 1223 1175.5±102.0 (10.2±0.66)×107c d 30 5.88 1142 1098.5±92.9 (5.67±0.40)×107d e 35 6.86 752 720.5±67.6 (1.87±0.16)×107e e 40 6.99 747 717.5±61.8 (2.31±0.13)×107e f 45 8.06 500 472.5±61.2 (1.21±0.18)×107f 50 8.48 389 363.0±58.0g (0.97±0.07)×107f

Maximum deformation (mm) Modal Mean Value 5.78 5.6±0.37a 5.81 5.6±0.49a 6.10 5.8±0.68a 9.69 9.0±1.41b 19.23 18.4±1.62c 20.22 19.0±2.76c 31.25 29.6±3.50d 34.79 33.6±2.42e

Dimensions: Biscuit breadth 2.87cm, thickness 0.64cm and distance between supporting edges 3cm Note: Values in column with different superscripts differ significantly at 5% level of significance Effect on breaking force The breaking force was found to decrease linearly with the moisture content and also with ERH up to moisture content of 5.88% i.e. ERH of 30% with very high correlation coefficient (>0.99). This shows that breaking force can be a good indicator of the moisture content of the biscuit. From the Regression analysis, the straight line equation of breaking force obtained were tabulated below: Table .4. Linear Regression analysis of graph of breaking force with EMC and ERH Relation of breaking Range Equation Regression coeff. force with (r) Moisture Content 3.27 to 5.88% y = -155.31 x + 2038 - 0.995 ” 6.86 to 8.48% y = - 225.62 x +2309.5 -0.998 Equilibrium RH 10 to 30% y = -20.137 x + 1735.9 - 0.998 ” 40 to 75% y = -35.2 x + 2127.3 -0.977 The dependable variable y is for breaking force, and x is corresponding independent variable. The ANOVA analysis of the breaking force data of different ERH indicated that they were significantly different from each other (p>0.05). The l.s.d showed that only data obtained for ERH of 35 and 40% were not significantly different from each other, at 5 % level of significance (p<0.05). From the comparison of the straight lines of two different ranges of moisture content in above table, it was observed that slope of second line was greater than that of first line, indicating the more prominent effect of moisture in second stage. The graph indicated a drastic variation in the breaking force in between ERH of 30 to 40%, where the sucrose crystallization had taken place.

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12 Glass transition region for sucrose

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14 Elasticity (x10 dyne cm )

Breaking force (x100 gm) and

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Breaking force (x100gm) and

Fig.4. Effect of ERH on mechanical properties of biscuit

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Fig.5. Effect of EMC on mechanical properties of biscuit

