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Microwave heating of apple mash to improve juice yield and quality
ARTICLE IN PRESS
Lebensm.-Wiss. u.-Technol. 37 (2004) 551–557
Microwave heating of apple mash to improve juice yield and quality K.A. Gerard, J.S. Roberts* Nabisco, Inc., 200 DeForest Avenue, East Hanover, NJ 07936, USA Department of Food Science and Technology, Cornell University, NYSAES, 630 W. North St., Geneva 14456, USA Received 11 March 2003; received in revised form 12 December 2003; accepted 17 December 2003
Abstract The effects of four heat treatments of apple mash on juice yield and quality were evaluated and compared to juice produced from unheated apple mash at 21 C. Fuji and McIntosh apple mashes were heated to bulk temperatures of 40 C, 50 C, 60 C and 70 C in a 2450 MHz microwave oven at 1500 W. Juice yield increased when mash was heated before pressing. Cider produced from the heated mashes had comparable pH, titratable acidity, and sensory characteristics to cider produced from room temperature mashes; however, total phenolic and flavonoid content of the juice increased with increasing mash temperature. Soluble solids and turbidity also increased as treatment temperature increased. r 2004 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. Keywords: Microwave; Extraction; Phenolics; Cider
1. Introduction A large body of research indicates that the consumption of foods high in phenolics and flavonoids contributes to health and may reduce the risk of heart disease and cancer (Sichel, Corsaro, Scalia, Di billio, & Bonomo, 1991; Negre-Salvayre & Salvayre, 1992; Knekt, Jarvinen, Reunanen, & Maatela, 1996; Guhr & Lachance, 1997; Zhishen, Mengcheng, & Jianming, 1999). Raw apples contain high concentrations of phenolics, and have shown benefits to health in reducing the risk of cancer and cardiovascular heart disease (Burda, Oleszek, & Lee, 1990; Knekt et al., 1996; Pearson, Tan, German, Davis, & Gershwin, 1999; Eberhardt, Lee, & Liu, 2000). Fuji, McIntosh, Red Delicious, Granny Smith, & Liberty apples all have over 100 mg total phenolics per 100 g of apple, Golden Delicious apples have 82.2 mg phenolics per 100 g of apple, and the Empire apples have only 50.9 mg phenolics per 100 g of apple (Lee & Smith, 2000). However, the concentration of phenolics and flavonoids in juice and cider are drastically reduced after juice processing (Lea & Timberlake, 1978; Spanos, Wrolstad, & Heatherbell, 1990; Lu & Foo, 1997). A study of *Corresponding author. Tel.: +1-315-787-2496; fax: +1-315-7872284. E-mail address: [email protected] (J.S. Roberts).
Granny Smith apples indicated that after mashing and pressing 1350 g of apple, only 6.0 mg of quercetin glycosides and 4.8 mg of phloretin glycosides were present in the liter of juice produced (Spanos et al., 1990). This represents a reduction of 95 mg/100 mg due to juice extraction. In another study, the flavonoid content of five apple varieties and juice from these varieties was tested. After juicing, only 9.9–12.7 mg/ 100 mg of the flavonols present in the whole apple were present in the juice (Price, Prosser, Richetin, & Rhodes, 1999). Lu and Foo (1997) identified and quantified phenolic compounds in apple pomace, which is the waste product of apple juice processing. The total amount of epicatechin, caffeic acid, phloridzin, phloretin glycosides and quercetin glycosides was 7.42 g of phenolics per kg of pomace, which indicated that up to 99 mg/100 mg of phenolics remain in the pomace after juicing. Heat and enzyme treatments of apple mashes have been investigated to improve phenolic content, yield, and quality of juice. Lea and Timberlake (1978) examined the effect of warm anaerobic incubation of apple pulp on juice quality. Warm incubation of apple mash at 40 C for 4 h increased yield slightly and increased extraction of phenolics by up to 50 mg/ 100 mg. However, the juice tasted bitter and astringent due to the increases in oligomeric and polymeric procyanidins. Genovese, Elustondo, and Lozano (1997)
0023-6438/$30.00 r 2004 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2003.12.006
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evaluated the effect of steam heating of crushed apples on juice yield, color, and cloud stability. Steam heating of crushed apples at 70 C for 15–20 s controlled enzymatic browning and stabilized cloudiness, yet decreased the soluble solids of the juice. Spanos et al. (1990) produced apple juice concentrate in a diffusion extraction apparatus at 55 C, 63 C, 67 C, and 73 C. Juice yields from the diffusion extractor were much higher than for pressed juices. At 73 C, diffusion extracted juices contained 3 times the cinnamics and 5 times the phloretin glycosides as pressed juices. However, diffusion extraction equipment is expensive and can only be used to produce juice concentrate, and produces bitter, astringent juice. The use of enzymes in juice processing can result in greatly improved juice yields as well as quality improvements (Lea & Timberlake, 1978; Beveridge & Harrison, 1986; Chang, Siddiq, Sinha, & Cash, 1995; Cliff, Dever, & Gayton, 1991). However, the use of commercial enzymes does not produce juices with increased phenolic content. Due to the health benefits of phenolic compounds and flavonoids, it is desirable to develop methods to increase the extraction of phenolics and flavonoids from apples during juice processing. Heat treatment of fruit mash has proven effective for increasing the concentration of phenolic compounds in fruit juices as well as yield. However, most current heat treatments produce juice with unacceptable analytical and sensory properties. Microwave energy has the advantage of heating solids rapidly and uniformly, thus inactivating enzymes more quickly and minimize browning. Therefore, the objective of this research was to evaluate the effect of microwave heat treatment of apple mash on juice yield, quality, and content of total phenolics and flavonoids in the juice.
2. Materials and methods Apples (Malus X domestica Borkh) were mashed and then pressed at five different temperatures: 21 C, 40 C, 50 C, 60 C, and 70 C. The resulting cider was evaluated based on yield, quality, and phenolic and flavonoid content. A flow diagram of the juicing process and evaluation is shown in Fig. 1. Physicochemical properties, such as pH, acidity, and soluble solids, and turbidity were immediately evaluated on the cider after juice extraction. Unpasteurized cider was stored at 10 C to evaluate total phenolics and flavonoids. Sensory analysis was conducted on pasteurized cider. 2.1. Apple selection and storage Fuji and McIntosh apple cultivars were chosen for this experiment due to their high phenolic content and antioxidant activity, as well as for their use in cider production. Fuji apples grown in Geneva, NY and
Fuji & Macintosh Apples Washed and Stored
Apples Ground in a Mixer with Ascorbic Acid
Apple Mash Heated in the Microwave to 40OC, 50OC, 60OC, and 70OC
Control Mash is Pressed
Heated Mash is Pressed
Juice Yield Evaluation
Unpasteurized Cider
pH, Titratable Acidity, Soluble Solids, and Turbidity Evaluation
Juice is Pasteurized at 71.1 OC for 6 seconds
Sensory Evaluation
Cider Stored at -10 OC
Cider is Thawed and Total Phenolic and Total Flavonoid Assay Performed
Fig. 1. Flow diagram of apple cider processing and experimental design.
