Texture of Vacuum Microwave Dehydrated Apple Chips As Affected by Calcium Pretreatment, Vacuum Level, and Apple Variety P

JFS: Food Engineering and Physical Properties

Texture of Vacuum Microwave Dehydrated Apple Chips as Affected by Calcium Pretreatment, Vacuum Level, and Apple Variety P. W. Y. S HAM, C.H. SCAMAN, T.D. DURANCE

ABSTRACT: The effects of calcium pretreatment, vacuum level, and apple variety on the texture of apple chips, processed using a combination of air drying and vacuum microwave dehydration (VMD), were determined. Pre-

treatment of apple slices by immersion in 1-5% aqueous CaCl2 significantly increased crispness of chips as determined by instrumental and sensory analysis; however above 1%, chips were perceived as bitter. Higher vacuum applied during VMD significantly lowered density and increased crispness of chips. This effect was mediated by the vaporization of water in the interior of the chip, which caused expansion of the tissue. Chips made from Fuji apples had higher calcium contents, and were crisper than Red and Golden Delicious apple chips. Microstructure of the chips evaluated by scanning electron microscopy indicated that chips with thicker cell walls and large internal voids were crisper. Keywords: vacuum microwave dehydration, apple chips, calcium, texture, structure

Introduction wave drying process can be utilized to expand the structure PPLES ARE PERCEIVED AS A HEALTHY FOOD (LEE AND MAT- of some products to yield a puffy texture which is similar to Atick 1989) and are a desirable ingredient for fruit-based that created by frying (Durance 1997; Lin and others 1998 & snacks. The production of crisp apple chips with low fat and 1999; Durance and Liu 1996). Therefore, with the use of vac- high fiber content are expected to be highly desirable by uum microwave drying, a unique oil-free, crisp dehydrated consumers (Matz 1993). Apple chips can be dehydrated by apple chip product may be developed. various conventional methods but each of these has limita- A better understanding of the physical and textural chang- tions. Reduction of nutritional value and development of un- es that foods undergo during vacuum microwave drying is desirable colors, flavors, and textures are observed in oil- needed, so that processing parameters can be readily manipu- fried or hot air-dried apple chips, and other fruit and lated to obtain the desired characteristics in the final product. vegetables products (Matz 1993; Yongsawatdigul and Gu- In this study, the textural characteristics of vacuum micro-

nasekaran 1996). Although freeze-drying usually yields prod- wave dehydrated apple chips were specifically analyzed in re- Food Engineering and Physical Properties ucts with a good appearance, it results in a non-crisp lation to calcium pretreatment, vacuum level during dehydra- sponge-like structure and may result in flavor losses (Flink tion, and apple variety. Calcium has widely been reported to 1975). Also, freeze-drying is time consuming and expensive play an important role in preserving structural integrity and since the process cannot be readily carried out on a continu- mechanical strength of cell walls (Poovaiah and others 1988). ous basis. Infiltration of calcium to increase the firmness of fresh and Vacuum microwave dehydration (VMD) offers an alterna- processed fruit products, particularly apples, is well docu- tive way to improve the quality of dehydrated products. The mented (Glenn and Poovaiah 1990; Poovaiah 1986; Stow 1989). vacuum permits water to vaporize at a lower temperature Therefore, the effect of calcium treatment (1-5% CaCl2) prior and faster rate than at atmospheric pressure (Petrucci and to dehydration on the textural characteristics of apple chip Clary 1989). Microwaves penetrate to the interior of the food was studied. Also, the effect of vacuum level employed during causing water to boil within the food. This results in a greatly VMD on apple chips texture was investigated to determine the increased vapor pressure differential between the center and influence on puffing and crispness of the product. Further- the surface of the product, allowing rapid transfer of mois- more, since different varieties of apples have different structure out of the food. The moisture is removed from the dry- tural features and processing qualities (Lapsley and others ing chamber by the water-ring pump, which maintains the 1992; Trakoontivakorn and others 1988; Summers 1994), the vacuum. Therefore, when vacuum and microwave heating textural characteristics of apple chips made from 3 commonly are combined, foods can be dried rapidly without exposure available apple varieties— Golden Delicious, Red Delicious, to high temperatures. Further, the reduced exposure to air and Fuji— were examined. during drying may help to reduce oxidative deterioration, and color, flavor, and nutrient properties of products can be Materials and Methods largely preserved. Cranberries (Yongsawatdigul and Gu- nasekaran 1996), shrimp (Lin and others 1999), krill (Durance Source of Materials 1997), potato chips (Durance and Liu 1996), bananas (Drou- Apples (Golden Delicious, Red Delicious, and Fuji) were zas and Schubert 1996), and grapes (Petrucci and Clary 1989) purchased from local stores. Only unbruised, sound fruits have been successfully dehydrated by VMD, resulting in high were selected. Food grade calcium chloride was obtained quality products. It has been shown that the vacuum micro- from General Chemical Canada Ltd., Mississauga, Ontario.

