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ARTICLE IN PRESS

LWT 40 (2007) 1578–1586 www.elsevier.com/locate/lwt

Aroma of dehydrated pear products Drazˇenka Komes, Tomislav Lovric´, Karin Kovacˇevic´ Ganic´ Faculty of Food Technology and Biotechnology, University of Zagreb, Pierottijeva 6, p. p. 625, HR-10001 Zagreb, Croatia Received 5 July 2006; received in revised form 12 December 2006; accepted 21 December 2006

Abstract Pear purees and cubes were dehydrated with sugars (sucrose and trehalose) addition. Freeze drying and foam-mat drying were used. Manual solid-phase microextraction (SPME) coupled with gas chromatography (GC–flame ionization detector (FID) and GC–MS) was applied to determine the changes in retention of aroma compounds in dehydrated pear purees and cubes. The best retention of aroma compounds in dehydrated pear purees was noticed in the case when freeze drying and trehalose addition were combined. In dehydrated pear cubes, previously dipped in trehalose solution, the highest aroma retention was also determined. This study showed possible application of trehalose as potentially beneficial food ingredient, with the aim to improve the quality of dehydrated fruit products, especially their aroma, and to produce superior dried fruit products or ingredients, which are widely used in food formulation. r 2007 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. Keywords: Aroma retention; Fruit; Pear; SPME; Trehalose; Volatiles

1. Introduction Recently, much attention has been paid to the quality feature, especially aroma, of dehydrated products (Tsami & Katsioti, 2000). Dehydrated fruit pieces and purees are widely used as ingredients for many food products, such as fruit ice cream, yoghurt, cereal, bakery goods and others in food formulation. Because of increased consumers’ demand for attractive colour and flavour or specific texture, the retaining of these food attributes is very important objective of any processing methods (Moreno, Bugueno, Velasco, Petzold, & Tabilo-Munizaga, 2004). Dehydration is one of the preservation techniques for foods and food materials, but like many other evaporative concentration processes it is often linked with reduced product quality resulting from large losses of aroma compounds. In regard to the low processing temperatures, applied in freeze drying, thermal degradation reactions are excluded and high aroma retention is attainable in a porous product with excellent rehydration properties (Beaudry, Raghavan, Ratti, & Rennie, 2004; Coumans, Kerkhof, & Bruin, 1994; Sabarez, Price, & Korth, 2000). Corresponding author. Tel.: +385 1 4816 252; fax: +385 1 4826 251.

E-mail address: [email protected] (D. Komes).

In order to explain the retention of homogeneously dissolved volatile aroma components during drying of liquid food materials, the concept of selective diffusion and the microregion were proposed. The selective diffusion concept is built on the observed fact that the diffusion coefficient of water in concentrated solutions behaves quite different from that of other substances such as aroma components. Both diffusion coefficients are strongly dependent on concentration but the decrease with increasing solids content of the aroma components diffusion coefficient is substantially stronger than that of water. This behaviour is fairly general and not very dependent on different molecular size: even the diffusion coefficient of oxygen shows this behaviour. The microregion concept of Flink and Karel basically assumes that during freezing and subsequent drying ‘‘microregions’’ are formed inside the liquid food in which aroma molecules are trapped (Bruin, 2000). Since positive influence of sucrose and some other sugars on aroma retention are also already known (Lovric´ & Pozderovic´, 1986; Lovric´, Pilizˇota & Abramovic´, 1983), particular attention has been paid to the positive protective aspects of trehalose, which is currently being considered as a potentially beneficial food additive (Macdonald & Johari, 2000).

0023-6438/$30.00 r 2007 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2006.12.011

