MICROWAVE-VACUUM DRYING OF CRANBERRIES: PART 11. QUALITY EVALUATION J. YONGSAWATDIGUL Seafood Laboratory Oregon State University Astoria, OR 97103 and S . GUNASEKARAN' Department of Biological Systems Engineering University of Wisconsin-Madison 460 Henry Mall Madison. W 53706 Accepted for Publication October 10. 1995 ABSTRACT Color, rexture and water activity of microwave-vacuum dried cranbem'es were evaluated and compared with the correspondingpropellies of hot-air dried and store-bought cranberries. The microwave drying was done both in thepulsed and continuous modes. The microwave-vacuum dried cranbem'es were redder and had a softer texture than those dried by the conventional hot-air method. The storage stabilig of the product dried by microwave-vacuum method was comparable to that of conventionallydried cranbem'es. Themicrowave operating conditions have an fleet on the quality of the dried cranberries. INTRODUCTION The conventional hot-air method of drying fruits often degrades product quality. Van Arsdel et al. (1973) indicated that hot-air drying can cause heat damage and adversely affect flavor, color, size, texture and nutritional value of 'Author for correspondence. Journal of Food Processing and Preservation 20 (1996) 145-156. All Rights Reserved. Copyright 1996 by Food & Nutrition Press, Inc.. Trumbull, CT 06611 145
MICROWAVE-VACUUM DRYING OF CRANBERRIES: …cranberries, which were presumably processed by hot-air drying, was greater than the redness value of the cranberries dried conventionally
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MICROWAVE-VACUUM DRYING OF CRANBERRIES: PART 11. QUALITY EVALUATION
Seafood Laboratory Oregon State University
Astoria, OR 97103
S . GUNASEKARAN'
Department of Biological Systems Engineering University of Wisconsin-Madison
460 Henry Mall Madison. W 53706
Accepted for Publication October 10. 1995
Color, rexture and water activity of microwave-vacuum dried cranbem'es were evaluated and compared with the corresponding propellies of hot-air dried and store-bought cranberries. The microwave drying was done both in the pulsed and continuous modes. The microwave-vacuum dried cranbem'es were redder and had a softer texture than those dried by the conventional hot-air method. The storage stabilig of the product dried by microwave-vacuum method was comparable to that of conventionallydried cranbem'es. The microwave operating conditions have an fleet on the quality of the dried cranberries.
The conventional hot-air method of drying fruits often degrades product quality. Van Arsdel et al. (1973) indicated that hot-air drying can cause heat damage and adversely affect flavor, color, size, texture and nutritional value of
'Author for correspondence.
Journal of Food Processing and Preservation 20 (1996) 145-156. All Rights Reserved. Copyright 1996 by Food & Nutrition Press, Inc.. Trumbull, CT 06611 145
146 J. YONGSAWATDIGUL AND S. GUNASEKARAN
the products. Case-hardening is a common defect particularly found in dried fruits due to rapid drying. As drying progresses, the rate of water evaporation is faster than the rate of water diffusion to the product surface. Therefore, the outer skin becomes dry and acts as a water barrier, causing a wet interior. Furthermore, loss of volatile compounds inevitably occurs during drying. Since the products are exposed to high temperature for a long period, these volatile compounds are vaporized and lost with water vapor. This causes a significant loss of characteristic flavor in dried products. High temperature and long drying time also degrade the product's original color. Therefore, alternative, energy- efficient drying methods are necessary for the food industry in order to manufacture products of high quality.
Vacuum drying is an alternative method for drying fruits. The vacuum allows water to vaporize at a lower temperature than at atmospheric conditions. Therefore, fruits can be dried without exposure to high temperature. Moreover, the absence of air during dehydration diminishes oxidation reactions. Because of these advantages, the color, texture and flavor of dried products are improved. Ismail (1989) patented a process for production of semi-moist cranberries that involves vacuum drying.