All other chemicals were of reagent grade. and dissolved in 5 ml of 6N HCl. The suspension was warmed to effect a complete solution. The samples were Sample Preparation then diluted with 1% LaCl3 solution (Fisher) and analyzed Part 1. Calcium Pretreatment. 1 kg of Golden Delicious for calcium content with a Perkin Elmer Atomic Absorp- apples was prepared for each drying process. Apples were tion Spectrophotometer (Woodbridge, Ont., Canada). All washed, sliced to a thickness of 4 mm, steam blanched for 2 calcium values were reported on a dry weight basis. min and immediately dipped in 0, 1, 2, 3, 4, or 5% calcium Instrumental Textural Analysis: The TA XT2 Texture Ana- chloride solutions at room temperature with constant stir- lyzer (Stable Micro System Ltd., Surrey, England) was used to ring for 1 min. Excess solution was shaken from the slices. measure the slope (g/mm) of the first peak produced during Apple slices were air dried on a Vers-a-belt dryer (Wal-Dor the punch and die test. Each chip samples was centered on a Industries Ltd., New Hamburg, Ontario) at 70 °C, with an air metal base with a 1 cm hole, and a circular, flat ended punch flow rate of 1.1m3/min for about 30 min to a moisture con- (5 mm diameter), moving at 7.0 mm/sec was used to break tent of ~50% on a dry weight basis (db). Samples were then the chip. The slope of the first peak recorded in the force/ placed in the high-density polyethylene drying drum in a 2 deformation curve was calculated from the baseline to the kW maximum power microwave vacuum chamber (EnWave peak height, using the software, XT.RA Dimension (Version Corp., Port Coquitlam, B.C.). The drum was rotated at a rate 3.7) (Stable Micro System Ltd., Surrey, England). The slope of of 5 revolutions per minute. After a vacuum of 28 inches of the first peak was essentially linear from baseline to the peak Hg was achieved, samples were vacuum microwave dried at height. 15 measurements were obtained from each of 3 repli- a microwave power of 1.5 kW for 4 minutes followed by an cate samples. Therefore, the slope values were the mean of additional 1 to 2 minutes, until a final moisture content of 45 measurements. ~5% db was obtained. Vacuum was maintained in the cham- Density: The density of apple chips was measured by im- ber by a water-ring vacuum pump, model LEMC 60 (Sihi mersing the weighed samples (~ 20 g) in a 500 ml graduated Pumps Ltd., Guelph, Ont., Canada). Dehydrated samples cylinder pre-filled with 200 ml of dried flax seed, followed by were packed in individual cheesecloth bags, which were then tapping the cylinder lightly against the counter top until no placed in an airtight bag. They were allowed to equilibrate at volume change was observed. Volumes were obtained by