ARTICLE IN PRESS D. Komes et al. / LWT 40 (2007) 1578–1586

Trehalose is a non-reducing, bland, non-toxic, dietary disaccharide, which does not change significantly the flavour of food to which it is added. It is not rare but is rather commonly found in nature including in mushrooms, honey and baker’s yeasts, some of which are known to contain almost 20% of trehalose on a dry solid basis and are usually eaten (Colaco & Roser, 1994; Ekdawi-Sever, Conrad, & de Pablo, 2001). When sucrose and its solutions were compared with trehalose, the former demonstrated higher water diffusion coefficients, lower Tg, lower densities and higher intramolecular hydrogen bonding. Also, trehalose appears to exhibit a higher hydration capacity than sucrose. These characteristics play an important role in preservation process (Colaco & Roser, 1994). Glass-softening temperature, Tg, of trehalose is the highest among sugars and its value decreases on hydration, or increases on dehydration, as sensitively, as those of maltose, sucrose and glucose (Macdonald & Johari, 2000). Trehalose has also been introduced commercially to the US as a food ingredient by Cargill Health & Food Technologies and also recognized by The Food and Drug Administration as GRAS. The object of the present study was to investigate the influence of the addition of different sugars, especially trehalose, on the aroma retention in dehydrated samples of ‘‘Packham’s Triumph’’ pears. Besides apples, pears (Pyrus communis L.) belong to the most important fruit crops of Rosaceae plants grown in Croatia. Complete information involving aroma compounds associated with pear fruit is relatively limited (Lopez, Miro, & Graell, 2001; Suwanagul & Richardson, 1998) compared to many other fruits. About 70 aroma compounds were identified in ‘‘Packham’s Triumph’’ pears (Chervin, Speirs, Loveys, & Patterson, 2000; Suwanagul & Richardson, 1998). The volatile profiles of this variety of pears are characterized by volatile compounds like esters, alcohols, hydrocarbons, aldehydes and ketones. Esters comprise 60–98% of total volatiles among the varieties. Hexyl acetate was observed to be present as a major ester of pear but strong fruity note of pears is a result of the presence of butyl acetate and ethyl butanoate (Berger, 1991). The aromatic ester, 2-phenylethyl acetate, was noticed to contribute a sweet, rose-, honey-like aroma of pear. It was reported to be a flavour constituent of various other fruits such as apple, plum, mango, strawberry and apricot (Berger, 1991; Komes, Lovric´, Kovacˇevic´ Ganic´, Gajdosˇ Kljusuric´, & Banovic´, 2005). Straight-chain alcohols with 2–8 carbons are present as the second largest group in the profile. Several aldehydes and ketones, as well as other aroma compounds, are also found in some varieties but are present in very small amounts. Headspace solid-phase microextraction (SPME), as a solventless sample preparation technique (Vas, Ko˜teleky, Farkas, Dobo, & Vekey, 1998; Zhang & Pawliszyn, 1993), in combination with gas chromatography (GC–flame ionization detector (FID) and GC–MS), was applied to

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determine the changes in retention of fruit volatiles in dehydrated pear products. 2. Materials and methods 2.1. Sample preparation The present study was carried out with 10 samples, including pear cubes and pear purees (10% solids). Pears (P. communis L. var. ‘‘Packham’s Triumph’’) were purchased from a local market. Preparation of pear cubes consisted of hand peeling and cutting the pears into uniform cubes of 5 mm, followed by dipping in sugar solution with added ascorbic acid (1%) as inhibitor of surface browning. According to Luh, Kean, and Woodroof (1986), two samples of pear cubes were immersed in 251 Brix sugar (sucrose—sample no. 9 or trehalose— sample no. 10) solution at room temperature and held with constant agitation for 1 h. After draining on mash trays to eliminate excess moisture, fruit cubes were dehydrated in a cabinet drier. Besides the pear cubes (samples no. 8, 9 and 10), the second group of samples was pear purees, which were dehydrated by using foam-mat drying (samples no. 2, 3 and 4) and freeze drying (samples no. 5, 6 and 7). In order to compare the influence of different sugars on flavour retention, pear purees were also prepared with and without the addition of sugars (trehalose, sucrose), as previously described for cubes. Preliminary experiments were carried out in order to evaluate the content of added sugars as well as optimal process conditions required to achieve a significant aroma retention of the product. The amount of added sugar (trehalose or sucrose) in freeze-dried purees was 8% (wet basis). Sugar addition to foam-mat dried products was lower (4%) because of difficulties in the foaming step, where 0.02% Hamulsion CNF (Hahn, Lubeck) and 2% carboxymethylcellulose sodium salt as thickening agents were also used. Foam-mat drying of purees and dehydration of cubes were performed in a cabinet drier (adapted for foam-mat drying) in triplicate for each sample. For pear cubes the following conditions were applied: 100 1C kept for 3 min, 80 1C kept for 3 min, 70 1C kept for 3 min and 60 1C for 60 min. Dehydrated pear cubes reached approximately 7.7% of moisture content. The moisture loss was monitored by periodically weighing the tray. Purees were foam-mat dried to approximately 7.3% moisture content. Temperature was programmed as follows: 100 1C kept for 4 min, 80 1C kept for 3.5 min, then decreased to 65 1C and maintained for 45 min. Freeze drying was performed in freeze drier GAMMA 2-20 (Martin Christ, GmbH, Osterode am Harz, Germany) in triplicate for each sample. The freeze drier consisted of a circular chamber with nine steel trays to hold the samples. Samples were dried until they reached 2% of moisture content. It required a drying time of the order of 27 h. The following conditions were applied: the freezing temperature was 40 1C, the temperature of sublimation was from 20