McIntosh apples grown in Fullerton, NY were picked in September 2000, washed, randomized, and stored at 2 C. Apples were removed from storage 12–24 h before processing and allowed to come up to room temperature, which was 21 C, before being mashed. Firmness of each batch of apples was determined at room temperature using a Fruit Pressure Tester (McCormick, Model FT 327 (apple and pears), Yakima, WA). A small disc of skin was cut off of six randomly selected apples. The force required to pierce the apple flesh with the hand-held penetrometer was recorded. The results of 6 measurements were averaged. 2.2. Juice processing Commercial production of apple cider using rackand-frame presses result in juice yields around 70 g/100 g apples when using fresh apples, but juice yield can drop as low as 60 g/100 g apples when using stored apples than have lost firmness (Rutledge, 1996). A lab-scale rack and frame press was designed, constructed and used in this study, and is shown in Fig. 2. The racks were made into a lattice design using rock maple wood strips 0.21 0.02 0.008 m with 0.007 m gaps between each strip to allow the juice to flow through. The frame, which is used to form the apple mash in the press cloth between each rack, was also made of rock maple with inner square dimensions of 0.18 0.18 m and outer square dimension of 0.21 0.21 m. To determine
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0.21 m
Magnetron 2450 MHz
0.21 m
Microwave Oven Cavity (0.88 x 0.88 x 0.88 m)
Directional Coupler
FiberOptic Temperature Probes
Fiso OSR
Power Meters 0.008 m
0.02 m
0.007 m gap
Mash
(a) Controller 0-2.5 kW
• apple mash in cheesecloth
cider
(b) Fig. 2. Lab-scale rack-and-frame press: (a) dimensions of a rack, and (b) apple mash racked in a hydraulic press.
whether this lab-scale press performed as a large-scale press, juice yields were compared with both reported yields in literature as well as to yields from a pilot-scale rack-and-frame press (Orchard Equipment and Supply Co., Conway, MA). The racks on this pilot-scale press measured 0.56 0.56 m. Apple mash was made on a large scale using a hammer mill (W.J. Fitzpatrick Co., Model D Comminuting Machine, Chicago, IL) with blunt hammers at 4600 rpm. Approximately 70 kg of mash was loaded in the pilot-scale rack-and-frame press, and juice yields were determined. Four replicates were conducted. Lab-scale juice processing parameters were optimized to mimic large-scale juice production. Apples (3.2 kg) were ground in a commercial food processor (Robot Coupe Inc., RV6N, Ridgeland, MS) at 1750 rpm for 60 s. To evaluate the effectiveness of pressing apple mash at elevated temperatures to extract phenolic compounds into cider, 53.3 mg ascorbic acid per 100 g mash was added to the mash to retard browning and oxidation of phenolic compounds. Apple mashes were heated using a custom built microwave oven, as shown in Fig. 3. The aluminum microwave cavity measures 0.88 0.88 0.88 m (Microwave Research Center, Marlborough, NH) and is equipped with two aluminum mode stirrers operating at 30 and 35 rpm. The oven was powered by a 2450 MHz magnetron with continuous, variable power from 0 to 2500 W (Gerling Applied Electronics, Inc., Modesto, CA).
Fig. 3. Microwave oven apparatus.
The oven cavity is connected to the magnetron using WR-284 waveguide components, and input and reflected power were measured by power meters (Model 435 B, Hewlett Packard Corp., Santa Clara, CA) connected to a directional coupler in the waveguide. Roberts and Gerard (2004) studied the heating characteristics of apple mash using microwave energy, such as regional heating effects, load size, mash depth, and power absorption. The mash to be heated was placed in a rectangular polypropylene container (0.533 0.375 0.0810 m). The rectangular shape is necessary to mimic a belt that would be used in a microwave tunnel; however, the corner and edge heating would be significantly reduced with a moving belt system (Gerling, 2003). For this study, the corners and edges of the container were shielded with 0.0508 m wide aluminum tape to reduce over-heating effects at the corners and edges. Roberts and Gerard (2004) showed that 3 kg of apple mash at a depth of 0.016 m heated in the microwave oven using 1500 W for 4.0 min, 7.1 min, 10.9 min, and 16.2 min were the optimum parameters to achieve bulk temperatures of 40 C, 50 C, 60 C, and 70 C, respectively. The control mash was not heated. Both the control and heated apple mashes were loaded into the lab-scale rack and frame. Three 1-kg batches of the mash were placed in cheesecloth and stacked between the racks. The prepared mash was placed in the press (Carver, Inc., #3925, Wabash, IN) and 2170 kPa of pressure was applied to the mash for 2 min. Four replicates at each temperature and for each mash variety were performed, so the experimental design was 5 (temperatures) 2 (apple varieties) 4 (replicates). 2.3. Juice pasteurization A minimum volume of 8 L is necessary to pasteurize juice in a Microthermics pasteurizer (Model 25HV,
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Raleigh, North Carolina), so juice from a set of four pressing replicates at each temperature were combined to meet this requirement. The Fuji and McIntosh cider pressed from mash temperatures of 21 C, 40 C, 50 C, 60 C, and 70 C were pasteurized for Eschericia coli at 71 C for 6 s (Mazzotta, 2001) using the Microthermics unit. After pasteurization, the juices were bottled in plastic pint containers and refrigerated at 4 C until time of sensory analysis.
tional 6 min, 2 ml of 1 N NaOH was added and the solution was diluted to a total volume of 10 ml with distilled water. Absorbance was measured at 510 nm using a double-beam UV spectrophotometer. Flavonoid concentration was determined by using a catechin standard curve. Both total phenolic and total flavonoid assays were performed in quadruplicate.
