QUALITY AND STABILITY OF BLUEBERRY JUICE BLENDED WITH APPLE, GRAPE AND CRANBERRY JUICE
GARY MAIN, MIKE FAUPEL, JUSTIN MORRIS2 and RONALD MCNEW
University of Arkansas Institure of Food Science and Engineering
2650 North Young Avenue Fayetteville, AR 72704
Accepted for Publication March 7, 2000
Sensory quality and storage stability of blueberry juice blends were investigated with the goal of maintaining dominant blueberry flavor, aroma and color. Apple juice, Concord and Venus grape juices and cranberry juice cocktail were blended at 75, 50 and 25% with blueberry juice. As the percent of blueberry juice decreased, the intensity of blueberry-related sensory attributes decreased. The 25% Concord and Venus blends were the only blends that resulted in blueberry flavor similar to the reference. Apple and cranberry juice cocktail blended with blueberry juice produced several blends with good flavor and aroma, but they were not readily characterized as blueberry juice. All blends at the 25% level produced blueberry color equal to or greater than the reference. The juice samples were evaluated initially and after three months of storage at 37C. After storage, all blends had decreased red color.
Arkansas blueberry (Vaccinium species) growers produce more fruit than can be sold on the fresh market. This excess fruit problem is further exacerbated when the market is flooded with blueberries produced in northern regions. The excess fruit includes blemished and broken berries that are not suitable for the fresh market. This fruit is sometimes frozen for use in products like jams and pies. In the United States, large volume, northern blueberry producers determine the market price of frozen blueberries. Frozen blueberries are difficult to sell profitably due to supply exceeding demand and high storage cost. For this reason, 20-25 % of the Arkansas blueberry crop has been left unpicked in recent
Published with the approval of the director of the Arkansas Agricultural Experiment Station. Author to whom correspondence should be sent.
Journal of Food Quality 24 (2001) 111-125. All Rights Reserved. mCopyright 2001 by Food & Nutrition Press, Inc. I TrumbuN. Connecticut. 111
112 G. MAIN, M. FAUPEL, J. MORRIS and R. MCNEW
years. Broadening secondary markets for frozen blueberries has been considered as one alternative for alleviating this situation. Use of excess fruit in a juice product is especially attractive since machine-harvested fruit can be used. Blueberry juice products may provide a profitable market outlet for blueberry producers in the Ozarks because of an existing fruit juice industry (Price and Morris 1991).
Blueberry juice is an expensive product because blueberries are priced higher than most other fruits. Fortunately, blueberry juice shares a characteristic with cranberry juice, in that both have strong flavors and can be improved by diluting and/or blending with other juices (Luh 1980). Strong-flavored fruit juices can often be improved by dilution with mild flavored, low acid juices such as apple and retain much of their original flavor.
Blueberries contain high levels of antioxidants, including flavonoids that can be used to promote value-added blueberry products as health-promoting foods. Blueberries are used in native and folk medicine in North America and Europe (Kalt 1997). Prior ef ul. (1998) evaluated different cultivars of blueberries and found that blueberries are one of the richest sources of antioxidants of the fresh fruits and vegetables studied. Blueberries have high levels of antioxidant compounds including anthocyanins (Prior ef ul. 1998), quercetin (Bilyk and Sapers 1986), and ellagic acid (Wang et al. 1994).
The Arkansas Blueberry Growers Association SuccessfulIy marketed a blend of blueberry and Reliance grape juice (Price and Morris 1991). This blend was tested in a consumer acceptability test with Reliance grape juice, Venus grape juice and a blueberry/Venus juice blend. The blueberry/Venus blend compared favorably in attributes of color, flavor and sweetness to a Reliance grape juice that was commercially marketed (Price and Morris 1991). The Venus and Reliance cultivars were developed and released by the University of Arkansas breeding program (Moore 1983; Moore and Brown 1977). Other blueberry juice products have also been produced on an experimental level (Dumeny 1978; Tipton ef u1. 1999). Currently, at least one blueberry juice blend (blueberry/cranberry juice cocktail) is being marketed nationally, and a blueberry/Reliance grape juice blend is marketed in Arkansas. Unfortunately, Reliance is a table grape variety, and most grapes are used for fresh market. Reliance is only available for juice production on a large scale when environ- mental factors, such as hail, leave the grape unsuitable for the fresh market.