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to 20 1C under a vacuum of 1.030 mbar and the temperature of isothermal desorption was from 20 to 30 1C under a vacuum of 0.001 mbar. 2.2. Rehydration ratio The rehydration ratio (RO) is the ratio of the mass of the rehydrated sample to the mass of the dehydrated sample (McMinn & Magee, 1997). In this work, RO was determined by placing 5 g of dehydrated pear cubes in 70 mL of distilled boiling water for 5 min. The sample was then transferred to a 7.5-cm Buchner funnel covered with Whatman no. 1 filter paper, filtered under slight vacuum and then weighed. RO was calculated from the ratio of the drained weight of rehydrated pear cubes to the initial weight of the dehydrated sample. 2.3. Headspace solid-phase microextraction (HS-SPME)

comparison with the mass spectrum of authentic references. All analyses were carried out in triplicate for each sample. Data presented are means of peak area ratio7 standard deviation or the percentage. The peak area ratio was calculated by the dividing the peak area of individual compounds by the peak area of an IS (3-nonanone). The percentage refers to the retention of total aroma in dehydrated pear samples in relation to the data obtained for total aroma in the pears before drying. 2.5. Chemicals The standards of aroma compounds and trehalose were purchased from Merck (Darmstadt, Germany). Sucrose was obtained from Kemika (Zagreb, Croatia), Hamulsion CNF was from Hahn (Lubeck, Germany) and carboxymethylcellulose sodium from Kristall-Chemie (Wiener Neudorf, Austria).

The used SPME device was a Sulpeco (Bellefonte, PA) manual SPME holder 57330-U. Fused silica fibre coated with polydimethylsiloxane (PDMS) with 100 mm film thicknesses (Sulpeco) was used for extraction and concentration of volatile compounds. The fibre was preconditioned at 250oC for 1 h in the inlet of the GC prior to sampling as instructed by the manufacturer. The rehydration of samples (20 mL) was followed by the addition of internal standard (IS), 3-nonanone (0.5 ppm v/v) and 4 g of NaCl p.a., which were also added to the original, not dried puree. Then, samples were capped in 50-mL glass vials, warmed to 50 1C in water bath and gently mixed. Samples were equilibrated for 10 min prior to insertion of the fibre. The SPME fibre was exposed to the sample headspace at 50 1C for 30 min and immediately transferred to the GC injection port at 200 1C for 3 min in splitless mode. Blank runs were performed regularly prior to sample analysis to ensure the removal of impurities.

2.6. Statistics

2.4. Chromatography

3. Results and discussion

All samples were analysed with a Varian 3300 gas chromatograph equipped with an FID. Compounds were separated on a DB 624 column (30 m  0.32 mm i.d., 1.8 mm film thickness; J&W Scientific, Folsom, CA). Nitrogen was used as a carrier gas at the flow rate of 1 mL/min. A split/splitless injector was used (ratio 1:5) and maintained at 200 1C. The detector was kept at 250 1C. Temperature programme was as follows: 3 min at 40 1C, from 40 to 190 1C at 5 1C/min and 10 min at 190 1C. The same conditions were applied for the GC–MS analysis on a Hewlett-Packard 5890 gas chromatograph with a 5970 series mass selective detector. The mass spectrometer was operated in electron impact mode (70 eV) and the masses were scanned over the range of 30–250m/z. Carrier gas was helium. Compounds were identified using the nbs75k library of mass spectra and by

In order to evaluate the influence of the addition of trehalose on the improvement of quality of dehydrated fruit products, this study was based on the differences in retention of aroma compounds determined in dehydrated pear samples with the addition of sugars compared to pears before dehydration. The retention of total aroma in dehydrated pear purees and cubes in comparison with pears before drying (100%) is shown in Table 1. Among the samples of the dehydrated pear cubes, the best total aroma retention (46%) was obtained in the sample previously dipped in trehalose solution, followed by pear cubes dipped in sucrose solution with 38% of aroma retention and the lowest retention was determined in pear cubes without previous dipping in sugar solution (31%). Besides this higher retention of total aroma, the highest RO

In order to determine the influence of the addition of sugars as well as the dehydration process, on the aroma retention in dehydrated pear samples, ANOVA and Duncan’s multiple range test (Montgomery, 2000), the similarity coefficient (Datta & Nakai, 1992) and multivariate analysis (Esbensen, 2000) were applied. The peak area ratio of the identified aroma compounds of pears as a numerical value was used. The value of similarity coefficient ranges from 0 to 1 with 1 meaning dehydrated pear samples are completely identical to pears before drying and 0 meaning that there is no similarity. The computer program Mathematica 5.0 (Wolfram Research, USA) was employed to determine this coefficient. Principal component analysis (PCA) was performed with the Statistica 6.0 (StatSoft, Inc., Tulsa, OK, USA).