2.6. Sensory evaluation 2.4. Evaluation of juice yield and physicochemical properties The fresh juice was collected and weighed to determine juice yield. The pH, titratable acidity, soluble solids, and turbidity of both fresh and pasteurized juices were evaluated. The pH was measured using a Fisher AR50 meter (Fisher Scientific, Pittsburgh, PA). Titratable acidity was determined by titrating 10 ml of juice with 0.1 N NaOH to an endpoint pH of 8.2. Results were expressed with respect to malic acid (g/ml). Each test was performed in duplicate and results were averaged. Soluble solids were measured by placing a few drops of juice on a hand-held refractometer (Fisher Scientific, #02422, Pittsburgh, PA). Two measurements were averaged and expressed in g/100 g. Turbidity was measured using a Hach Model 2100 AN ratio turbidimeter with a 24-mm cuvette (Hach Co., Loveland, CO). This instrument measures from 0 to 10,000 nephelometric turbidity units (NTUs). 2.5. Total phenolic and total flavonoid assays Samples of both fresh and pasteurized juices were frozen to 10 C and later thawed to perform total phenolic and total flavonoid assays. Total phenolic concentrations were determined following procedures outlined by Singleton and Rossi (1965). One milliliter of diluted cider (3 ml cider/25 ml distilled water) was placed in a 25-ml flask. Distilled water was added to bring the total volume to 5 ml. Next, 1 ml Folin-Coicalteau reagent (Sigma-Aldrich, Milwaukee, MI) was added. After 6 min, 10 ml of Na2CO3 (7 g/100 ml distilled H2O) was added and the solution was diluted to a total volume of 25 ml with distilled water. After 1.5 h, absorbance was measured at 750 nm using a doublebeam UV spectrophotometer (Lambda 4A, Perkin Elmer, Norwalk, CT). Phenolic concentration was determined using a gallic acid standard curve. Total flavonoid concentrations were determined using procedures outlined by Zhishen et al. (1999). One milliliter of diluted cider (5 ml cider/25 ml distilled water) was placed in a 10-ml flask. Distilled water was added to bring the total volume to 5 ml. Next, 0.3 ml of NaNO2 (5 g/100 ml distilled H2O) was added. After 5 min, 0.3 ml of AlCl3 (10 g/100 ml distilled H2O) was added. After an addi-
Triangle differences tests were performed with the pasteurized McIntosh and Fuji juices to determine if panelists were able to detect differences between cider processed from mash at room temperature to cider processed from heated mash. These triangle tests followed the procedures outlined by (Meilgaard, Civille, & Carr, 1999) and applied by (Aigster, Sims, Staples, Schmidt, & O’Keefe, 2000). These tests were conducted in the Sensory Laboratory in the Department of Food Science and Technology. Panelists were experienced with apple cider but untrained with respect to this particular difference tests, and the panelists consisted of students, staff, and faculty, all ranging in age between 21 and 60 years. The tests were conducted under red light to eliminate influence of the cider color on the panelists’ evaluation. Approximately 25 ml of cider sample was filled in plastic containers and coded using a 3-digit random number. Samples were served at room temperature (22 C). Three samples were presented to the panelists, two samples were similar and the other was different. The order in which the samples were presented was randomized (AAB, ABA, BAA, BBA, BAB, or ABB). Panelists were asked to taste the samples in the order they were given and to determine the odd sample. Between each sample, the panelists were to cleanse their palate with an unsalted cracker and water. Three triangle test sessions were conducted on separate days, and each session evaluated only one cider with two treatments: (1) Fuji cider pressed from mash at 21 C and Fuji cider pressed from mash at 40 C, (2) Fuji cider pressed from mash at 21 C and Fuji cider pressed from mash at 60 C, (3) McIntosh cider pressed from mash at 21 C and McIntosh cider pressed from mash at 40 C, and (4) McIntosh cider pressed from mash at 21 C and McIntosh cider pressed from mash at 60 C. Each of the 4 triangle test sessions was replicated.
2.7. Statistical analysis The means for all quality attributes, total phenolics and flavonoids, and yield for the cider produced at the five mash temperatures were compared using Least Significant Difference (LSD) with Pp0:05:
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3. Results and discussion 3.1. Evaluation of juice yield and physicochemical properties The cider produced using the lab-scale press resulted in juice yields ranging between 71.9 and 72.8 g/100 g apples, as shown in Table 1 under the 21 C mash column for both Fuji and McIntosh apples. The average yield from the pilot-scale rack-and-frame press was 74.373.4 g/100 g apples. The yields from the lab-scale rack-and-frame are within the range of yields from the pilot-scale rack-and-frame press as well as from reported yields for a rack-and-frame press (Downing, 1989; Rutledge, 1996). Therefore, the effect of mash temperature on juice yield and quality was able to be determined using the lab-scale equipment. Juice yield increased when the mash was heated before pressing (Table 1). There were significant increases in Fuji juice yield among all temperatures except between the yields from mash heated to 60 C and 70 C. McIntosh juice yields from heated mash were significantly greater than the juice yields from room temperature mash. However, there was no significant difference in the McIntosh juice yields between mashes heated to 40 C, 50 C, 60 C, or 70 C. Fuji and McIntosh control mash both yielded juice of approximately 72.0 g/100 g apples, which is the expected yield when producing cider using a rack and frame press (Downing, 1989; Rutledge, 1996). When heated to 60 C, the Fuji mash yielded juice of 80.3 g/100 g apples and the McIntosh mash yielded juice of 75.3 g/100 g apples. The observed difference in
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juice yield when the two mashes were heated is probably the result of differences in firmness between the two apple varieties. The average Fuji apple firmness was 7.38 kg, while the average McIntosh firmness was 5.52 kg. McIntosh is considered a soft cultivar, which explains the low firmness value (Downing, 1989). Thus, the effect of heating was greater for the firmer apple than for the softer apple. There was no significant difference in pH or titratable acidity between treatments (Table 1). Average pH for the Fuji and McIntosh juices were 4.14 and 3.52, respectively, while average titratable acidity was 0.22 g/ ml for Fuji and 0.42 g/ml for McIntosh. However, the heat treatments did have significant effects on soluble solids and turbidity. For both the McIntosh and Fuji juices, soluble solids increased significantly (Pp0:05) with increasing temperature (Table 1). The Fuji and McIntosh mash heated to 60 C and 70 C produced juices with significantly higher soluble solids than the juice from the mash at 21 C. Turbidity of the Fuji and McIntosh juices increased with mash temperature (Table 1). There were significant increases in turbidity of the Fuji juices produced as mash temperature increased, but there was no significant difference between juice produced from mashes at 60 C and 70 C. Likewise, there were significant increases in turbidity of the McIntosh juices as mash temperature increased, with no significant difference between the 50 C and 60 C treatments. Pasteurization also resulted in greater turbidity. This increase in turbidity is the result of increased interactions of haze-active (HA) proteins and HA polyphenols
Table 1 Extraction yields and physicochemical properties of juices pressed at various mash temperatures Mash temperature during pressing 21 C
40 C
50 C
60 C
70 C
Fuji Yield (g/100 g) pH Malic acid (g/100 ml) Soluble solids (g/100 g) Turbidity (NTUs) Phenolics (mg/ml) Flavonoids (mg/ml)
71.970.6d 4.1270.04a 0.2170.01a 13.570.1d 21997120c 0.9070.06c 0.4370.01d
73.970.9c 4.1370.03a 0.2270.01a 13.770.3c,d 25857150b 0.8570.01c 0.4670.02c,d
77.270.7b 4.1470.04a 0.2170.01a 13.870.1b,c,d 3137779a 0.9670.04c 0.4770.02c
80.370.3a 4.1670.03a 0.2270.01a 14.070.1a,b 34527120a 1.0870.03b 0.5470.03b
79.971.4a 4.1370.05a 0.2270.02a 14.2 70.4a 33587180a 1.2170.06a 0.6270.03a
McIntosh Yield (g/100 g) pH Malic acid (g/100 ml) Soluble solids (g/100 g) Turbidity (NTUs) Phenolics (mg/ml) Flavonoids (mg/ml)
72.870.5b 3.4970.08a 0.4370.02a 11.970.2d 1949761d 1.0370.05d 0.6970.03d
74.970.5a 3.4870.07a 0.4170.01a 12.070.1b,c 2514795c 1.0770.05d 0.7470.02c
74.771.1a 3.5170.09a 0.4270.03a 12.170.3b,c 31027110b 1.1970.01c 0.8270.01b
75.372.0a 3.5670.11a 0.4270.02a 12.570.3a 32567190b 1.4470.08b 0.8370.01b
76.271.5a 3.5970.14a 0.4370.02a 12.8 70.3a 3477782a 1.6670.05a 0.8770.02a
Mean7standard deviation for n ¼ 4 experiments and Pp0:05: a,b,c,d Different letters across the same row indicate a significant difference in descending order (least significant difference at Pp0:05).