In recent years, microwave drying has gained popularity as an alternative drying method in the food industry. Microwave drying is rapid, more uniform and energy efficient compared to conventional hot-air drying (Decareau 1985). Applying microwave energy under vacuum affords advantages of both vacuum drying and microwave drying -- improved energy efficiency and product quality. Several fruits, grains and other products have been successlklly dried by the microwave-vacuum application (Anon. 1988; Wadsworth et al. 1990).
Therefore, microwave-vacuum drying was investigated as a potential method for obtaining high-quality dried cranberries. In our previous paper (Yongsawat- digul and Gunasekaran 1996), we presented the energy use and efficiency of microwave-vacuum drying. f i e microwave energy was applied in both pulsed mode and continuous mode by appropriately controlling the times the microwave magnetron was turned on. The pulsed microwave drying (PD) was more efficient than continuous drying (CD). In both cases, drying efficiency improved when lower pressure (i.e., higher vacuum) was applied. However, microwave drying is known to result in a poor quality product if not properly applied (Jolly 1986). Surface scorching is one of the most common problems (Gunasekaran 1990). Therefore, we investigated certain quality characteristics (color, texture and water activity) of microwave-vacuum dried cranberries and compared them with the corresponding properties of hot-air dried (in our laboratory) and commercially -dried (presumably using hot air) cranberries.
MICROWAVE-VACUUM DRYING PART I1 147
MATERIALS AND METHODS
Frozen cranberries (Vaccinium macrocarpon Air.) of Searles variety obtained from Ocean Spray Co., Babcock, WI, were used. The cranberries were thawed and pretreated in high fructose corn syrup solutions (30"B and 60"B) for 24 h and then rinsed with hot water (75f5C) to prevent stickiness due to excessive fructose on the skin. Infusing sugar solution into cranberries before drying is known to give the dried product a soft texture (Aurand 1989; Ismail 1989). Pretreated cranberries were dried using a laboratory-scale microwave- vacuum oven (Zwag, Model Labotron 500). In this oven, microwave power can be selected to be either 250 W or 500 W of continuous output. A thick-walled glass bell jar is placed at the center of a turntable. The space enclosed by the bell jar can be evacuated. Pressure inside the bell jar can be adjusted from 0 to 100 mm Hg absolute. The oven can be operated either in a continuous or a pulsed mode. In the pulsed mode, the magnetron is alternately turned on and off, corresponding to a set time ranging from 0.1 s to 200 s. Two vacuum levels were investigated (5.33 kPa and 10.67 kPa) for both continuous drying (CD) and pulsed drying (PD). The CD was performed at 250 and 500 W power levels and the PD was performed at 250 W.
A convective hot-air oven (Blue M) was used for conventional drying of cranberries. The initial moisture contents of the pretreated cranberries were 76% and 62% for 30"B and 60"B fructose solutions, respectively. They were dried to a final moisture content of 15 % . Additional details of the drying process have been presented in Part I (Yongsawatdigul and Gunasekaran 1996). The dried product was stored for 90 days to determine the effect of storage on color and water activity. Dried cranberries were also purchased from a local grocery store for comparison of quality parameters. The drying method used for the commercial product was unknown but presumed to be hot-air drying.
The color of dried cranberries is formed by red anthocyanins and yellow flavonoids (Francis and Clydesdale ef al. 1975), and it is one of the major attributes used to evaluate their quality. The color of the cranberries (fresh, fructose-pretreated, experimentally-dried and commercially -dried) was objectively measured using a Hunter Tristimulus Colorimeter (Model 25A-9). The color of the experimentally-dried cranberries was evaluated at 0. 15, and 90 days after drying. A cylindrical plastic dish (5.8-cm diameter x 4.0-cm deep) containing the dried samples was placed at the light port (5-cm diameter). Each sample was measured for color values four times. The instrument was initially standardized with a pink standard plate (YCIE = 46.8, XCIE = 54.6, ZCIE
148 J. YONGSAWATDIGUL AND S. GUNASEKARAN
46.9). The color values were obtained as a lightness (L*), redness (a*) and yellowness (b*). The data were transformed to hue (H") and saturation (C*). Color difference (AE) was also calculated by using the color values of fresh (i.e., frozen-thawed) cranberries as a standard.