The storage stability of blueberry juice blends is not documented. One method to determine the potential stability of a beverage is to store samples at an elevated temperature. After a 3-month period, under elevated storage temperature, if no defects are observed, the product can be determined to have a shelf-life of 10-12 months (Woodroof and Phillips 1981). Storage studies of grape beverages show a rapid loss of quality at elevated storage temperatures
BLUEBERRY JUICE BLENDED WITH OTHER FRUIT JUICES 113
(King ef al. 1988; Morris ef at. 1986). In addition, some blended juices have been reported to be more stable than others (Sistrunk and Morris 1982).
The objectives of this study were to produce blueberry juice blends and to investigate the quality and storage stability of these blends.
MATERIALS AND METHODS
Juices from blueberries and Concord and Venus grapes were prepared using similar methods (Moms 1998). Concord grapes (Vifis kubmscuna) were hand harvested at optimum maturity at the University of Arkansas research vineyards at Fayetteville. Frozen, machine-harvested highbush blueberries (Vaccinium corymbosum) were provided by the Arkansas Blueberry Growers Association, Lowell, AR. The frozen blueberries were crushed in a stemmer- crusher with rollers set at 3-mm. The crushed fruit was heated and continuously stirred in a steam kettle to 60C. A pectinolytic enzyme (Clarex L@, Solvay Enzyme Products, Inc. Elkhart, IN) was added at a rate of 155 m L / l O O O Kg of fruit. The fruit mass was stirred for 20 min and then cooled to 35C before pressing at 42 kg/cm2 in a bladder press. The juice was placed in a 2C-room overnight to promote settling of particulates and was then filtered using a diatomaceous earth filter. The juice was pasteurized through a heating coil at 91C, then immediately passed through a cooling coil until it reached 30C. The juice was stored at 2C in clear 3.78-L glass bottles.
Venus grape juice produced by the Ozark Table Grape Growers Associa- tion, Searcy, AR, was used in the Venus blends. Speas Farm@ brand natural, unsweetened apple juice from concentrate was used in blends containing apple juice. Since 100% cranberry juice is too strongly flavored, cranberry blends were prepared using Ocean Spray@ brand cranberry juice cocktail. A six- member sensory panel selected the commercial juices used in the experiment as being the best juices from brands locally available. Juices from the same commercial lots were used in all preparations. All commercial juices had been filtered and pasteurized at or prior to bottling.
Blueberry juice was blended with each of the other four products (Concord grape, Venus grape, apple or cranberry juice cocktail) at mixtures of 75, 50 and 25% blueberry for 12 blends.
Three-liter batches of each blend were made. All blends were adjusted to achieve a soluble solids:acid ratio of 22 to 26, that is considered optimum for fruit juices (Rathburn and Morris 1990). Sucrose was used for sugar adjustment, and citric acid was used for adjustment of acidity. High fructose corn syrup
114 G. MAIN, M. FAUPEL, J . MORRIS and R. MCNEW
could also be used as a sweetener but was not used in this study to avoid a dilution effect associated with addition of a liquid. After adjustment, each juice blend was filled into 125-mL glass bottles and pasteurized at 91C. The bottles were sealed, inverted for five minutes and then placed in water to rapidly reduce the temperature. The juice was then stored at either 21 or 37C.
Titratable acidity, pH, soluble solids, tristimulus color and absorbance were determined for each juice blend at time of bottling and after three months of storage at 37C. This storage temperature and time simulate long term storage at 21C (Kwoh 1972; Kwolk and Bookwalter 1971). Titratable acidity was determined by diluting 5 mL of juice with 125 mL deionized water and titrating to pH 8.2 with 0.1NNaOH. Soluble solids were measured with a refractometer, and pH was measured using a pH meter with a combination electrode. Tristimulus color was measured with a Gardner Color Difference Meter (CDM) that was standardized to a dark red plate having values of L = 23.4, a = 24.8, b = 6.8. A white plate was placed on the optical glass cup containing 5.0 mL of the juice, and values of L, a and b were recorded. Hue angle was calculated as the angle the tangent of which was b/a. A spectrophotometer was used to measure absorbance at wavelength settings of 420 and 520 nm. These values provide an index of browning and anthocyanin color respectively. All instruments were standardized before use, and samples were analyzed in triplicate.
Potential panel members from the Food Science Department attended training sessions, trial evaluations, and participated in discrimination testing to determine their ability to perceive the differences in blueberry and/or grape juices. Twelve panelists were selected to participate in the juice sensory analysis. Descriptive terms were generated by panelists during training sessions. A commercial 70% blueberry juice/30% Reliance grape juice was used as a sensory standard because of the mild sensory characteristics of Reliance grapes. Reference marks were provided for the blueberry/Reliance standard for each attribute, with positioning of reference marks chosen according to objective analysis and preliminary sensory observations.