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when they are heated (Siebert & Lynn, 2000). It was noted that turbidity of the Fuji juices was higher than turbidity of the McIntosh juices. Apple cultivar and maturity affect interactions of HA proteins and HA polyphenols. The Fuji and McIntosh apples were processed at similar maturity, so the difference in turbidity is probably the result of cultivar differences. Previous research demonstrates that in comparison to McIntosh apples, Fuji apples have much higher levels of HA proteins, which contribute to increased turbidity (Siebert & Lynn, 2000). 3.2. Total phenolics and total flavonoids The concentration of total phenolics and total flavonoids in both Fuji and McIntosh juices increased with treatment temperature (Table 1). Phenolic concentrations of Fuji and McIntosh juices increased by up to 41 and 65 mg/100 ml, respectively. There were significant increases in total phenolics with increasing mash temperatures of the Fuji and McIntosh juices, with the exception between the unheated and 40 C juice extractions. There was no significant difference in flavonoid concentration of the untreated and 40 C Fuji juices, or between Fuji juice from mash temperatures of 40 C and 50 C. There were significant differences in total flavonoids between all treatments of the McIntosh juice. While the increase total phenolics and flavonoids is significant, the overall extraction is low. 3.3. Sensory evaluation Table 2 shows the results of the triangle taste test conducted. For a significant difference at Pp0:05; 12 out of the 19 panelist needed to correctly distinguish between the cider pressed at room temperature to the cider pressed at elevated temperatures (Meilgaard et al., 1999). For both Fuji and McIntosh ciders, there was no significant difference (Pp0:05) between the cider pressed at room temperature to the cider pressed at
Table 2 Triangle taste testa of cider from heated mash and cider from unheated mash (control) Control (21 C) v. Cider from heated mash
Correct responses
Incorrect responses
Significant difference? (Pp0:05)b
Fuji (40 C) Fuji (60 C) Macintosh (40 C) Macintosh (60 C)
10 9 6 10
9 10 13 9
No No No No
a
The number of untrained panelists was 19 for each test. The minimum number of correct responses, c; required for a significant difference at (Pp0:05) is c ¼ 12 for n ¼ 19 (Meilgaard et al., 1999). b
40 C and 60 C. An important note was that all of the panelists that correctly identified the odd sample noted that the cider pressed at elevated temperatures were similar to or better than the cider pressed at room temperature. The sensory test was conducted under red light conditions because there was an obvious color difference between the cider pressed at room temperature to the cider pressed at 40 C and 60 C. The cider pressed at the elevated temperatures were much lighter in color than the cider pressed at room temperature, and this effect my be the result of polyphenoloxidase being inactivated during the heat treatment of the mash. However, proper color measurement and analysis could not be conducted since ascorbic acid was added to the mash to preserve as much phenolics within the mash as possible.
4. Conclusions In this study, juice from four heat treatments (40 C, 50 C, 60 C, and 70 C) of Fuji and McIntosh apple mashes were compared to juice from unheated mash. These results demonstrate that microwave heating mash increase juice yields. Microwave heat treatment of the mash also increased extraction of phenolics and flavonoids from apple mash and resulted in juice with increased concentrations of total phenolics and flavonoids. All of the juices produced from heated mash were of a high quality, with comparable pH and titratable acidity. Soluble solids and turbidity increased in the juice with increasing mash temperature. Sensory panelists were unable to detect differences between the cider produced at room temperature to the ciders produced at 40 C and 60 C. Therefore, microwave heating of apple mash before juice extraction resulted in a high quality juice with increased phenolic and flavonoid content as well as increased juice yield. These results also showed that heating the apple mash to 60 C is the optimum temperature for improving juice quality and yield. Heating the mash to 70 C did not increase Fuji or McIntosh yield and had little effect on total phenolics and flavonoids. However, heating the mash to 70 C required additional processing time, 16.2 min compared to 10.9 min to achieve mash bulk temperature of 60 C. Mash heated to 60 C resulted in maximum yields with significant increases in phenolic and flavonoid content. Juice quality was comparable to control pressed juices. Results for juice quality and yield varied between the two varieties. Further research should be conducted to determine the effects of microwave heat treatment on other apple varieties. When stored for long periods of time, apples begin to soften, increase in soluble pectin, and are more difficult to press resulting in poor yields. The results of the McIntosh pressing showed improved
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juice yields and quality from heating the mash when the condition of the apple is poor for traditional pressing. Further research should be conducted to determine the effects on juice yield and quality of heated and unheated mash from fresh apples and from apples at periodic storage times.
References Aigster, A., Sims, C., Staples, C., Schmidt, R., & O’Keefe, S. F. (2000). Comparison of cheeses made from milk having normal and high oleic fatty acid compositions. Journal of Food Science, 65, 920–924. Beveridge, T., & Harrison, J. E. (1986). Pear juice production from heated pear mashes. Canadian Institute of Food Science and Technology Journal, 19, 12–16. Burda, S., Oleszek, W., & Lee, C. Y. (1990). Phenolic compounds and their changes in apples during maturation and cold storage. Journal of Agriculture and Food Chemistry, 38, 945–948. Chang, T., Siddiq, M., Sinha, N. K., & Cash, J. N. (1995). Commercial pectinases and the yield and quality of Stanley plum juice. Journal of Food Process and Preservation, 19, 89–101. Cliff, M., Dever, M. C., & Gayton, R. (1991). Juice extraction process and apple cultivar influences on juice properties. Journal of Food Science, 56, 1614–1627. Downing, D. (Ed.), (1989). Processed Apple Products. New York: Van Nostrad Reinhold. Eberhardt, M. V., Lee, C. Y., & Liu, R. H. (2000). Antioxidant and anticancer activities of fresh apples. Nature, 405, 903–904. Genovese, D. B., Elustondo, M. P., & Lozano, J. E. (1997). Color and cloud stabilization in cloudy apple juice by steam heating during crushing. Journal of Food Science, 62, 1171–1175. Gerling, J. (2003). Personal communication. Guhr, G., & Lachance, P. A. (1997). Role of phytochemicals in chronic disease prevention. In P. A. Lachance (Ed.), Nutraceuticals: Designer foods III garlic, soy, and licorice (pp. 311–364). Trumbull: Food and Nutrition Press, Inc. Knekt, P., Jarvinen, R., Reunanen, A., & Maatela, J. (1996). Flavonoid intake and coronary mortality in Finland: A cohort study. British Medical Journal, 312, 478–481. Lea, A. G. H., & Timberlake, C. F. (1978). The phenolics of ciders: Effect of processing conditions. Journal of Agricultural and Food Chemistry, 29, 484–492.