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Effect on Elasticity It was observed that elasticity doesn’t follow the straight-line function with both moisture content and ERH. Elasticity was found to decrease very slowly at first, up to the moisture content of 5.13% i.e. ERH of 25%. Above this, elasticity was found to decrease at a very high rate, linearly and steeply. The decrease in elasticity is the result of the plasticizing effect of water, the more the moisture content more plastic the material will be. It can be seen clearly in the graph. In the region of 10% to 35% ERH, ANOVA analysis showed significant difference between different observation (p>0.05) with coeff. of variation below 8.89%.Above this region, the ANOVA analysis showed no significant difference in the observation of 35% and 40% ERH, and again that of 40% and 45% ERH (p<0.05). This indicates the plateau region, called rubbery plateau, which can be observed in graph. Literature shows that elasticity is a good measure of crispiness characteristics of the crispy material. For a material to be crispy, it must have the certain degree of elasticity with brittle failure (Jowitt, 1979). It indicates the importance of study of elastic nature of biscuit sample. Hence, the legal limit i.e. 6% moisture content level can be considered to control the crispy characteristic of biscuits. Effect on maximum deformation before breaking It was observed that deformation was very small until the moisture content of 5.13% (i.e. 25% ERH). There was little or no significant effect of moisture sorption up to that level. ANOVA analysis also indicated no significant difference between the deformation data obtained up to that level. Crispiness is always associated with the brittle failure and also the high rates of failure and strain energy release so, for crispiness, deformation before breaking should be low (Jowitt, 1979). The increase in deformation is always associated with the rubbery characteristics or plasticity of the material. Hence, deformation study shows that the crispy or brittle characteristic of the biscuit retains very well up to 5.88% moisture content above which there is linear increase in deformation and so rubbery characteristic. So, biscuit should not have moisture content of more than 6 %. Above that level, the deformation was found to increase sharply with sorption of moisture. A very good linear relationship has been found between the moisture content and the deformation before breaking. The correlation coeff. is very high (r = 0.99). The observed regression equation is: y = 0.946 x – 4.63 , (r = 0.99) Where y is for deformation and x is for moisture content. Discussion on the effect of ERH on Mechanical Properties of Biscuits The overall result can be correlated with the physical state of sucrose in the biscuit sample. Up to nearly 6% moisture content, sucrose is completely in glassy (amorphous) state and the slope of breaking force, elasticity and deformation curve are all small. This is because, during baking, the drying rate is such a high that sugar can’t crystallize and remain as amorphous structure. The glassy state is brittle and hard and so the crispy characteristics of the biscuit retain (Peleg & Bagley, 1999). That is why all mechanical parameters were changing at very small rate with sorption of moisture in this region. The large change in all mechanical properties between moisture content of 5.88% and 6.86% (i.e. ERH if 30% and 35%) was found to be correlated with the phase change in the sugar from glassy state to rubbery state i.e. glass transition of sucrose. The sugar remained at glassy state up to moisture content of 5.88% and above that, it was converted to super cooled amorphous solutions or very viscous liquid which have the properties similar to rubber so also called rubbery state (Karel, 1993). This is the reason to have a significant great change in the mechanical properties of biscuit. After the glass transition, crystallization is the time dependent phenomenon. The crystallization of sugar has been found to be in between 35% and 40% ERH (moisture content of 6.88% and 6.988%) from the sorption curve. Palmer et al. (1956) also studied the crystallization of sucrose at 24oC and nearly 32.5% RH. During crystallization moisture is released and so moisture content decreases and again the sorption occurs at 40% RH. It has been found that there is very small change in moisture content 7

between ERH of 35% and 40% and also, all the mechanical properties have been found to be insignificant (p<0.05). This may be due to the fact that mechanical properties are more related to moisture content than ERH. After crystallization, again the mechanical properties have been found to change with increase in moisture content but the effect is more prominent. This fact may be explained from the sorption studies of glassy and crystalline sugar. Amorphous sucrose begins absorbing water already at aw = 0.1 with major rise in moisture content at aw = 0.3, but crystalline sucrose remains eventually dry until about aw = 0.84 and meet amorphous sucrose at aw = 0.9 (Peleg and Bagley, 1999). This means, glassy state absorbs water and less water is available for polymer to change in mechanical properties, while crystalline sugar doesn’t absorb it and more free moisture to affect the polymer to change in mechanical properties such as breaking force (hardness), elasticity and deformation before breaking (Crispiness). This shows that sucrose combined with water contributes to the plasticization of biopolymer because the sugar will shift the transition towards lower temperature. The plasticizing effect of sugar will depend on the water / sugar / biopolymer mass ratio. In this way, in many different cereals based foods containing relatively high percentage of sugar (such as cookies, crackers and cakes) this carbohydrate contributes significantly to the overall texture to the food by affecting the thermo mechanical behaviour of the products.

Conclusions The Mechanical properties (texture) of biscuit are affected significantly by the phase transition of sucrose. The crispy and brittle characteristics of the biscuit retain only if sucrose remains in glassy state but if phase transition of sucrose from glassy to viscous liquid (i.e. rubber) and to crystalline phase occurs then, there will be corresponding significant change in the mechanical properties of biscuit, which contribute to the kinaesthetic sensation perceived during eating. It was also concluded that 6 % moisture level was adequate to control the changes in texture of biscuit. The phase transition of sucrose was observed above 35% ERH, so package design should not allow to increase the internal humidity level above that one, so as to control the effect of moisture on biscuit quality.