Food Engineering and Physical Properties room temperature for 14 d to the same water activity prior measurement of the flax seed displacement. The data were to analysis. All experiments were performed in triplicate. expressed as the weight of sample (g) per volume (ml). Mea- Part 2. Vacuum Level and Drying Technique. 1 kg of surements were performed in triplicate. Golden Delicious apples were prepared as mentioned in Part Scanning Electron Microscopy (SEM): The structure of the 1, with the following change. Instead of a CaCl2 solution, all dehydrated apple chips was examined using scanning elec- apple slices were dipped in water, at room temperature, for 1 tron microscopy. Apple chips were first dipped in liquid ni- min after steam blanching. trogen and then immediately fragmented into small pieces. To examine the effect of vacuum level, blanched apple Apple chip fragments were attached to SEM stubs and subse- slices were air dried for approximately 30 min to ~50% db. quently coated with gold (~ 25 nm), using the Nanotech The partially dried apple slices were then VMD at vacuum SEMPREP II Sputter Gold Coater, and, finally, stored under levels of 7, 14, 21, 24, and 28 inches of Hg with a constant mi- desiccation (1 to 2 days) until examined by the scanning elec- crowave power of 1.5 kW to obtain apple chips with a mois- tron microscope (Stereoscan 250, Cambridge Instruments ture content of ~5% db. Drying times varied from approxi- Ltd., Cambridge, UK). Polaroid pictures were taken and pro- mately 10 min at 7 in of Hg, to 4 min at 28 in of Hg. cessed as specified by the manufacturer. For the study of the effect of drying techniques, prepara- Sensory Evaluation of Crispness: Quantitative descriptive tion of air-dried apple chips were carried out on a Vers-a- analysis (Stone and others 1974) was carried out to deter- belt dryer (Wal-Dor Industries Ltd., New Hamburg, Ontario). mine the effect of calcium pretreatment on bitterness, and Apple slices were air dried for approximately 3.5 h at 70 ЊC calcium pretreatment and apple variety on the crispness of with air flow rate at 1.1m3/min to a moisture content of ~5% apple chips. Panelists with sensory evaluation experience db. For the preparation of freeze-dried apple chips, apple were recruited from the staff and students of the University slices were freeze-dried under vacuum (100 mic Hg) with a of British Columbia. Selected panelists were then trained in chamber temperature of 20 ЊC and a condenser temperature sensory descriptive evaluation. The sensory analysis was car- of –55 ЊC for approximately 10.5 h. ried out in triplicate. Commercial deep fried Golden Deli- Part 3. Apple Varieties. Red Delicious, Fuji, and Golden cious apple chips (Seneca Foods Corp., Marion, N.Y., U.S.A.) Delicious varieties were used to prepare VMD apple chips as were provided as a reference standard for moderately crisp described above in Part 1, except that blanched apple slices chips. Panels were conducted under red fluorescent light to were dipped in 0 and 1 % CaCl2 solutions for 1 min. minimize the effect of the apple chip color on the assess- ment of the texture attributes. Analysis The panelists were asked to bite through the whole samples Moisture Content Determination. Samples, in triplicate, with their front teeth to evaluate the crispness. A 15 cm un- were weighed into pre-weighed, labeled aluminum weigh structured line scale, with anchor points at 1.5 cm from each boats and then dried in vacuum oven at 70 ЊC and a vacuum end and at the midpoint was used to evaluate the sensory level of 27 in of Hg for 16 h. Upon completion of the drying, crispness. Panelists indicated the intensity of each attribute by weight boats were cooled in a desiccator and weighed. The making a vertical line across the unstructured line. Quantita- moisture content of the sample on a dry weight basis (% db) tion was made by measuring the distance from the left end was then calculated from the difference between the wet and point of the line to each point marked by the panelist. dry weights divided by the dry weight of the sample. Statistical Analysis: Data were analyzed using ANOVA Calcium Content Analysis: Apple chip samples (~ 1 g) (Minitab Inc., State College, Pa., U.S.A., 1994). Data for calci- were ashed in a muffle furnace at 500 ЊC for 24 hours so um levels of chips made from Fuji, Red Delicious, and Gold- that only white ash remained. The residues were cooled en Delicious apples in Part 3 was log transformed prior to