Sensory qualities evaluated were: intensity of blueberry flavor, oxidized flavor, body, astringency, blueberry color, visual browning and blueberry aroma. Each attribute was charted on an unstructured 15-cm line with anchor words on each end (e.g., not sweet, very sweet). Three coded 15-mL juice samples were presented in clear plastic cups to panelists in individual booths at each evaluation session, along with a reference sample. GE Chroma 50 lights
BLUEBERRY JUICE BLENDED WITH OTHER FRUIT JUICES 115
that provide lighting conditions similar to noonday sun were used, and samples were presented at room temperature in random order. Panelists were provided with crackers and water for cleansing their palate between samples. Flavor attributes were evaluated separately from visual and aroma attributes. Panelists were asked to place a vertical mark and the sample number on the line to indicate the sensory rating for each juice. The distance of each mark from the end of the line was measured in centimeters. The highest possible intensity score for all attributes was 15 and the lowest, zero.
Statistical analysis was done using the ANOVA procedure (SAS Institute, Inc. 1985). Sensory data were analyzed as a randomized block design with blocks being sensory panelists. Objective data were analyzed within each juice type as a 3 x 2 factorial with 3 levels of juice and 2 storage times. Least significant difference at p 5 0.05 was used to separate means.
Blueberry blends should retain a recognizable blueberry flavor, aroma and color in order to market them as blueberry products. In all blends, as the percentage of nonblueberry juice increased, there was a decrease in the blueberry-related sensory attributes. Apple juice blends were higher in browning and oxidized flavor as compared with the reference (Fig. 1). For blueberry flavor, all levels of apple juice blends were rated lower than the reference. The apple juice flavor and aroma masked that of blueberry with the 75 % apple blend overwhelming the blueberry characteristics. The blueberry color of the 25 and 50% apple blends was equal to or higher than the reference, and these two blends were less astringent than the reference sample.
Concord juice blends were more blueberry-like than apple blends. All levels of Concord blends were equal to the reference for browning, astringency and body (Fig. 2). Blueberry flavor and color of the 25% Concord blend was equivalent to the reference. Blueberry intensity attributes varied with percentage of Concord juice in the blend. Concord character attributes overwhelmed the blueberry aroma, color and flavor at the higher percentages of Concord juice in the blends.
The 25% Venus blend had blueberry aroma and flavor similar to the reference, and the 25, 50 and 75% blends were similar to each other in blueberry flavor (Fig. 3). All levels of Venus blends were equal to the reference blueberry color and similar to the reference in browning and astringency.
116 G . MAIN, M. FAUPEL, J. MORRIS and R. MCNEW
Blueberry Flavor (1.8)
Percentage Apple Juice --+--75% -.. - 50% A 25% -8- Reference
Reference sample is a commercial 70% blueberry juice and 30% 'Reliance' grape juice blend Values in ( ) are LSD at p 5 0.05 for that attribute. N S = not significant
FIG. 1. SENSORY PROFILES OF BLUEBERRY/APPLE JUICE BLENDS
All cranberry blend levels were similar to the reference for browning, astringency and body (Fig. 4). The 25 and 50% cranberry blends matched or exceeded the blueberry color intensity of the reference. All levels of cranberry blends had less blueberry flavor than the reference. The flavors of the cranberry blends were intrusive and the blueberry character was not strong enough for the juice to be characterized as blueberry.
BLUEBERRY JUICE BLENDED WITH OTHER FRUIT JUICES 117
Blueberry Flavor (1.5)
T Blueberry Aroma (1.0)
Oxidized Flavor (0.6)
Percentage Concord Juice -.+-. 75% --..- 50% A 25% -8- Reference
Reference sample is a commercial 70% blueberry juice and 30% Reliance grape juice blend Values in ( ) are LSD at p i 0.05 for that attribute. NS = not significant
FIG. 2. SENSORY PROFILES OF BLUEBERRY/CONCORD GRAPE JUICE BLENDS
All of the juices in the study experienced color degradation during storage. Three months storage at elevated temperature is a harsh treatment that simulates a shelf-life of 10-12 months. The juices were stored without the addition of antioxidants. The addition of sulfur dioxide or other antioxidants might have increased the shelf-life.
118 G. MAIN, M. FAUPEL, J. MORRIS and R. MCNEW
Blueberry Flavor (2...