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Lee, C. Y., & Smith, N. L. (2000). Apples: An important source of antioxidants in the American diet. New York Fruit Quarterly, 8, 15–17. Lu, Y., & Foo, L. Y. (1997). Identification and quantification of major polyphenols in apple pomace. Food Chemistry, 59, 187–194. Mazzotta, A. S. (2001). Thermal inactivation of stationary-phase and acid-adapted Escherichia coli O157:H7, Salmonella, and Listeria monocytogenes in Fruit Juices. Journal of Food Protection, 64, 315–320. Meilgaard, M., Civille, G. V., & Carr, B. T. (Eds.), (1999). Sensory Evaluation Techniques. (3rd ed.). Boca Raton, FL: CRC Press. Negre-Salvayre, A., & Salvayre, R. (1992). Quercetin prevents the cytotoxicity LDL on lymphoid cell lines. Free Radical Biology and Medicine, 12, 101–106. Pearson, D. A., Tan, C. H., German, J. B., Davis, P. A., & Gershwin, M. E. (1999). Apple juice inhibits human low density lioprotein oxidation. Life Sciences, 64, 1913–1920. Price, K. R., Prosser, T., Richetin, A. M. F., & Rhodes, M. J. C. (1999). A comparison of the flavonol content and composition in dessert, cooking and cider-making apples; distribution within the fruit and effect of juicing. Food Chemistry, 66, 489–494. Roberts, J. S. & Gerard, K. A. (2004). Development and evaluation of microwave heating of apple mash as a pretreatment to pressing. Journal of Food Process Engineering (in press). Rutledge, P. (1996). Production of non-fermented fruit products. In D. Arthey, & P. R. Ashurst (Eds.), Fruit processing (pp. 70–96). London: Chapman and Hall. Sichel, G., Corsaro, C., Scalia, M., Di billio, A. J., & Bonomo, R. P. (1991). In vitro scavenger activity of some flavonoids and melanins against oxygen. Free Radical Biology and Medicine, 11, 1–8. Siebert, K. J., & Lynn, P. Y. (2000). Apple cultivar and maturity affect haze-active protein and haze-active polyphenol concentrations in juice. Journal of Food Science, 65, 1386–1390. Singleton, V. L., & Rossi, J. A. (1965). Colorimetry of total phenolics with phosphomolybdic-phosphotungstic acid reagents. American Journal of Enology and Viticulture, 16, 144–158. Spanos, G. A., Wrolstad, R. E., & Heatherbell, D. A. (1990). Influence of processing and storage on the phenolic composition of apple juice. Journal of Agricultural and Food Chemistry, 38, 1572–1579. Zhishen, J., Mengcheng, T., & Jianming, W. (1999). The determination of flavonoid contents in mulberry and their scavenging effects on superoxide radicals. Food Chemistry, 64, 555–559.
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Microwave heating of apple mash to improve juice yield and quality K.A. Gerard, J.S. Roberts* Nabisco, Inc., 200 DeForest Avenue, East Hanover, NJ 07936, USA Department of Food Science and Technology, Cornell University, NYSAES, 630 W. North St., Geneva 14456, USA Received 11 March 2003; received in revised form 12 December 2003; accepted 17 December 2003
Abstract The effects of four heat treatments of apple mash on juice yield and quality were evaluated and compared to juice produced from unheated apple mash at 21 C. Fuji and McIntosh apple mashes were heated to bulk temperatures of 40 C, 50 C, 60 C and 70 C in a 2450 MHz microwave oven at 1500 W. Juice yield increased when mash was heated before pressing. Cider produced from the heated mashes had comparable pH, titratable acidity, and sensory characteristics to cider produced from room temperature mashes; however, total phenolic and flavonoid content of the juice increased with increasing mash temperature. Soluble solids and turbidity also increased as treatment temperature increased. r 2004 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. Keywords: Microwave; Extraction; Phenolics; Cider
1. Introduction A large body of research indicates that the consumption of foods high in phenolics and flavonoids contributes to health and may reduce the risk of heart disease and cancer (Sichel, Corsaro, Scalia, Di billio, & Bonomo, 1991; Negre-Salvayre & Salvayre, 1992; Knekt, Jarvinen, Reunanen, & Maatela, 1996; Guhr & Lachance, 1997; Zhishen, Mengcheng, & Jianming, 1999). Raw apples contain high concentrations of phenolics, and have shown benefits to health in reducing the risk of cancer and cardiovascular heart disease (Burda, Oleszek, & Lee, 1990; Knekt et al., 1996; Pearson, Tan, German, Davis, & Gershwin, 1999; Eberhardt, Lee, & Liu, 2000). Fuji, McIntosh, Red Delicious, Granny Smith, & Liberty apples all have over 100 mg total phenolics per 100 g of apple, Golden Delicious apples have 82.2 mg phenolics per 100 g of apple, and the Empire apples have only 50.9 mg phenolics per 100 g of apple (Lee & Smith, 2000). However, the concentration of phenolics and flavonoids in juice and cider are drastically reduced after juice processing (Lea & Timberlake, 1978; Spanos, Wrolstad, & Heatherbell, 1990; Lu & Foo, 1997). A study of *Corresponding author. Tel.: +1-315-787-2496; fax: +1-315-7872284. E-mail address: [email protected] (J.S. Roberts).
Granny Smith apples indicated that after mashing and pressing 1350 g of apple, only 6.0 mg of quercetin glycosides and 4.8 mg of phloretin glycosides were present in the liter of juice produced (Spanos et al., 1990). This represents a reduction of 95 mg/100 mg due to juice extraction. In another study, the flavonoid content of five apple varieties and juice from these varieties was tested. After juicing, only 9.9–12.7 mg/ 100 mg of the flavonols present in the whole apple were present in the juice (Price, Prosser, Richetin, & Rhodes, 1999). Lu and Foo (1997) identified and quantified phenolic compounds in apple pomace, which is the waste product of apple juice processing. The total amount of epicatechin, caffeic acid, phloridzin, phloretin glycosides and quercetin glycosides was 7.42 g of phenolics per kg of pomace, which indicated that up to 99 mg/100 mg of phenolics remain in the pomace after juicing. Heat and enzyme treatments of apple mashes have been investigated to improve phenolic content, yield, and quality of juice. Lea and Timberlake (1978) examined the effect of warm anaerobic incubation of apple pulp on juice quality. Warm incubation of apple mash at 40 C for 4 h increased yield slightly and increased extraction of phenolics by up to 50 mg/ 100 mg. However, the juice tasted bitter and astringent due to the increases in oligomeric and polymeric procyanidins. Genovese, Elustondo, and Lozano (1997)
0023-6438/$30.00 r 2004 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2003.12.006
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evaluated the effect of steam heating of crushed apples on juice yield, color, and cloud stability. Steam heating of crushed apples at 70 C for 15–20 s controlled enzymatic browning and stabilized cloudiness, yet decreased the soluble solids of the juice. Spanos et al. (1990) produced apple juice concentrate in a diffusion extraction apparatus at 55 C, 63 C, 67 C, and 73 C. Juice yields from the diffusion extractor were much higher than for pressed juices. At 73 C, diffusion extracted juices contained 3 times the cinnamics and 5 times the phloretin glycosides as pressed juices. However, diffusion extraction equipment is expensive and can only be used to produce juice concentrate, and produces bitter, astringent juice. The use of enzymes in juice processing can result in greatly improved juice yields as well as quality improvements (Lea & Timberlake, 1978; Beveridge & Harrison, 1986; Chang, Siddiq, Sinha, & Cash, 1995; Cliff, Dever, & Gayton, 1991). However, the use of commercial enzymes does not produce juices with increased phenolic content. Due to the health benefits of phenolic compounds and flavonoids, it is desirable to develop methods to increase the extraction of phenolics and flavonoids from apples during juice processing. Heat treatment of fruit mash has proven effective for increasing the concentration of phenolic compounds in fruit juices as well as yield. However, most current heat treatments produce juice with unacceptable analytical and sensory properties. Microwave energy has the advantage of heating solids rapidly and uniformly, thus inactivating enzymes more quickly and minimize browning. Therefore, the objective of this research was to evaluate the effect of microwave heat treatment of apple mash on juice yield, quality, and content of total phenolics and flavonoids in the juice.
