Chapter 18
Flavor—Package Interaction Assessing the Impact on Orange Juice Quality 1
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G. Sadler, M. Parish , J. Davis, and D. Van Clief 1
National Center for Food Safety and Technology, 6502 South Archer Road, Summit-Argo, IL 60501 Citrus Research and Education Center, University of Florida, 700 Experimental Station Road, Lake Alfred, FL 33850
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Nitrogen-purged, orange juices (high oil, medium oil, low oil, and thermally abused) stored in glass with sterile strips of polymers (low density polyethylene, oriented polypropylene, nylon, polyethylene terephthalate, or ethylene vinyl alcohol co-polymer) did not suffer detectable sensory change during storage despite oil losses as great as 41%. However, d-limonene absorption greatly increased microbial proliferation in juice systems containing LDPE.
There has been a steady increase in the use of plastics in packaging since the 1960*8. The tendency of aroma compounds to transfer between packages in close storage on store shelves was identified in the late 196Cs (1). In the early 198(ys, research speculated that volatile absorption by packaging materials might influence flavor intensity or alter characteristic flavor profile of foods packaged in plastic materials (2). In these studies, terpene hydrocarbons were observed to be absorbed at exceptionally high rates. This appeared to be especially important for citrus. Cold pressed citrus peel oils can contain over 95% d-limonene and smaller percentages of other terpene compounds. Citrus processors often add cold pressed peel oil to orange juice to optimize flavor. Early citrus flavor work identified d-limonene as an important component of citrus flavor. It therefore appeared likely that limonene absorption would be a good candidate for absorption-related flavor loss. In reality, d-limonene and other unoxidized terpenes impart no significant flavor to citrus products (3,4). Early reports of d-limonene's importance to flavor can be traced to the former difficulty of removing active flavor compounds from d-limonene distillates. When d-limonene is present above its sensory threshold it is often considered a flavor defect (3). Volatiles with polar functional groups (esters, carbonyl compounds, alcohols, etc) are more often associated with desirable citrus flavor. These compounds tend to
0097-6156/95/0596-0202512.00/0 © 1995 American Chemical Society Rouseff and Leahy; Fruit Flavors ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
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be much less soluble in packaging materials than terpenes (5,6,7,8,9,10). A great deal of experimental sophistication has been used to study the interaction of these classes of compounds with a broad assortment of food-contact polymers. Although analytical methods are able to detect trivial levels of absorption they provide limited insight into the sensory consequences of volatile uptake. Surprisingly little work has been devoted to understanding the sensory impact of volatile absorption. Early studies compared orange juice flavor differences between laminated paper/foil drink boxes and product stored in glass for at 25° C. Although researchers noted a significantly greater shelf life for the product in glass, they cautioned that oxygen exposure for product in juice boxes was much greater than for product stored in glass.(ll) Therefore, the component of shelf life attributable to volatile absorption could not be distinguished from the portion of shelf life dictated by oxidation. A recent study (12) indicated absorption of citrus aroma volatiles into a polyethylene/barrier laminated carton at 4°C did not significantly differ in sensory quality from their 4°C glass control. At this lower temperature, volatile absorption and oxygen permeation/reaction would be substantially slower than at 25° C. Improved flavor stability would be reasonably expected. Although the results of this study are likely solid, they are mitigated somewhat by the use of juice with mediocre initial sensory scores. This compresses the range of hedonic response available to sensory panelists. The olfactory effect of volatile absorption has been analyzed using gas chromatography-olfactometry (4). Extracts of juices with and without prior polymer contact were subjected to G C separation. Peaks were analyzed by FID and human olfaction. Despite d-limonene losses of 70-80%, the odor intensity of polymer-treated juices were not perceptibly lower than polymer-free controls. These studies suggest that volatile absorption, while instrumentally quantifiable, may not have significant sensory impact. This belief is also held by some citrus processors. Others feel volatile absorption may not be detectable in typical juices, but may be apparent in juices with very high or very low levels of peel oil. Juices with underlying flavor defects, such as oxidized or thermally abused flavor, have also been speculated to exhibit greater absorption-related flavor differences. The effect of volatile absorption on quality attributes other than flavor have received little attention. The absorption of some flavors have been shown to increase the oxygen permeability of host polymers. This method has been exploited to determine the diffusion coefficient and relative magnitude of interaction in some polymer/flavor couplets (8). However, no effort has been made to determine whether this phenomenon significantly influences quality. To date no work has examined the impact of volatile absorption on chemical and microbial properties of juice. However, anecdotal information among producers of unpasteurized juice holds that high peel oil juices have greater microbial stability than juices with typical peel oil. Dipentene, (a mixture of d-limonene and its 1isomer) has been used as an antimicrobial agent for many years. Treatment of citrus waste effluents and fermentation of citrus molasses are slowed by excessive levels of d-limonene. Therefore, it is not unreasonable to believe d-limonene may have some impact on the microbial environment in citrus juices. If d-limonene does influence microbial numbers then its removal through interaction with package materials may lead to shorter shelf life.