H" = t a d (b*/a*) C* = [(a*)2 + (b*)2]1R AE* = [(AL*)' + (Aa*)2 + (Ab*)7ln
where: AL* = L*-,, - L* rM
Ab* = b*r,,,,,,le - b* Aa* = a*,,,+ - a* rondard
Maximum shear-force required for cutting individual cranberries was presumed as a texture index of the dried product. This was measured using an Instron Universal Testing Machine (Model 1130). A v-shaped cutter probe designed for this purpose was attached to the Instron crosshead and the crosshead speed was set at 4 cdmin. The texture measurement of experimental (microwave-vacuum and hot-air dried) samples were performed within 24 h after drying. Before the texture measurement of the commercial samples, moisture content of the samples was increased from 11.4% (wet basis) to 15.0% (wet basis) to match the moisture content of experimental samples by exposing them to high humidity environment maintained by a saturated solution of potassium sulphate (K,SO,). Texture measurement of all samples were replicated ten times. The effects of variable factors on cutting forces were statistically analyzed by using ANOVA (Box et al. 1978).
Water Activity Measurement
Water activity of all the cranberry samples was determined as a measure of their storage stability using a Decagon (model CX- 1) water activity meter. The water activity of the experimental samples were determined at 0, 15, and 90 days after drying. The sample was manually sliced into tiny pieces and immediately loaded into a sample cup to prevent water loss. The sample used was just enough to cover the bottom of the sample cup (about one to two g r a m .
MICROWAVE-VACUUM DRYING PART Il
RESULTS AND DISCUSSION
Mean color values of cranberries dried by the CD are shown in Table 1. The color values obtained from fresh cranberries were used as a standard for calculating a color difference value (AE). It is seen that color difference (AE) increased and saturation values (C*) decreased significantly (P r0.05) when higher pressure was applied. The redness value (a*) decreased as pressure level increased. Table 2 shows the color values of the cranberries dried by PD. Redness, yellowness, color differences and saturation were significantly (P S0.05) affected by pressure, initial moisture content and power-on time. The effect of pressure on each color value was the same as what was described for CD. It is obvious that a higher pressure level seems to diminish the redness of the cranberries dried by both CD and PD. Francis and Clydesdale (1975) reported that redness of cranberries was strongly related to anthocyanin content. Hence, it can be presumed that oxidation of anthocyanin pigments is facilitated at the higher pressure since such a condition creates higher temperature and much more oxygen available for enhancing oxidation of the anthocyanins. The redness, yellowness and saturation values of the lower moisture cranberries (pretreated with 60"B of fructose solution) were greater than those of higher moisture cranberries (pretreated with 30"B of fructose solution). Furthermore, the color difference value was smaller. This may be because oxidation of anthocyanins is retarded since lower moisture cranberries are exposed to microwaves for a shorter time. The color change value (AE) was smaller in cranberries dried at longer power-on time setting. The redness was also greater in such a dried sample. These results support the notion that shorter drying time decreases the oxidation of anthocyanins.
Mean color values of fresh, pretreated and dried cranberries prepared by various drying methods are shown in Table 3. The redness, yellowness and color difference values of cranberries slightly decreased when they were pretreated with fructose solution and greatly decreased after they were dried regardless of the drying method. The redness value of the commercially dried cranberries, which were presumably processed by hot-air drying, was greater than the redness value of the cranberries dried conventionally in this study. This may be due to varietal or other variations of the cranberries used or different processing conditions.
The redness values of cranberries dried by CD were greater than those dried by PD. It may be explained that degradation of anthocyanins in cranberries dried by CD was less because total drying time of CD was significantly shorter than that of PD. Exposure of cranberries to drying conditions for a short time could
s of aa
ies dried by CD (s
r 0 d
ean (i s
(P s 0.