ARTICLE IN PRESS D. Komes et al. / LWT 40 (2007) 1578–1586 Table 1 Total aroma retention (%) in dehydrated pear samples in comparison with pears before drying (100%)

Foam-mat dried puree Freeze-dried puree Dried cubes

Without sugar addition

With sucrose addition

With trehalose addition

31

29

38

62

66

92

31

38

46

of 3.8% at 5 min was noticed in the sample of pear cubes dehydrated after dipping in the trehalose solution. The RO of the other dehydrated samples previously dipped in sucrose solution and without previous dipping in the sugar solution were 3.4% and 2.9%, respectively. The higher RO indicates that dehydrated pear cubes have fine porous structure, which contributes to their better reconstitution keeping compartmental properties of these products. A short-time reconstitution capacity may be advantageous for dried pear cubes when they are used for breakfast cereals, since they are consumed with milk within minutes of mixing (Beaudry et al., 2004; Feng & Tang, 1999). As can be seen in Table 1, the retention of total aroma in dehydrated pear purees was in the range from 29% in foam-mat dried pear puree with sucrose addition to 92% in freeze-dried puree with trehalose addition. The retention of fruit volatiles in dehydrated pear purees varied in relation to the dehydration process and the kind of sugar added. As could be expected, the retention of flavour compounds in freeze-dried puree with the addition of sugar was the best, whether compared to the samples without sugar addition or to foam-mat dried samples (Bruin, 2000; Erba, Forni, Colonello, & Giangiacomo, 1994). The least loss of total aroma (8%), as well as of individual fruit volatiles, was obtained in the case when freeze drying and trehalose addition were combined (Tables 1 and 3). This means that the addition of sugar into the pear puree prior to dehydration as well as process pressure prevented loss of a part of fruit volatiles and increased the retention of its characteristic aroma in dehydrated products, which is in agreement with the results obtained in previous research (Komes, Lovric´, Kovacˇevic´ Ganic´, & Gracin, 2003; Lovric´ & Pozderovic´, 1986). Nineteen compounds were identified from the headspace sample of ripened ‘‘Packham’s Triumph’’ pears. Compounds were divided into groups of esters and alcohols, carbonyls and terpenes. The results are shown as the peak area ratio of aroma compounds with their corresponding relative standard deviation. The peak area ratio was calculated by dividing the peak area of individual compounds by the peak area of an IS, 3-nonanone. ‘‘Packham’s Triumph’’ pears are known to have very similar flavour to Bartlett pears, which are the most aromatic and favourite pears. As can be seen, most of the identified compounds of ‘‘Packham’s Triumph’’ pears were

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esters and alcohols. According to literature data, hexyl acetate and butyl acetate were the two major esters, which accounted for more than 80% of the profile (Chervin et al., 2000; Suwanagul & Richardson, 1998). In agreement with the previous result (Suwanagul & Richardson, 1998), when compared to the amount of butyl acetate, the amount of hexyl acetate was on an average twice as high in pears before dehydration. The retention of the aromatic ester, 2-phenylethyl acetate with its flowery and sweet-like aroma, was also detected. Besides typical previously reported esters, methyl and ethyl butanoate, propyl acetate, n-amyl acetate, butyl butanoate, ethyl hexanoate, ethyl octanoate and ethyl decanoate were determined. In foam-mat dried pear puree without sugar addition, all esters, except butyl butanoate, were retained in the average percentage of 33%. The addition of sucrose contributed to their preservation of 37% and the addition of trehalose resulted in their retention of 32%. From the results presented in Table 2, it is evident that neither the addition of sucrose nor the addition of trehalose in combination with foam-mat drying had any influence on retention of propyl acetate, whereas methyl butanoate was retained in puree with sucrose addition in the percentage of 15% and of 25% in puree with trehalose addition. Very small amount of butyl and hexyl acetate was determined in foammat dried pear purees, whereas their retention as high as 90% in freeze-dried purees was determined. In freeze-dried pear puree with trehalose addition the retention of most esters was almost 90% in comparison with pears before dehydration, except volatile esters such as ethyl butanoate, methyl butanoate and propyl acetate, which were retained in smaller percentage (Table 3). In dehydrated pear cubes, the highest percentage of retention among esters was shown by ethyl hexanoate, butyl butanoate, ethyl octanoate and ethyl decanoate (Table 4). In the case of pear cubes dipped in trehalose solution, more than 50% of these esters were determined. The decrease of the content of esters, as well as of other aroma compounds, was related to the decrease in quality of the product (Bruin, 2000). In this case, the addition of sugars, especially trehalose, has an important role. The obtained results of the statistical analysis (ANOVA and Duncan’s test) showed that in comparison with the pears before dehydration, all dehydrated samples showed significant differences (Po0.05) related to the retention of total aroma. It is interesting that by freeze drying only the retention of propyl acetate was not statistically different among the purees with the addition of different sugars (Table 3). Ester synthesis in pears was reported to be derived from b-oxidation and lipoxygenase-catalyzed oxidation of fatty acids. The varieties, which contained high amounts of unsaturated fatty acids or those having enzyme systems favouring this type, are likely to produce esters with a high degree of unsaturation and yield a large number of esters, more than those varieties that contained less unsaturated fatty acids or contained mainly saturated fatty acids. Alcohols constitute the second largest group of aroma compounds in

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1582 Table 2 Peak area ratio in foam-mat dried pear purees No.