Acknowledgement I would like to express sincere gratitude to my guide Pro. Dr. Dilip Subba (Asst. Dean, Central Campus of Technology, Dharan), other lecturers of the campus and my colleagues for their constant support, inspiration, encouragement and help during my dissertation.

References Arora, C.L. (2001). B.S.C. Physics Volume I, S. Chand & Company Ltd. RamNagar, NewDelhi110055. AOAC (1975). Official methods of analysis, 12th ed., Association of Official Analysis Chemists, Washington DC. Bourne, M.C., Mayer, J.C. and Hand, D.B. (1966). Measurement of food texture by a Universal testing machine, Food Technology. 20,170, In : Sherman P. (1979). Food Texture and Rheology. Academic Press (London) Ltd. Brennan, J.G., Jowitt, R. and Williams, A. (1974). Sensory and Insturmental measurements of “brittleness” and “Crispiness” in biscuits, Presented at the Fourth International Congress of Food Science and Technology, Madrid, Spain, Sept. 22-27, In : Sherman P. (1979). Food Texture and Rheology. Academic Press (London) Ltd. deMan, J.M. and Stanley, D.W. (1984). Mechanical properties of food. In : Gruenwedel, D.W. and Whitaker, J. R. (1984). Food analysis, Principles and Techniques, Vol. I. Marcel and Dekker, INC. New York and Basel. Iles, B.C. and Elson, C.R. (1972). Crispiness, BFMIRA Research Reports No. 190, In : Sherman P. (1979). Food Texture and Rheology. Academic Press (London) Ltd. 8

Jowitt, R. (1979). An engineering approach to some aspects of food texture. In : Sherman P. (1979). Food Texture and Rheology. Academic Press (London) Ltd. Karel, M. (1993). Application of state diagrams in food processing. 1993 Food Preservation 2000 conference, Vol. II, Natick, Massachusetts, USA. Karel, M (1993). Fundamentals of dehydration process. Advances in pre-concentration and dehydration of foods. Applied Science Publishers Ltd., London. Kase, S. (1953). A Theoritical Analysis of the distribution of tensile strengtyh of vulcanised rubber. J. Polym. Sci. 11 (5): 425-431. In : Gruenwedel, D.W. and Whitaker, J. R. (1984). Food analysis, Principles and Techniques, Vol. I. Marcel and Dekker, INC. New York and Basel. Mitchell, J.R. (1984). Rheological Techniques. In : Gruenwedel, D.W. and Whitaker, J. R. (1984). Food analysis, Principles and Techniques, Vol. I. Marcel and Dekker, INC. New York and Basel. Paine, F.A. and Paine, H.Y. (1983). A handbook of food packaging, Dried and chemically preserved foods, publ., The council of the institute of Packaging. Palmer, K.J., Dye W.B. and Black D. (1956). X-ray diffractometer and microscopic investigation of cryustallization of amorphous sucrose, J. Agric. Food Chem. 4:77. In : Rahman & Labuza (1999). Handbook of Food preservation, Marcel Dekker, Inc. New York and Basel. Peleg & Bagley (1999). Physical Properties of Foods. Applied Scienced Publishers Ltd., London. Rahman & Labuza (1999). Handbook of Food preservation, Marcel Dekker, Inc. New York and Basel. Rangana (2000). Handbook of Analysis and Quality Control for fruits and vegetable products. Tata McGraw-Hill Publishing Company Limited, New Delhi. Vickers, Z. (1979). Crispiness and Crunchiness of foods. In : Sherman P. (1979). Food Texture and Rheology. Academic Press (London) Ltd.

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