152 J . YONGSAWATDIGUL AND S. GUNASEKARAN
TABLE 3. MEAN ( f STANDARD ERROR) COLOR VALUES OF CRANBERRIES
PRETREATED AND DRIED BY VARIOUS METHODS
' Aveage color values of all cranberries used in the experiment (68 samples).
Average color values of cranberries pretreated with either 30'B or 60"B of fructose solution (68 samples).
Average color values of cranberries dried by hot& drying (4 samples).
' Average color values of m b e r r i a dried by PD operating at 24 diffumt conditions (48 samples).
(1 6 samples). ' Avemge color values of cranberries dried by CD operating at eight different conditions
minimize anthocyanin loss. Color differences in the products dried conventional- ly and those dried by CD can be visually differentiated (Yongsawatdigul 1992). It is clear that microwave-vacuum drying improved the color quality of the dried cranberries when compared to the conventional method. After the dried products were stored at room temperature (20f3C) for 15 and 90 days, the redness values constantly decreased. The drying process and storage time apparently contributed to the degradation of anthocyanins in cranberries, causing a decrease in redness.
MICROWAVE-VACUUM DRYING PART II 153
TABLE 4. CUTTING FORCE OF CRANBERRIES DRIED BY VARIOUS DRYING METHODS
Texture of the dried cranberries was measured as the maximum force required to cut a single cranberry. Table 4 shows the data obtained from samples dried by various drying methods. At the same moisture level, the force required to cut the commercially dried cranberries and the product dried by
154 J. YONGSAWATDIGUL AND S. GUNASEKARAN
hot-air drying was comparable, while the cranberries dried by either CD or PD required less cutting force. This implies that microwave-vacuum dried cranberries are softer compared to the product dried conventionally. Although the Cranberries dried by CD required only slightly greater cutting force (mean = l7.99 N) than those dried by PD (mean = 15.40 N), it was observed (by sensory perception) that the cranberries dried by the CD were much harder than those dried by the PD method. The temperature of the cranberries dried by the CD is higher than when dried by the PD, which may have caused the tougher texture.
Furthermore, cranberries dried by using higher pressure, higher microwave power, longer power-on time and shorter power-off time setting seemed to be harder because all of these operating conditions caused a high product temperature. The cutting force of cranberries pretreated with higher fructose concentration (60"B) was greater than those pretreated in 30"B fructose solution. This may be because more sugar coats on the surface and infuses into the cranberries, resulting in a stickier and tougher texture.
TABLE 5. WATER ACTIVITY OF CRANBERRIES DRIED BY DIFFERENT DYRING METHODS
AND STORED FOR DIFFERENT DURATIONS
'Average water activity of cranberries dried by pd (48 samples) *Average water activity of cranberries dried by CD (16 samples) 'Average water activity of cranberries dried by hot-air oven (4 samples)
MICROWAVE-VACUUM DRYING PART I1 155
The water activity of all the cranberry samples is presented in Table 5 . Water activity values of dried cranberries pretreated with 30"B of fructose syrup (0.53-0.65) were slightly greater than those of cranberries pretreated with 60"B (0.41-0.52). Since all samples had similar moisture content, it is reasonable to conclude that a higher concentration of fructose decreases the amount of free water, which subsequently provides a more microbially-stable product. Concentration of sugar solution used in the pretreatment step was cited as a causative of sugaring defect in raisin and other dried fruits (Woodroof and Luh 1986; Bolin 1976). However, we did not notice any sugaring in the cranberries pretreated with 60"B solution, even after 90 days. After the dried Cranberries were stored for 15 and 90 days at room temperature (20f3C), their water activity values slightly increased. The change in water activity with respect to storage time was somewhat similar among the cranberries dried by CD, PD and conventional drying. This suggests that microbial stability of cranberries dried by either CD or PD is about the same as for those dried conventionally.
The quality of dried cranberries depends on the drying method and the drying conditions used. In general, microwave-vacuum drying resulted in a better quality product (judging by the color and texture) than conventional hot-air drying. The redness values of the cranberries decreased with storage time. The storage stability of dried cranberries as measured by water activity is about the same regardless of the drying method or conditions used.
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