2. Materials and methods Apples (Malus X domestica Borkh) were mashed and then pressed at five different temperatures: 21 C, 40 C, 50 C, 60 C, and 70 C. The resulting cider was evaluated based on yield, quality, and phenolic and flavonoid content. A flow diagram of the juicing process and evaluation is shown in Fig. 1. Physicochemical properties, such as pH, acidity, and soluble solids, and turbidity were immediately evaluated on the cider after juice extraction. Unpasteurized cider was stored at 10 C to evaluate total phenolics and flavonoids. Sensory analysis was conducted on pasteurized cider. 2.1. Apple selection and storage Fuji and McIntosh apple cultivars were chosen for this experiment due to their high phenolic content and antioxidant activity, as well as for their use in cider production. Fuji apples grown in Geneva, NY and
Fuji & Macintosh Apples Washed and Stored
Apples Ground in a Mixer with Ascorbic Acid
Apple Mash Heated in the Microwave to 40OC, 50OC, 60OC, and 70OC
Control Mash is Pressed
Heated Mash is Pressed
Juice Yield Evaluation
Unpasteurized Cider
pH, Titratable Acidity, Soluble Solids, and Turbidity Evaluation
Juice is Pasteurized at 71.1 OC for 6 seconds
Sensory Evaluation
Cider Stored at -10 OC
Cider is Thawed and Total Phenolic and Total Flavonoid Assay Performed
Fig. 1. Flow diagram of apple cider processing and experimental design.
McIntosh apples grown in Fullerton, NY were picked in September 2000, washed, randomized, and stored at 2 C. Apples were removed from storage 12–24 h before processing and allowed to come up to room temperature, which was 21 C, before being mashed. Firmness of each batch of apples was determined at room temperature using a Fruit Pressure Tester (McCormick, Model FT 327 (apple and pears), Yakima, WA). A small disc of skin was cut off of six randomly selected apples. The force required to pierce the apple flesh with the hand-held penetrometer was recorded. The results of 6 measurements were averaged. 2.2. Juice processing Commercial production of apple cider using rackand-frame presses result in juice yields around 70 g/100 g apples when using fresh apples, but juice yield can drop as low as 60 g/100 g apples when using stored apples than have lost firmness (Rutledge, 1996). A lab-scale rack and frame press was designed, constructed and used in this study, and is shown in Fig. 2. The racks were made into a lattice design using rock maple wood strips 0.21 0.02 0.008 m with 0.007 m gaps between each strip to allow the juice to flow through. The frame, which is used to form the apple mash in the press cloth between each rack, was also made of rock maple with inner square dimensions of 0.18 0.18 m and outer square dimension of 0.21 0.21 m. To determine
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0.21 m
Magnetron 2450 MHz
0.21 m
Microwave Oven Cavity (0.88 x 0.88 x 0.88 m)
Directional Coupler
FiberOptic Temperature Probes
Fiso OSR
Power Meters 0.008 m
0.02 m
0.007 m gap
Mash
(a) Controller 0-2.5 kW
• apple mash in cheesecloth
cider
(b) Fig. 2. Lab-scale rack-and-frame press: (a) dimensions of a rack, and (b) apple mash racked in a hydraulic press.
whether this lab-scale press performed as a large-scale press, juice yields were compared with both reported yields in literature as well as to yields from a pilot-scale rack-and-frame press (Orchard Equipment and Supply Co., Conway, MA). The racks on this pilot-scale press measured 0.56 0.56 m. Apple mash was made on a large scale using a hammer mill (W.J. Fitzpatrick Co., Model D Comminuting Machine, Chicago, IL) with blunt hammers at 4600 rpm. Approximately 70 kg of mash was loaded in the pilot-scale rack-and-frame press, and juice yields were determined. Four replicates were conducted. Lab-scale juice processing parameters were optimized to mimic large-scale juice production. Apples (3.2 kg) were ground in a commercial food processor (Robot Coupe Inc., RV6N, Ridgeland, MS) at 1750 rpm for 60 s. To evaluate the effectiveness of pressing apple mash at elevated temperatures to extract phenolic compounds into cider, 53.3 mg ascorbic acid per 100 g mash was added to the mash to retard browning and oxidation of phenolic compounds. Apple mashes were heated using a custom built microwave oven, as shown in Fig. 3. The aluminum microwave cavity measures 0.88 0.88 0.88 m (Microwave Research Center, Marlborough, NH) and is equipped with two aluminum mode stirrers operating at 30 and 35 rpm. The oven was powered by a 2450 MHz magnetron with continuous, variable power from 0 to 2500 W (Gerling Applied Electronics, Inc., Modesto, CA).
Fig. 3. Microwave oven apparatus.
The oven cavity is connected to the magnetron using WR-284 waveguide components, and input and reflected power were measured by power meters (Model 435 B, Hewlett Packard Corp., Santa Clara, CA) connected to a directional coupler in the waveguide. Roberts and Gerard (2004) studied the heating characteristics of apple mash using microwave energy, such as regional heating effects, load size, mash depth, and power absorption. The mash to be heated was placed in a rectangular polypropylene container (0.533 0.375 0.0810 m). The rectangular shape is necessary to mimic a belt that would be used in a microwave tunnel; however, the corner and edge heating would be significantly reduced with a moving belt system (Gerling, 2003). For this study, the corners and edges of the container were shielded with 0.0508 m wide aluminum tape to reduce over-heating effects at the corners and edges. Roberts and Gerard (2004) showed that 3 kg of apple mash at a depth of 0.016 m heated in the microwave oven using 1500 W for 4.0 min, 7.1 min, 10.9 min, and 16.2 min were the optimum parameters to achieve bulk temperatures of 40 C, 50 C, 60 C, and 70 C, respectively. The control mash was not heated. Both the control and heated apple mashes were loaded into the lab-scale rack and frame. Three 1-kg batches of the mash were placed in cheesecloth and stacked between the racks. The prepared mash was placed in the press (Carver, Inc., #3925, Wabash, IN) and 2170 kPa of pressure was applied to the mash for 2 min. Four replicates at each temperature and for each mash variety were performed, so the experimental design was 5 (temperatures) 2 (apple varieties) 4 (replicates). 2.3. Juice pasteurization A minimum volume of 8 L is necessary to pasteurize juice in a Microthermics pasteurizer (Model 25HV,
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Raleigh, North Carolina), so juice from a set of four pressing replicates at each temperature were combined to meet this requirement. The Fuji and McIntosh cider pressed from mash temperatures of 21 C, 40 C, 50 C, 60 C, and 70 C were pasteurized for Eschericia coli at 71 C for 6 s (Mazzotta, 2001) using the Microthermics unit. After pasteurization, the juices were bottled in plastic pint containers and refrigerated at 4 C until time of sensory analysis.
tional 6 min, 2 ml of 1 N NaOH was added and the solution was diluted to a total volume of 10 ml with distilled water. Absorbance was measured at 510 nm using a double-beam UV spectrophotometer. Flavonoid concentration was determined by using a catechin standard curve. Both total phenolic and total flavonoid assays were performed in quadruplicate.