Compounds

Pear puree before drying (1) Peak area ratio7RSD (%)

Puree without sugar addition (2) Peak area ratio7RSD (%)

Puree with sucrose addition (3) Peak area ratio7RSD (%)

Puree with trehalose addition (4) Peak area ratio7RSD (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Methyl butanoate Propyl acetate Ethyl butanoate Hexanal Butyl acetate 2-hexenal 1-hexanol n-amyl acetate Butyl butanoate Ethyl hexanoate Hexyl acetate 1-octanol Linalool 2-phenylethanol Ethyl octanoate a-terpineol Citronellol 2-phenylethyl acetate Ethyl decanoate

0.0475.9 0.02711.9 0.1075.8 0.0476.6 0.9277.5 0.0374.3 0.0874.9 0.0679.5 0.0377.3 0.0176.7 2.2472.5 0.0174.8 0.0273.8 0.0273.8 0.0175.3 0.0275.0 0.0273.2 0.0374.6 0.0273.8

— 0.0174.2 0.00577.4 0.0273.9 0.0573.6 0.0276.5 0.00375.91 0.0277.2 0.02715.1 0.0178.72 0.0172.63 — 0.005711.4 0.00474.84 0.0176.3 0.0175.3 0.00375.7 0.0173.0 0.0177.35

0.0171.9 — 0.0777.0 0.00373.8 0.0672.3 0.0174.8 0.003710.81 0.00474.6a 0.0574.6 0.0178.72 0.0375.43,b 0.00277.8 0.0175.6 0.00475.94 0.0173.9c 0.0176.5 0.0174.6 0.0273.9 0.0173.55,d

0.0179.7 — 0.0774.0 0.0175.5 0.0574.0 0.00374.2 0.0172.7 0.00274.8a 0.0573.2 0.0178.3 0.0272.9b — 0.0176.4 0.0173.4 0.0176.1c 0.0174.0 0.00371.0 0.00373.0 0.0178.0d

There is no statistically difference (Duncan’s test, Po0.05) in retention of aroma compounds between the purees with sucrose and trehalose addition denoted by the same letter a,b,c,d and between the purees without addition and with sucrose addition denoted by numbers 1,2,3,4,5, RSD—relative standard deviation.

Table 3 Peak area ratio in freeze-dried pear purees No.

Compounds

Pear puree before drying (1) Peak area ratio7RSD (%)

Puree without sugar addition (5) Peak area ratio7RSD (%)

Puree with sucrose addition (6) Peak area ratio7RSD (%)

Puree with trehalose addition (7) Peak area ratio7RSD (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Methyl butanoate Propyl acetate Ethyl butanoate Hexanal Butyl acetate 2-hexenal 1-hexanol n-amyl acetate Butyl butanoate Ethyl hexanoate Hexyl acetate 1-octanol Linalool 2-phenylethanol Ethyl octanoate a-terpineol Citronellol 2-phenylethyl acetate Ethyl decanoate

0.0475.9 0.02711.9 0.1075.8 0.0476.6 0.9277.5 0.0374.3 0.0874.9 0.0679.5 0.0377.3 0.0176.7 2.2472.5 0.0174.8 0.0273.8 0.0273.8 0.0175.3 0.0275.0 0.0273.2 0.0374.55 0.0273.77

0.0174.9 0.0173.6 0.0277.5 0.00373.9 0.5172.21 0.0272.52 0.0171.5 0.0473.63 0.0172.8 0.00473.6 1.1578.4 0.00176.3 0.0171.9 0.0173.7 0.00475.9 0.0172.2 0.0173.3 0.0177.53 0.0173.40

0.0172.9 0.00572.0a 0.0376.4 0.0174.7 0.5172.21 0.0172.02 0.0174.4 0.0472.33 0.0276.6 0.0171.2 1.5779.8 0.00273.7 0.0172.0 0.0177.3 0.00574.0 0.0172.6 0.0175.1 0.0375.9 0.0272.5

0.0174.2 0.0172.3a 0.0477.3 0.0274.5 0.8674.4 0.0274.8 0.0373.1 0.0576.2 0.0276.7 0.0177.7 1.9674.9 0.00371.8 0.0272.5 0.0173.3 0.0179.1 0.0173.2 0.0175.2 0.0374.4 0.0275.3

There is no statistically difference (Duncan’s test, Po0.05) in retention of aroma compounds between the purees with sucrose and trehalose addition denoted by the same letter a and between the purees without addition and with sucrose addition denoted by numbers 1,2,3, RSD—relative standard deviation.

pear profiles. They account for 2–14% of total aroma compounds (Suwanagul & Richardson, 1998). From the class of alcohols, 1-hexanol, 1-octanol and 2-phenylethanol

were identified. 1-octanol demonstrated the highest retention in freeze-dried purees, especially in the puree with trehalose addition (84%). An interesting feature of the

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Table 4 Peak area ratio in dehydrated pear cubes No.

