Effect of Irradiation and Other Processing Treatments on the Flavor

Chapter 11

Effect of Irradiation and Other Processing Treatments on the Flavor Quality of Apple Cider 1

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Terri D. Boylston , Loretta R. Crook , Cheryll A. Reitmeier , and Fransiska Yulianti

Downloaded by UNIV OF OKLAHOMA on April 29, 2013 | http://pubs.acs.org Publication Date: September 30, 2008 | doi: 10.1021/bk-2008-0988.ch011

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Department of Food Science and Human Nutrition, 2312 Food Sciences Building, Iowa State University, Ames,IA50011-1061 Glanbia Nutritionals Research and Development, 50 Falls Avenue, Suite 255, Twin Falls, ID 83301

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Irradiation is an effective non-thermal processing treatment to inactivate foodborne pathogens and spoilage microorganisms in apple cider, although it can affect the flavor quality. The effects of irradiation, sorbate addition, and packaging treatment on the flavor quality of apple cider during storage were investigated. Free radicals generated during irradiation were quenched by sorbate to minimize the loss of esters and other volatile compounds that contribute to apple aroma. In the absence of sorbate, packaging materials and environments which minimize oxygen exposure were critical in preserving the desirable flavor quality of the irradiated apple cider during storage. Development of processing treatments to improve the safety of apple cider must consider their effect on the flavor compounds that impact consumer acceptability.

© 2008 American Chemical Society In Food Flavor; Tamura, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

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Introduction Apple cider is a fresh-pressed, unfermented pulp-containing juice made from chopped and pressed apples. Esters, alcohols, and aldehydes are among over 200 volatile compounds that contribute to characteristic apple flavor (/). Researchers have identified specific compounds to be of particular importance to apple flavor, including butyl acetate, 2-methylbutyl acetate, hexyl acetate, ethyl 2-methylbutanoate, ethyl butanoate, butanol, hexanol, hexanal, and trans-2hexenal (7-5). Synthesis of these flavor compounds occurs in the maturing and ripening fruit, with cultivar differences accounting for differences in the relative composition of volatile flavor compounds and overall flavor characteristics (4). During the processing of the apples into apple cider, oxidation of unsaturated fatty acids to C-6 aldehydes and reduction of these aldehydes to alcohols further contribute to the development of flavor (6). Differences in apple variety, maturity, and quality, cider processing parameters, and storage conditions contribute to variability in the flavor characteristics and quality of apple juice and cider (7). Recent foodborne illness outbreaks associated with of E. coli 0157:H7, Cryptosporidium, and other foodborne illnesses in apple cider (5-/7) and other juices have contributed to major changes in the apple cider industry. The FDA has mandated that a warning label be placed on fruit and vegetable juice products that have not been processed to achieve a 5-log reduction in the most resistant pathogen present, currently recognized to be C. parvum (12-13). The recognized treatment for the apple cider to achieve the mandated 5-log reduction in microbial load is pasteurization at a minimum of 70° C for 6 seconds (13). Although many producers currently pasteurize apple cider to comply with federal regulations, undesirable changes in flavor and appearance result from the heat treatment (14, 15). Pasteurization of apple juice results in a decreased fruitaroma score accompanied by a decreased content of esters and an increased cooked aroma, attributed to the formation of furfural and hydroxymethylfurfural during heating (2). In addition, 5-methyl-2-furfural, benzaldehyde, and 2,4decadienal, compounds that are formed during heating, have been identified in cooked apple slices (16). Non-thermal processing treatments have been proposed as alternatives to pasteurization, causing inactivation of pathogenic and spoilage microorganisms with fewer changes in texture, color, and flavor. A 5-log microbial reduction in apple cider has been achieved with doses of 1.8 - 2.5 kGy (77, 18). Irradiation of apple cider at 2 and 4 kGy resulted in a decreased content of esters characteristic of apple flavor (19). Cardboard and musty flavors have also been detected by sensory panelists in irradiated apple cider containing potassium sorbate (18, 19). Sorbates and benzoates, approved for use as chemical preservatives in apple cider, decrease spoilage and increase shelf-life, but contribute undesirable taste characteristics (20). Sorbates are most effective against the growth of yeasts and

In Food Flavor; Tamura, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.


123 molds with their activity against bacteria, including E. coli 0157:H7, more limited (21-23). pH has a significant impact on the effectiveness of the preservative since the undissociated form of the acid has the greatest antimicrobial effect (20, 21, 24). Processing treatments to improve safety, such as the irradiation of apple cider, must ultimately meet the consumers' demands for high flavor quality and acceptability. At Iowa State University, extensive research has been conducted to investigate the effects of irradiation, sorbate treatment, and packaging on the microbial safety and flavor quality of apple cider (25-27). In this chapter, we will highlight the results from these studies, focusing on the effects of these processing treatments on the flavor quality of apple cider, as assessed using instrumental and sensory evaluation techniques.