1342 JOURNAL OF FOOD SCIENCE—Vol. 66, No. 9, 2001 Vacuum Microwave Dried Apple Chips . . . analysis due to non-homogeneity of the variance. Differenc- sensory analysis (Figure 2). It has long been known that Ca es among mean values were established using Tukey’s multi- confers rigidity to cell walls and appears to serve as an inter- ple comparison test (Peterson 1985). Mean values were con- molecular binding agent that stabilizes pectin-protein com- sidered significantly different when P Յ 0.05. Pearson plexes of the middle lamella in plant tissue (Demarty and correlations were done on treatment means. others 1984; Dey and Brinson 1984; Fry 1986; Grant and oth- Z-Score Transformation of Data from Sensory Analysis: ers 1973). In previous studies, it was observed that infiltration The significant panelist effect was observed when statistical of calcium into apples resulted in the immediate deposition analysis of the sensory results was carried by ANOVA. It was of calcium at the cell wall site (Glenn and Poovaiah 1990; observed that the panelists were using different parts of the Monsalve and others 1993; Siddiqui and Bangerth 1996) and scale during the sensory trials. Therefore, in order to stan- promoted cross-linking of pectic polymers. dardize the sensory scores from different panelists, z trans- ANOVA of the raw sensory crispness scores indicated a formation using Equation (2) was then performed on the significant panelist effect and a Ca pretreatment by panelist sensory data, as described by Reid and Durance (1992). effect. It was noted that despite training and use of references samples, panelists tended to used different portions of the z = (x Ϫ X) / s.d. (2) line scale; therefore z transformation was used to force all panelist scores into the same scale. The sample Ca content where z is the transformed score, x is the actual scores from was observed to be highly correlated to both the z scores of a panelist for a sensory attribute, X and s.d. are the mean and sensory crispness (r = 0.954, p = 0.001) and the instrumental the standard deviation of all the sensory scores for that sen- compression slope (r = 0.976, p = 0.000). The extensive cross- sory attribute from the same panelist, respectively. The linking of pectic polymers by Ca facilitated the formation of transformed scores (that is, z scores) were calculated and a cell wall network with increased mechanical strength. analyzed statistically using ANOVA to assess the effect of dif- However, increasing sample Ca content above 22800 ␮g/g db ferent treatments on the sensory crispness of the apple chip did not result in an appreciable increase in either the sensory products. crispness rating or the instrumental slope readings. Further, while chips dipped in 0 or 1% CaCl2 were not perceived as Results and Discussion having a bitter taste, the 2% or higher CaCl2 treatments resulted in a significant (p Ͻ 0.05) increase in bitter perception The Effects of Calcium Pretreatment in the chips (Figure 3). Therefore, the application of a 1% Dipping the apple slices in solutions with increasing calci- CaCl2 pretreatment to the apple slices was selected to pro- um chloride (CaCl2) concentration resulted in apple chip duce apple chips with a significant increase in crispness but samples with higher calcium (Ca) content (Figure 1). At the without a detrimental effect on the taste. lower (0 to 2%) CaCl2 concentrations, a linear increased in Crispness is a desired textural characteristic of chips. It Ca content of chips was observed. At the higher CaCl2 con- has been reported that crispness is characterized by resis- centrations (3 to 5% CaCl2), the increase in sample Ca con- tance to deformation under load up to the point of sudden tent was still linear but with a much lower slope than in the fracture, and that this characteristic can be best measured lower CaCl2 concentration range. This was probably due to by the slope of the force/deformation curve (Bourne and the saturation of available Ca binding sites in the samples. others 1966). This was in agreement with what was observed Increasing the Ca content of the VMD apple chips resulted in this experiment where the instrumental slope of the

in significantly higher slope readings of the force/deforma- Food Engineering and Physical Properties tion curve in instrumental analysis and crispness ratings by