Compounds

Pear puree before drying (1) Peak area ratio7RSD (%)

Cubes without sugar addition (8) Peak area ratio7RSD (%)

Cubes with sucrose addition (9) Peak area ratio7RSD (%)

Cubes with trehalose addition (10) Peak area ratio7RSD (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Methyl butanoate Propyl acetate Ethyl butanoate Hexanal Butyl acetate 2-hexenal 1-hexanol n-amyl acetate Butyl butanoate Ethyl hexanoate Hexyl acetate 1-octanol Linalool 2-phenylethanol Ethyl octanoate a-terpineol Citronellol 2-phenylethyl acetate Ethyl decanoate

0.0475.9 0.02711.9 0.1075.8 0.0476.6 0.9277.5 0.0374.3 0.0874.9 0.0679.5 0.0377.3 0.0176.7 2.2472.5 0.00474.8 0.0273.8 0.0273.8 0.0175.3 0.0275.0 0.0273.2 0.0374.6 0.0273.8

0.00277.2 0.00178.1 0.0175.5 0.00475.2 0.0376.3 0.00277.4 0.00477.6 0.00475.31 0.0174.1 0.00775.82 0.1575.6 0.0175.13 0.0174.74 0.00375.0 0.00873.5 0.0173.4 0.00275.55 0.00474.76 0.0173.6

0.00674.0a 0.00577.5 0.02710.5 0.00277.1 0.0174.1 0.00473.3 0.0176.7 0.0177.81,b 0.0179.3 0.0173.72,c 0.3272.1 0.0176.33 0.0173.64 0.00575.5 0.0175.7 0.0178.3 0.00374.15 0.00472.86 0.0276.2

0.0175.4a 0.00372.9 0.0274.4 0.00473.2 0.0772.6 0.0175.9 0.0172.7 0.0176.5b 0.0271.8 0.0173.1c 0.4272.5 0.0174.0 0.0275.1 0.00273.4 0.0173.1 0.0176.0 0.00473.5 0.0176.5 0.0174.6

There is no statistically difference (Duncan’s test, Po0.05) in retention of aroma compounds between the purees with sucrose and trehalose addition denoted by the same letter a,b,c and between the purees without addition and with sucrose addition denoted by numbers 1,2,3,4,5.

varieties with the high alcohol content is lower production of esters, indicated by the number of the esters in the profile. This may be the reflection of low activity of endogenous esterifying enzyme systems in converting alcohols to esters (Suwanagul & Richardson, 1998). Unlike other pome fruits such as apples and quinces, pears produced very small amounts of aldehydes. Hexanal and 2-hexenal were the only aldehydes found among recently studied pear profiles (Chervin et al., 2000; Suwanagul & Richardson, 1998) and they were also determined in the present study. The addition of trehalose in pear puree dehydrated by freeze drying high retention of 2-hexenal (73%) was demonstrated, whereas in the foammat dried puree with trehalose the retention of these aldehydes was the lowest (9%). From the group of terpenes, a-terpineol, linalool and citronellol were identified. The addition of sucrose in the puree dehydrated by foam-mat drying contributed to the highest preservation of terpenes. The amount of 76% of a-terpineol, 50% of linalool and 38% of citronellol was retained. When compared to the pear puree without any additions, the addition of trehalose in the course of foam-mat drying did not affect the retention of terpenes. In the case of freeze drying, the retention of terpenes was the highest in dehydrated puree with the previously added trehalose. In dehydrated pear cubes, terpenes demonstrated the lowest retention in the samples of cubes previously dipped in a solution without sugar, followed by cubes with sucrose with somewhat higher retention, whereas the highest retention of these compounds was determined in cubes with trehalose.

Table 5 Similarity coefficient of the dehydrated pear purees and cubes in comparison with pears before drying

Foam-mat dried puree Freeze-dried puree Dried cubes

Without sugar addition

With sucrose addition

With trehalose addition

0.63

0.69

0.83

0.93

0.97

0.99

0.77

0.74

0.85

In order to compare the obtained data regarding the retention of aroma compounds in dehydrated purees and cubes, and the pears before dehydration, the similarity coefficient was applied (Datta & Nakai, 1992). The highest value for the similarity coefficient amounting to 0.99 was determined for the freeze-dried puree with trehalose (Table 5). The same results were obtained for pear cubes in comparison with the pears before dehydration. The value of 0.85, determined for dehydrated pear cubes previously dipped in trehalose solution, is the closest to similarity coefficient of pears before dehydration whose value is 1. These results confirm the positive influence of trehalose addition on the improvement of the quality of dehydrated products. On the basis of the results presented in Tables 2–4, the retention of each individual flavour compound in all studied samples of pears was compared in relation to the pears before drying by applying the multivariate analysis, PCA (Esbensen, 2000). Multivariate analysis of the quantitative data classified the various samples