2.6. Sensory evaluation 2.4. Evaluation of juice yield and physicochemical properties The fresh juice was collected and weighed to determine juice yield. The pH, titratable acidity, soluble solids, and turbidity of both fresh and pasteurized juices were evaluated. The pH was measured using a Fisher AR50 meter (Fisher Scientific, Pittsburgh, PA). Titratable acidity was determined by titrating 10 ml of juice with 0.1 N NaOH to an endpoint pH of 8.2. Results were expressed with respect to malic acid (g/ml). Each test was performed in duplicate and results were averaged. Soluble solids were measured by placing a few drops of juice on a hand-held refractometer (Fisher Scientific, #02422, Pittsburgh, PA). Two measurements were averaged and expressed in g/100 g. Turbidity was measured using a Hach Model 2100 AN ratio turbidimeter with a 24-mm cuvette (Hach Co., Loveland, CO). This instrument measures from 0 to 10,000 nephelometric turbidity units (NTUs). 2.5. Total phenolic and total flavonoid assays Samples of both fresh and pasteurized juices were frozen to 10 C and later thawed to perform total phenolic and total flavonoid assays. Total phenolic concentrations were determined following procedures outlined by Singleton and Rossi (1965). One milliliter of diluted cider (3 ml cider/25 ml distilled water) was placed in a 25-ml flask. Distilled water was added to bring the total volume to 5 ml. Next, 1 ml Folin-Coicalteau reagent (Sigma-Aldrich, Milwaukee, MI) was added. After 6 min, 10 ml of Na2CO3 (7 g/100 ml distilled H2O) was added and the solution was diluted to a total volume of 25 ml with distilled water. After 1.5 h, absorbance was measured at 750 nm using a doublebeam UV spectrophotometer (Lambda 4A, Perkin Elmer, Norwalk, CT). Phenolic concentration was determined using a gallic acid standard curve. Total flavonoid concentrations were determined using procedures outlined by Zhishen et al. (1999). One milliliter of diluted cider (5 ml cider/25 ml distilled water) was placed in a 10-ml flask. Distilled water was added to bring the total volume to 5 ml. Next, 0.3 ml of NaNO2 (5 g/100 ml distilled H2O) was added. After 5 min, 0.3 ml of AlCl3 (10 g/100 ml distilled H2O) was added. After an addi-
Triangle differences tests were performed with the pasteurized McIntosh and Fuji juices to determine if panelists were able to detect differences between cider processed from mash at room temperature to cider processed from heated mash. These triangle tests followed the procedures outlined by (Meilgaard, Civille, & Carr, 1999) and applied by (Aigster, Sims, Staples, Schmidt, & O’Keefe, 2000). These tests were conducted in the Sensory Laboratory in the Department of Food Science and Technology. Panelists were experienced with apple cider but untrained with respect to this particular difference tests, and the panelists consisted of students, staff, and faculty, all ranging in age between 21 and 60 years. The tests were conducted under red light to eliminate influence of the cider color on the panelists’ evaluation. Approximately 25 ml of cider sample was filled in plastic containers and coded using a 3-digit random number. Samples were served at room temperature (22 C). Three samples were presented to the panelists, two samples were similar and the other was different. The order in which the samples were presented was randomized (AAB, ABA, BAA, BBA, BAB, or ABB). Panelists were asked to taste the samples in the order they were given and to determine the odd sample. Between each sample, the panelists were to cleanse their palate with an unsalted cracker and water. Three triangle test sessions were conducted on separate days, and each session evaluated only one cider with two treatments: (1) Fuji cider pressed from mash at 21 C and Fuji cider pressed from mash at 40 C, (2) Fuji cider pressed from mash at 21 C and Fuji cider pressed from mash at 60 C, (3) McIntosh cider pressed from mash at 21 C and McIntosh cider pressed from mash at 40 C, and (4) McIntosh cider pressed from mash at 21 C and McIntosh cider pressed from mash at 60 C. Each of the 4 triangle test sessions was replicated.
2.7. Statistical analysis The means for all quality attributes, total phenolics and flavonoids, and yield for the cider produced at the five mash temperatures were compared using Least Significant Difference (LSD) with Pp0:05:
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3. Results and discussion 3.1. Evaluation of juice yield and physicochemical properties The cider produced using the lab-scale press resulted in juice yields ranging between 71.9 and 72.8 g/100 g apples, as shown in Table 1 under the 21 C mash column for both Fuji and McIntosh apples. The average yield from the pilot-scale rack-and-frame press was 74.373.4 g/100 g apples. The yields from the lab-scale rack-and-frame are within the range of yields from the pilot-scale rack-and-frame press as well as from reported yields for a rack-and-frame press (Downing, 1989; Rutledge, 1996). Therefore, the effect of mash temperature on juice yield and quality was able to be determined using the lab-scale equipment. Juice yield increased when the mash was heated before pressing (Table 1). There were significant increases in Fuji juice yield among all temperatures except between the yields from mash heated to 60 C and 70 C. McIntosh juice yields from heated mash were significantly greater than the juice yields from room temperature mash. However, there was no significant difference in the McIntosh juice yields between mashes heated to 40 C, 50 C, 60 C, or 70 C. Fuji and McIntosh control mash both yielded juice of approximately 72.0 g/100 g apples, which is the expected yield when producing cider using a rack and frame press (Downing, 1989; Rutledge, 1996). When heated to 60 C, the Fuji mash yielded juice of 80.3 g/100 g apples and the McIntosh mash yielded juice of 75.3 g/100 g apples. The observed difference in
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juice yield when the two mashes were heated is probably the result of differences in firmness between the two apple varieties. The average Fuji apple firmness was 7.38 kg, while the average McIntosh firmness was 5.52 kg. McIntosh is considered a soft cultivar, which explains the low firmness value (Downing, 1989). Thus, the effect of heating was greater for the firmer apple than for the softer apple. There was no significant difference in pH or titratable acidity between treatments (Table 1). Average pH for the Fuji and McIntosh juices were 4.14 and 3.52, respectively, while average titratable acidity was 0.22 g/ ml for Fuji and 0.42 g/ml for McIntosh. However, the heat treatments did have significant effects on soluble solids and turbidity. For both the McIntosh and Fuji juices, soluble solids increased significantly (Pp0:05) with increasing temperature (Table 1). The Fuji and McIntosh mash heated to 60 C and 70 C produced juices with significantly higher soluble solids than the juice from the mash at 21 C. Turbidity of the Fuji and McIntosh juices increased with mash temperature (Table 1). There were significant increases in turbidity of the Fuji juices produced as mash temperature increased, but there was no significant difference between juice produced from mashes at 60 C and 70 C. Likewise, there were significant increases in turbidity of the McIntosh juices as mash temperature increased, with no significant difference between the 50 C and 60 C treatments. Pasteurization also resulted in greater turbidity. This increase in turbidity is the result of increased interactions of haze-active (HA) proteins and HA polyphenols