Figure 2—Mean slope of the force deformation curve (᭜) and mean crispness z scores (᭿) of VMD apple chips with different calcium content. For each attribute, any 2 values not followed by the same letter are significantly different at p 0.05. Error bars represent the standard error Figure 1—Calcium content of Golden Delicious apple chips of the mean. n = 45 for instrumental texture. n = 21 for vs %CaCl2 in pre-drying dip. n = 3. Error bars represent the crispness. Mean, raw crispness scores are recorded in standard error of the mean. brackets.

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Table 1—Density and compression slope of dried apple chips found when apple chip density was regressed against vacu- Apple Chip Density 1,2 Compression Slope 1,3 um level (Figure 5). When a higher vacuum level was employed during vacuum microwave drying, a larger puffing ef- Treatment (g/mm) (g/ml) fect was obtained, thus leading to a less dense apple chip Vacuum Microwave 0.19 (0.02) a 989 (79) b product. Previous studies demonstrated that the structure of Dried (28 inches Hg) carrot slices (Lin and others 1998) and potato slices (Durance a a Freeze Dried 0.14 (0.02) 367 (21) and Liu 1996) could be puffed and expanded by vacuum mi- Air Dried 0.43 (0.06) b 824 (64) c crowave drying, and the extent of puffing was dependent on 1 All measurements are recorded as mean (standard error). Any two values not followed by the same letter were significantly different at p<0.05. the vacuum level employed (Durance 1997). During vacuum 2 n=3 microwave heating, the absorption of microwave energy by 3 n=45 water molecules results in vaporization of water within the chip at a higher rate than diffusion to the surface. As a result, a large vapor pressure differential between the center and force/deformation curve was found to be highly correlated surface of products is obtained. The lower the pressure in (r = 0.997, p = 0.000) to the sensory crispness of the apple the chamber during the VMD process, the higher the pres- chips. Therefore, the slope of the force/deformation curve sure differential, resulting in a greater outward force causing was used as a measure of sensory crispness of apple chips in the apple chips to expand. Vickers and Bourne (1976) dem- other experiments. onstrated that a dry crisp food probably consists of cells or The effect of Ca on apple chip structure was observed un- cavities which are usually filled with air and a structural der the scanning electron microscope. The untreated (0% phase or cell walls that are formed by a brittle matrix. Mate- CaCl2) apple chip tissue had a honeycomb structure con- rials with these characteristics will produce crisp sounds that structed of closely connected cells (Figure 4a). In contrast to arise from collective breaking of individual cells. Therefore, it the thin and smooth cell wall structure observed in the un- seems that puffing is an important component contributing treated apple chip tissue, that of the Ca-pretreated tissues to crisp texture in a dried food product (Torreggiani and was thicker, and result in a more irregular network (Figure others 1995).