Rouseff and Leahy; Fruit Flavors ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
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FRUIT FLAVORS
The purpose of this work was to determine whether absorption of flavor by 4 common packaging polymers, low density polyethylene (LDPE), polyethylene terephthalate (PET), nylon, or ethylene vinyl alcohol copolymer (EVOH) is sufficient to influence flavor of high oil, low oil, typical oil or thermally abused juice; and to determine if d-limonene absorption influences microbial numbers at concentrations typically found in commercial orange juice.
Experimental Sensory Study Characteristics of Orange Juice Used in Sensory Testing. High (0.023%) and low (0.009%) oil lots of orange concentrate were acquired through a major citrus processor. Lots were specifically selected for high flavor scores during preliminary in-plant sensory screening. Moderate oil (0.016%) juice was produced by blending high and low oil lots. A l l lots contained similar proprietary levels of citrus essence. Juice Processing. Concentrates were reconstituted to 11.8°Brix with sterile distilled water. No subsequent thermal treatment was provided except for thermally abused samples which were prepared by recirculating reconstituted juice through a plate heat exchanger for 20 min. at 100° C. Polymer Treatment. Polymer strips (2 χ 10 cm) were cut from bulk rolls of LDPE, PET, nylon and EVOH. Nominal thickness for all films was 25 μ (1 mil) except for PET which was 23.4 μ (0.92 mil). A twist was introduced to the polymer strip which was stapled back on itself to form a kinked loop. The twist kept the strips from folding back upon themselves and limiting surface area available for absorption. Strips were sterilized in distilled water for 30 min. at 90° C. Strips of each material were aseptically transferred to milk dilution bottles containing 10 mL of 0.1% sterile peptone (Difco Laboratories, Detroit MI) broth. Duplicate aliquots (0.01 mL) were plated on orange serum agar (OSA) plates. Juice Storage. Juice (1600 mL) was stored in a sterile 2 L erlenmeyer flasks with 1589 cm of polymer. This represented 2 times the polymer surface to juice volume ratio found in standard gable-top containers (1.89 L). The headspace of the flasks were nitrogen purged to < 1% oxygen and capped with a saran-coated rubber stopper. Flasks were stored at 4.5 ± 0.5° C for 2 to 3 weeks. Flasks were agitated several times a week to allow all polymer surfaces access to flavor absorption. Controls were freee of polymer strips; but were otherwise subjected to all treatment and storage steps described above. 2
Juice Analysis, Chemical. Scott oil values were determine on all juices at the end of storage using analytical methods standard to the citrus industry (13). Juice Analysis, Sensory. Triangle taste panel studies examined sensory differences between polymer-treated juices and polymer-free controls. Fifteen to twenty-two
Rouseff and Leahy; Fruit Flavors ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
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experienced panelists were offered samples in random presentation. If a difference was perceived a statement of preference was requested. Triangle data was evaluated using tables of Roessler et al.(14). Samples were considered significantly different if they differed at the Ρ