Table 1 Extraction yields and physicochemical properties of juices pressed at various mash temperatures Mash temperature during pressing 21 C
40 C
50 C
60 C
70 C
Fuji Yield (g/100 g) pH Malic acid (g/100 ml) Soluble solids (g/100 g) Turbidity (NTUs) Phenolics (mg/ml) Flavonoids (mg/ml)
71.970.6d 4.1270.04a 0.2170.01a 13.570.1d 21997120c 0.9070.06c 0.4370.01d
73.970.9c 4.1370.03a 0.2270.01a 13.770.3c,d 25857150b 0.8570.01c 0.4670.02c,d
77.270.7b 4.1470.04a 0.2170.01a 13.870.1b,c,d 3137779a 0.9670.04c 0.4770.02c
80.370.3a 4.1670.03a 0.2270.01a 14.070.1a,b 34527120a 1.0870.03b 0.5470.03b
79.971.4a 4.1370.05a 0.2270.02a 14.2 70.4a 33587180a 1.2170.06a 0.6270.03a
McIntosh Yield (g/100 g) pH Malic acid (g/100 ml) Soluble solids (g/100 g) Turbidity (NTUs) Phenolics (mg/ml) Flavonoids (mg/ml)
72.870.5b 3.4970.08a 0.4370.02a 11.970.2d 1949761d 1.0370.05d 0.6970.03d
74.970.5a 3.4870.07a 0.4170.01a 12.070.1b,c 2514795c 1.0770.05d 0.7470.02c
74.771.1a 3.5170.09a 0.4270.03a 12.170.3b,c 31027110b 1.1970.01c 0.8270.01b
75.372.0a 3.5670.11a 0.4270.02a 12.570.3a 32567190b 1.4470.08b 0.8370.01b
76.271.5a 3.5970.14a 0.4370.02a 12.8 70.3a 3477782a 1.6670.05a 0.8770.02a
Mean7standard deviation for n ¼ 4 experiments and Pp0:05: a,b,c,d Different letters across the same row indicate a significant difference in descending order (least significant difference at Pp0:05).
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when they are heated (Siebert & Lynn, 2000). It was noted that turbidity of the Fuji juices was higher than turbidity of the McIntosh juices. Apple cultivar and maturity affect interactions of HA proteins and HA polyphenols. The Fuji and McIntosh apples were processed at similar maturity, so the difference in turbidity is probably the result of cultivar differences. Previous research demonstrates that in comparison to McIntosh apples, Fuji apples have much higher levels of HA proteins, which contribute to increased turbidity (Siebert & Lynn, 2000). 3.2. Total phenolics and total flavonoids The concentration of total phenolics and total flavonoids in both Fuji and McIntosh juices increased with treatment temperature (Table 1). Phenolic concentrations of Fuji and McIntosh juices increased by up to 41 and 65 mg/100 ml, respectively. There were significant increases in total phenolics with increasing mash temperatures of the Fuji and McIntosh juices, with the exception between the unheated and 40 C juice extractions. There was no significant difference in flavonoid concentration of the untreated and 40 C Fuji juices, or between Fuji juice from mash temperatures of 40 C and 50 C. There were significant differences in total flavonoids between all treatments of the McIntosh juice. While the increase total phenolics and flavonoids is significant, the overall extraction is low. 3.3. Sensory evaluation Table 2 shows the results of the triangle taste test conducted. For a significant difference at Pp0:05; 12 out of the 19 panelist needed to correctly distinguish between the cider pressed at room temperature to the cider pressed at elevated temperatures (Meilgaard et al., 1999). For both Fuji and McIntosh ciders, there was no significant difference (Pp0:05) between the cider pressed at room temperature to the cider pressed at
Table 2 Triangle taste testa of cider from heated mash and cider from unheated mash (control) Control (21 C) v. Cider from heated mash
Correct responses
Incorrect responses
Significant difference? (Pp0:05)b
Fuji (40 C) Fuji (60 C) Macintosh (40 C) Macintosh (60 C)
10 9 6 10
9 10 13 9
No No No No
a
The number of untrained panelists was 19 for each test. The minimum number of correct responses, c; required for a significant difference at (Pp0:05) is c ¼ 12 for n ¼ 19 (Meilgaard et al., 1999). b
40 C and 60 C. An important note was that all of the panelists that correctly identified the odd sample noted that the cider pressed at elevated temperatures were similar to or better than the cider pressed at room temperature. The sensory test was conducted under red light conditions because there was an obvious color difference between the cider pressed at room temperature to the cider pressed at 40 C and 60 C. The cider pressed at the elevated temperatures were much lighter in color than the cider pressed at room temperature, and this effect my be the result of polyphenoloxidase being inactivated during the heat treatment of the mash. However, proper color measurement and analysis could not be conducted since ascorbic acid was added to the mash to preserve as much phenolics within the mash as possible.
4. Conclusions In this study, juice from four heat treatments (40 C, 50 C, 60 C, and 70 C) of Fuji and McIntosh apple mashes were compared to juice from unheated mash. These results demonstrate that microwave heating mash increase juice yields. Microwave heat treatment of the mash also increased extraction of phenolics and flavonoids from apple mash and resulted in juice with increased concentrations of total phenolics and flavonoids. All of the juices produced from heated mash were of a high quality, with comparable pH and titratable acidity. Soluble solids and turbidity increased in the juice with increasing mash temperature. Sensory panelists were unable to detect differences between the cider produced at room temperature to the ciders produced at 40 C and 60 C. Therefore, microwave heating of apple mash before juice extraction resulted in a high quality juice with increased phenolic and flavonoid content as well as increased juice yield. These results also showed that heating the apple mash to 60 C is the optimum temperature for improving juice quality and yield. Heating the mash to 70 C did not increase Fuji or McIntosh yield and had little effect on total phenolics and flavonoids. However, heating the mash to 70 C required additional processing time, 16.2 min compared to 10.9 min to achieve mash bulk temperature of 60 C. Mash heated to 60 C resulted in maximum yields with significant increases in phenolic and flavonoid content. Juice quality was comparable to control pressed juices. Results for juice quality and yield varied between the two varieties. Further research should be conducted to determine the effects of microwave heat treatment on other apple varieties. When stored for long periods of time, apples begin to soften, increase in soluble pectin, and are more difficult to press resulting in poor yields. The results of the McIntosh pressing showed improved
ARTICLE IN PRESS K.A. Gerard, J.S. Roberts / Lebensm.-Wiss. u.-Technol. 37 (2004) 551–557
juice yields and quality from heating the mash when the condition of the apple is poor for traditional pressing. Further research should be conducted to determine the effects on juice yield and quality of heated and unheated mash from fresh apples and from apples at periodic storage times.
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