Food Engineering and Physical Properties 4b). Instead of a net-like pattern, cell wall breakage resulted A positive and significant linear relationship was found in large open structures in the Ca-pretreated tissues and between the slope of the force/deformation curve and the yielded chips with a crisper texture. In the Ca-pretreated ap- vacuum level employed during VMD (r = 0.911, p = 0.04) (Fig- ple tissues, the rigidity of cell wall structure conferred by the ure 5). This indicated that a crisper texture was obtained cross-linking of Ca ions to pectin resulted in an increase in when higher levels of vacuum were employed. Also, the the strength and a decrease in the flexibility of cell walls. slope of the force/deformation curve obtained by apple Therefore, with the same amount of expansion force provid- chips was found to be highly correlated to the chips’ densi- ed by water vapor generated during VMD, a greater loss of ties (r = –0.986, p = 0.002). Hence, with the application of the original apple structure was observed in the Ca-pretreat- higher vacuum levels, crisper apple chips were obtained as a ed apple chips. Also, the increase in cell wall rigidity may result of a greater degree of puffing. have prevented the collapse of sample structures during de- In evaluation of the effect of drying techniques on texture hydration. of apple chips, only chips produced from the highest level of vacuum, 28 in Hg (that is, VMD chips with highest crispness) Effects of Vacuum Levels and Drying Techniques were used for comparison to the air and freeze-dried ones An inverse linear relationship (r = -0.910, p = 0.04) was (Table 1). Due to the puffing effect provided by vacuum microwave drying at 28 in Hg, the density of vacuum microwave dried (VMD) apple chips was significantly lower than that of air-dried (AD) apple chips. Freeze-dried (FD) apple chips had the lowest mean density among the dried samples, but this was not significantly different from VMD chips (p Ͼ 0.05). During air drying, as the total water content is reduced, the liquid interface passes through the surface and creates very high surface tension forces that caused progres- sive contraction of the samples, and eventually shrinkage of tissue structures (Stanley and Tung 1976). Apples do not have the solid matrix to resist this collapse. However, during freeze-drying, the frozen apple slices remain rigid and moisture is sublimed directly from a solid state to a vapor state. This rigidity to a large extent prevents the collapse of the solid matrix after drying (Liapis 1987), leaving numerous voids within the structure. However, FD apple chips, with their very porous structure, were spongy and not crisp. The small- est slope among the dried samples was obtained by the FD chip samples in the instrumental test indicating that these chips were the least crisp (Table 1). The peak slope obtained by VMD apple chips was significantly higher than that of AD Figure 3—Mean bitterness z scores of VMD apple chips with different calcium content. n = 15. Any 2 values not followed apple chips. Apart from tissue shrinkage, the diffusion of sol- by the same letter are significantly different at p 0.05. Error utes during air drying may have an effect on the chip texture. bars represent the standard error of the mean. Mean, raw In the early stages of drying as moisture is removed from the bitterness scores are recorded in brackets. surface of the sample, removed moisture is replaced by the

1344 JOURNAL OF FOOD SCIENCE—Vol. 66, No. 9, 2001 Vacuum Microwave Dried Apple Chips . . . migrating liquid water from the interior of the sample. A lay- rior, and eventually causes the formation of a tough, leathery er of soluble solids is then left and builds up on or near the skin described as ‘case hardening’ (Hanson 1976). external surface of the product. This would then increase the Effects of different drying methods and vacuum levels on resistance to further escape of water from the product’s inte- apple chip structure were observed under the scanning elec- Food Engineering and Physical Properties

Figure 4—Scanning electron micrographs of apple chips. A . Golden Delicious vacuum (28mm Hg) microwave dried apple chips with 0% CaCl2 pretreatment. B Golden Delicious vacuum (28mm Hg) microwave dried apple chips with 1% CaCl2 pretreatment. C Golden Delicious air dried apple chips with 0% CaCl2 pretreatment. D Golden Delicious low vacuum (7mm Hg) vacuum microwave dried apple chips with 0% CaCl2 pretreatment. E Golden Delicious freeze dried apple chips with 0% CaCl2 pretreatment. F. Red Delicious vacuum (28 mm Hg) microwave dried apple chips with 0% CaCl2 pretreatment