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0.7

6 5 4

0.5

3

0.4 9

1

8

1

PC 2

PC 2

2

7

0

2 3

6

-1

5

0.3 0.2 0.1

4

-2

-0.1

-4

-0.2

-8

-6

-4

-2

0

2

4

6

PC 1

3 9

2 1 5

1 13 1618 110 914 17 7 12 15 2 6 48

0 -1 -2 11

-3 -4

-3

-2

-1

11

-8

-6

-4

-2

0

2

PC 1

Fig. 1. Plot of PC-1 vs. PC-2 for the dehydrated pear samples in relation to the pears before drying (the numbers of samples refer to Tables 2–4).

3

37 4 8691 14 13 16 2 19 17 10 15 1812

0.0

-3 -10

PC 2

5

0.6 10

0

1

2

PC 1 Fig. 2. Plot of PC-1 vs. PC-2 for aroma compounds in foam-mat dried pear purees (the numbers of compounds refer to Table 2).

according to processing conditions. The graphically displayed classifications corresponded to expected flavour quality. The obtained results of PCA for all studied samples of pears were projected on a two-dimensional plot defined by the first two principal components, PC-1 and PC-2 (Fig. 1). PC-1 is linear expression, which contains the maximum amount of variation among the content of aroma compounds in all samples (Fig. 1) as well as in foam-mat dried (Fig. 2) and freeze-dried (Fig. 3) samples. The second principal component (PC-2) is chosen orthogonal to first principal component, and contains the greatest remaining variation among the content of aroma compounds that is unrelated to PC-1. The samples were grouped according to the used method of dehydration. Oval forms shown on Fig. 1 separate pear purees into three categories, where samples no. 2–4 indicate foam-mat dried

Fig. 3. Plot of PC-1 vs. PC-2 for aroma compounds in freeze-dried pear purees (the numbers of compounds refer to Table 3).

purees, samples no. 5–7 indicate freeze-dried purees and samples no. 8–10 indicate dried pear cubes. Sample no. 1 indicates pear puree before drying (control sample). As could be expected, the freeze-dried pear puree with trehalose addition (sample no. 7) is the nearest to the pears before drying (sample no. 1), which is related to the lowest loss of aroma compounds during freeze drying in comparison with the pear puree before drying. These results can potentially help processors determine product quality without sensory evaluation measurements, and suggest changes in processing conditions to improve flavour of processed pear purees. The results obtained by applying PCA, separately for freeze dried and foam-mat dried purees, are shown in Figs. 2 and 3. Most components in foam-mat dried as well as in freeze dried are very close together, indicating that they provide similar information. Butyl acetate and hexyl acetate in freeze dried, as well as these two compounds, butyl butanoate and ethyl butanoate in foam-mat dried, are separated indicating that they show specifics regarding their retention depending on used method or sugar addition. By applying this analysis, ethyl butanoate, butyl acetate, hexyl acetate and butyl butanoate were shown to be the most represented aromatic compounds, which is in agreement with literature data (Chervin et al., 2000; Suwanagul & Richardson, 1998). The retention of some original compounds after drying may be related to their volatility. Sugar addition into the pear samples prior to dehydration prevented loss of a part of fruit volatiles and increased the retention of its characteristic aroma in dehydrated products, which is in agreement with the results obtained in previous research (Komes et al., 2003; Lovric´ & Pozderovic´, 1986). These results suggested the dependence of retention ability of some carbohydrates on their molecular weight because the addition of carbohydrates with higher molecular weight contributes to the higher retention of aroma compounds in