Overview of Methodology For these studies, fresh apple cider consisting of dominant apple cultivars, with (0.1%) and without (0%) potassium sorbate, was obtained from a local Iowa cider producer. Pasteurized apple cider was pasteurized by the producer at 79° C for 2 s. Raw apple cider was treated with electron beam irradiation at the Linear Accelerator Facility, Iowa State University, Ames, Iowa. For all experiments, the cider was irradiated at an expected dose of 2 kGy, based on previous research that demonstrated that a 2 kGy dose provides a 5-log reduction of E. coli 0157:H7 in apple cider (18). Samples were at room temperature for 20 min during irradiation. Cider was held at 4° C following treatment. Descriptive sensory evaluation, with a 10-member trained panel, evaluated the aroma and flavor characteristics of the apple cider. Selected attributes including sweetness, sourness, astringency, apple flavor, caramelized flavor, and musty flavor, were evaluated on a 15-cm line scale, with "0" corresponding to ratings of "none" and "15" corresponding to ratings of "intense". Protocols for sensory evaluation were approved by the Iowa State University Institutional Review Board (25). Solid-phase microextraction (SPME) techniques, with absorption onto a SPME fiber (2cm-50/30|im divinylbenzene/carboxen/polydimethylsiloxane; Supelco, Inc., Bellefonte, PA ) were applied for the isolation and concentration of volatile flavor compounds from the apple cider (19). Each sample was analyzed in duplicate. The volatile flavor compounds were thermally desorbed onto a fused-silica capillary column (SPB-5, 30m x 0.25mm x 0.25jim film thickness, Supelco, Inc.) via the splitless injection port of the gas chromatograph and detected using a flame ionization detector. Peak areas for the volatile compounds were determined. Volatile compounds were identified using authentic standards (Sigma-Aldrich, Milwaukee, WI; AccuStandard, Inc., New Haven, CT) and confirmed with GC-MS analyses (Micromass GCT, Waters


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Corp., Milford, MA). Kovats retention indices, based on hydrocarbon standards, were calculated for all volatile compounds. Mass spectra of the volatile compounds were compared to a spectral library (Wiley Library) and a flavor and fragrance database (FlavorWORKS, Flavometrics, version 2.0, Anaheim Hills, CA) for identification. Analysis of variance was conducted to determine the significance of main effects and interactions. Fisher's least square difference test was used for mean separation to determine significant treatment effects (SYSTAT, ver. 9.01, SPSS, Inc., Chicago, IL). A probability level of 0.05 was designated for significance.

Flavor Quality of Raw, Pasteurized, and Irradiated Apple Cider In this study, the flavor characteristics of raw, pasteurized, and irradiated apple cider, with and without sorbate were evaluated using sensory and instrumental methods. Samples to be irradiated were packaged in transparent low-density polyethylene (LDPE; Nasco-Whirl Pak, Fort Atkinson, WI.) bags. All samples were analyzed within 24 hr of irradiation treatment to minimize any storage effects. The experiment was designed as a 2-way factorial with processing treatment and sorbate addition as the main effects. Three replications were conducted. Processing treatment had a significant effect on apple flavor, as determined by the results of the descriptive sensory evaluation and the instrumental analysis of volatile flavor compounds. The intensity of the apple flavor sensory attribute (Figure 1) and the content of several volatile flavor compounds which are important contributors to apple flavor (Figure 2) were significantly lower in the irradiated cider than in the raw apple cider. These volatile flavor compounds included hexyl acetate, butyl 2-methylbutanoate, and hexyl 2-methylbutanoate. The apple flavor intensity and the contents of these volatile flavor compounds in the pasteurized apple cider were generally less than in the raw cider, but greater than in the irradiated apple cider. The contents of butyl acetate, 2-methylbutyl acetate, and hexanol, also important contributors to apple flavor, were not affected by processing treatment. The intensity of the sensory attributes, sourness, astringency, and caramelized flavor were not significantly affected by processing treatment. The addition of sorbate resulted in a significantly higher intensity of sweetness in the apple cider, but did not significantly affect the pH or soluble solids contents of the apple cider (25). The content of many of the volatile flavor compounds present in apple cider were not affected by sorbate addition in this study. The irradiation of apple cider containing sorbate resulted in the highest intensity of musty flavor of all the apple cider treatments (Figure 3). A model



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Figure 1. Intensity offlavor attributes in raw, pasteurized, and irradiated apple cider, as determined by the descriptive sensory evaluation panel. SW, sweetness; SO, sourness; AS, astringency; AP, apple flavor; CA, caramelized flavor. Means identified with different letters (a-b) are significantly different (P

Effect of Irradiation and Other Processing Treatments on the Flavor

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