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tron microscope. After air-drying, apple chips exhibited se- Table 2—Calcium content and compression slope of dif- vere tissue shrinkage and collapse, with almost no open ferent apple varieties with and without calcium pretreat- structures (Figure 4c). The structure of the VMD apple chips ment. 1,2 1,3 at 7-mm Hg vacuum level was similar to that of the air-dried CaCl2 Ca content Compression slope ones, except that a small amount of open structure could Variety % (µg/g dry basis) (g/mm) still be observed (Figure 4d). The less severe shrinkage in the Red Delicious 0 540 (6)a 866 (34) a VMD (7-mm Hg) apple tissue compared to that of the AD Red Delicious 1 11900 (50)b 1180 (43) b, c sample may be due to the shorter drying time, lower drying Fuji 0 690 (4) c 1035 (50) a, b Њ Fuji 1 13100 (80) d 1630 (71) d temperature (maintained at less than 60 C), and some tissue e a expansion from internal water vapor. In contrast, with the Golden 0 620 (4) 874 (72) Delicious application of full vacuum, the VMD (28-mm Hg) apple chip Golden 1 12500 (70) f 1275 (52) c tissue exhibited less shrinkage (Figure 4a); a honeycomb net- Delicious work structure of closely connected, and puffed cells were 1All measurements recorded as mean (standard error). Any values not observed. A similar honeycomb network was observed in the followed by the same letter are significantly different at p Յ 0.05. 2 n = 3. Statistical analysis was on log transformed data. freeze-dried sample, which exhibited the least amount of tis- 3 n=45 sue shrinkage or cell collapse (Figure 4e). The cell walls of freeze-dried samples looked comparatively smoother and thinner than those of the VMD samples, explaining the non- crisp and spongy texture obtained by the freeze-dried apple may be due to Fuji apples containing higher amounts of pec- chips. tin, and therefore more calcium binding sites, than the other two varieties. As found in Part 1, calcium content was highly The Effect of Apple Variety correlated to the instrumental slope (r = 0.857, p = 0.03) of Instrumental tests were carried out for the textural analy- the chips. sis of Golden Delicious, Red Delicious, and Fuji VMD chips. The microstructure of the chips of different apple variet- Calcium treatments (0 & 1% CaCl2) were performed on each ies was also studied. The Ca-pretreated (1% CaCl2) chip sam-

Food Engineering and Physical Properties apple variety. The 1% CaCl2 treatment was chosen in order to ples made from Red Delicious and Fuji varieties were ob- give a significant increase in crispness without a significant served to have thicker cell walls and larger empty regions increase in bitterness of the chips. The 1% CaCl2-pretreated than the nontreated ones, similar to results obtained with apple chips of all apple varieties were significantly crisper, as Golden Delicious apples in Part 1 of this study. Also, a more indicated by a higher compression slope than their non Ca- obvious puffing effect was observed in the Ca-pretreated pretreated (0% CaCl2) ones (Table 2). This is consistent with samples. Among the three varieties, similar structures were results presented in Part 1. obtained for Fuji and Golden Delicious apple chips, while Despite the different intrinsic calcium levels, there was no Red Delicious chips looked comparatively less puffy than the difference in the crispness of chips of different varieties giv- other two (Figure 4f). A slightly greater amount of cell col- en the 0% CaCl2 treatment. For Ca-pretreated samples, Fuji lapse was observed in both the Ca-pretreated and the non- apples yielded chips with significantly higher slope, and treated chips made with Red Delicious apples. therefore crispness, than the Ca-pretreated Golden Deli- cious, and the Red Delicious apples. Further, the calcium Conclusions content of the Fuji apple chip before the calcium pretreat- ONDITIONS REQUIRED TO OBTAIN CRISP APPLE CHIPS USING ment was found to be higher than that of the Golden Deli- vacuum microwave dehydration (VMD) were deter- cious and the Red Delicious chips. The same trend was ob- C mined. A 1% CaCl2-pretreatment of apple slices before VMD served for apple chips after the calcium pretreatment. This enhance the crispness of apple chip products, without having a negative effect on taste. Crispness was enhanced when the highest level of vacuum was applied, which may have resulted from the greater puffing effect. This correlated with irregular cellular structure and voids, caused by vapor pressure generated during the VMD process. Fuji apple chips were found to have a crisper texture than either Golden Delicious and Red Delicious chips, which may be related to higher en- dogenous levels of calcium binding components. By manipu- lation of sample and VMD conditions, textural parameters of products such as apple chips can be modified. Vacuum microwave drying is a technology capable of producing high quality apple chips.

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