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dehydrated products. However, the difference in flavour retention ability registered between sucrose and trehalose cannot be attributed to this factor, but to other more complex mechanisms. In addition, since the flavour volatiles are generally larger than water molecules, they may not readily diffuse (Rulkens, 1973) or are trapped (Flink & Karel, 1970) within the fruit matrix during drying. According to Saravacos (1993), carbohydrates are known as substances which lock-in volatile compounds. This shows that some original flavour compounds could be retained within the dried solid, which are responsible for the natural aroma of the product. Besides these theories and mechanisms of aroma retention during dehydration, aroma retention is certainly affected by the phenomenon of the reduction of molar ratio of the more volatile compounds in the vapour phase above the sample immediately after the addition of sucrose and trehalose before drying, as well as by the formation of microregions in the dried layer of the material. In order to explain the mechanism of the trehalose action three theories were suggested: water replacement hypothesis, glass transformation and chemical stability hypotheses. The ‘‘water replacement’’ hypothesis, first proposed by Crowe et al. (1994), assumes that sugars hydrogen bond to biomolecules during dehydratation or freeze.drying, acting as substitutes of hydration water molecules. Trehalose has superior effects in ‘‘destructuring’’ the network of water and in slowing down its dynamics. These two properties could play a key role in the understanding of the microscopic mechanisms of bioprotection. Beside many other characteristics, very interesting is property of glass transition which prevents the loss of small hydrophobic volatile esters during drying and storage, thus ensuring their release only on rewetting and dissolution of the glassy matrix. Unlike many other sugars that also undergo glass transition, trehalose produces glasses that are not hygroscopes (Ekdawi-Sever et al., 2001). By binding water molecules more tightly, the glass formed by trehalose could hinder molecular motion more effectively, possibly leading to its superior cryo- and lyoprotection. The amorphous matrix can exist either as a very viscous glassy state or as a more liquid-like rubbery state. Water plays the role of a plasticizer and moisture content has a strong influence on the transition temperature like the glass transition temperature, Tg, the crystallization temperature, Tcr, the melting temperature, Tm, of food material. When the matrix is in the glassy state, volatiles are encapsulated in the amorphous glass and low mobility leading to the increased stability of the material being preserved. Above the glassy state, temperature collapses and sometimes crystallization takes place with a characteristic crystallization time, Tcr, and the encapsulated volatiles are released (Bruin, 2000). Librizzi et al. (1999) suggest that the behaviour of trehalose samples in the amorphous–microcrystal and microcrystal–amorphous transition may be most relevant in determining the peculiarity of this sugar as bioprotectant.

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As conclusion it can be seen that the best retention of aroma compounds in dehydrated pear puree was obtained by trehalose addition, regardless of the dehydration process applied (freeze drying or foam-mat drying), although much higher retention of flavour volatiles was obtained in freeze-dried purees. The aforementioned products are free of preservatives, maintain their natural flavour and colour and have an agreeable texture and good rehydratability properties (Erba et al., 1994). Therefore, they could be widely used as ingredients in bakery products, ice cream or yoghurt. In order to obtain the complete information regarding the influence of trehalose addition on aroma retention, this research requires further studies. In this way, by varying the sweetness and acidity of the canning syrup according to the characteristics of the raw fruit, a more acceptable flavour in canned fruit can be obtained (Luh et al., 1986). Acknowledgements This work was supported by the Ministry of Science and Technology, Republic of Croatia. The authors also would like to thank Emil Zlaticˇ, M.Sc., from Biotecnical Faculty, University of Ljubljana, Slovenia, for his assistance during freeze drying. References Beaudry, C., Raghavan, G. S. V., Ratti, C., & Rennie, T. J. (2004). Effect of four drying methods on the quality of osmotically dehydrated cranberries. Drying Technology, 22(3), 521–539. Berger, R. G. (1991). Fruits I. In H. Maarse (Ed.), Volatile compounds in foods and beverages (pp. 287–304). New York: Marcel Dekker Inc. Bruin, S. (2000). Flavor retention in dehydration processes. In R. P. Singh, & M. A. Wirakartakusumah (Eds.), Advances in food engineering (pp. 15–32). Boca Raton, FL: CRC Press Inc., Corporate Blvd. Chervin, C., Speirs, J., Loveys, B., & Patterson, B. D. (2000). Influence of low oxygen storage on aroma compounds of whole pears and crushed pear flesh. Postharvest Biology and Technology, 19, 279–285. Colaco, C. A. L. S., & Roser, B. (1994). Trehalose—a multifunctional additive for food preservation. In R. Heis (Ed.), Food packaging and preservation (pp. 123–140). London: Blackie Academic and Professional Press. Coumans, W. J., Kerkhof, P. J. A. M., & Bruin, S. (1994). Theoretical and practical aspects of aroma retention in spray drying and freeze drying. Drying Technology, 12(1&2), 99–149. Crowe, J. H., Leslie, S. B., & Crowe, L. M. (1994). Is vitrification sufficient to preserve liposomes during freeze drying? Cryobiology, 31, 355–366. Datta, S., & Nakai, S. (1992). Computer-aided optimization of wine blending. Journal of Food Science, 57, 178–183. Ekdawi-Sever, N., Conrad, P. B., & de Pablo, J. J. (2001). Molecular simulations of sucrose solutions near the glass transition temperature. Journal of Physical Chemistry, 105, 734–742. Erba, M. I., Forni, E., Colonello, A., & Giangiacomo, R. (1994). Influence of sugar composition and air dehydration levels on the chemical–physical characteristics of osmodehydrofrozen fruit. Food Chemistry, 50, 69–73. Esbensen, K. H. (2000). Multivariate data analysis—In practice (pp. 1–115). Corvallis: Camo Inc. Feng, H., & Tang, J. (1999). Microwave and spouted bed drying of frozen blueberries the effect of drying and pretreatment methods on physical

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