Shelf life of citrus products: packaging and storage 12.1 INTRODUCTION We define “shelf life” as the postproduction length of time, until the quality and safety of a product are reduced below a specified level. “Spoilage” is any process that leads to the reduction of quality and safety level. The major types of spoilage are: microbial, enzymatic, chemical, and physical. Baring gross under-processing (or no processing as in unpasteurized juices), unsanitary conditions, and mishandling, microbial and enzymatic spoilage is unlikely to occur in citrus products. The main concern about spoilage during storage of citrus products is then chemical spoilage. Chemical spoilage of liquid citrus products includes nonenzymatic browning, color change for other reasons, loss of ascorbic acid, deterioration of taste and aroma, and degradation of the bioactive principles. Nevertheless cloud loss has been reported in some cases. It should be kept in mind that the pectin methylesterase complex comprises a highly thermostable isoenzyme that is particularly difficult to inactivate completely. Texture-related spoilage (eg, softening) is relevant in the case of products containing solids. Theories of chemical reaction kinetics (Berk, 2013) are widely applied in the research of shelf life of foods, including processed citrus products. Assuming that reversal of the spoilage reaction is usually impracticable, the starting point of the kinetic model can be formulated as follows:
dCA = k (CA )n dt
where CA is the concentration of A (eg, ascorbic acid or a numerical parameter used for the definition of color). k is the rate constant and n is the order of the reaction. For most reactions in food storage, first order (n = 1) or zero order (n = 0) is assumed, pending experimental verification. Integration of Eq. 12.1 with n = 1 gives
Citrus Fruit Processing. http://dx.doi.org/10.1016/B978-0-12-803133-9.00012-6 Copyright © 2016 Elsevier Inc. All rights reserved.
252 CHAPTER 12 Shelf life of citrus products: packaging and storage
CA( t ) CA(0)
= − kt
In the experiments, the product is stored at constant temperature and CA is determined periodically. Substitution into Eq. 12.2 leads to the calculation of the rate constant k at the storage temperature. Variations in the results are treated using statistics. Determination of k at a number of different temperatures permits calculation of the “activation energy” which is a measure of the dependence of the rate of the investigated spoilage on temperature. Although a number of different models exist for said calculation, the Arrhenius model (Eq. 12.3) is the most frequently applied:
k = A exp
where E is the activation energy (kJ mol−1), R is the gas constant, T is the absolute temperature (K), and A is a constant. It should be pointed out that food spoilage reactions are, in most cases, the outcome of chain reactions, progressing through many steps. Many intermediate substances, not all of which are known, are formed, modified, or destroyed. Usually, the initial composition and the final outcome only are known and measurable. Kinetic variables such as order of the reaction, rate constants, and energy of activation are, therefore, simply empirical values, often with no significance at the molecular level. The kinetic model provides, however, a useful tool for predicting shelf life under specified storage conditions (Berk and Mannheim, 1986; Toledo, 2007).
12.2 SHELF LIFE OF SINGLE STRENGTH JUICES Citrus fruits are seasonal. It follows that the production of not-fromconcentrate (NFC) juices is also seasonal. On the other hand, demand for this eminently popular product is practically steady around the year. Consequently, long-term storage of single strength NFC juice is today an industrial necessity (WFLO, 2008). The quantities to store are enormous, therefore storing in relatively small retail packages is impractical. For a certain time, the technology adopted for long-term storing large quantities of NFC juice was freezing the juice and storing it as large frozen blocks or slabs until needed for thawing and repackaging for the retail trade. This was very costly and cumbersome. Since the 1980s, the solution of the big industry to the problem of long-term, high-volume storage of citrus juice has been the use of refrigerated bulk storage in very large tanks. The juice is pasteurized, deaerated, chilled to 0–1°C and aseptically pumped into presterilized, refrigerated tanks. Storage tanks with capacities of one million
12.2 Shelf life of single strength juices 253
gallons (approximately 3,500 cubic meters) or more are in use. They are often made of epoxy resin lined carbon steel. The tanks are grouped into the so-called “tank farms,” outdoors or in refrigerated enclosures. Nitrogen is often sparged, to create an inert headspace and to prevent gravity separation of pulp. If the temperature is maintained slightly above the freezing point and oxygen is efficiently excluded, a shelf life of 1 year is guaranteed. Juice reconstituted from concentrate does not necessitate long-term storage as it can be produced at any time from stored concentrate, according to demand. Storage and packaging studies on citrus juices and concentrates have been carried out principally on products packed in retail size units. Several studies investigated the effect of processing conditions on shelf life. Pérez-Cacho and Rouseff (2008) reviewed the research on processing and storage effects on orange juice aroma. Mannheim and Havkin (1981) compared the quality of aseptic bottled orange juice to hot-filled bottled juice during storage. In this study, the quality of aseptic juice was judged better immediately after filling but the differences vanished after storage. Sadler et al. (1992) made microbial, enzymatic, and chemical comparisons on orange juice stored at 4°C without pasteurization, with light pasteurization (66°C, 10 s) and with full pasteurization (90°C, 60 s) aiming at the inactivation of pectinesterase. Oxygen permeability of packaging did not affect the quality of unpasteurized juice. However, lightly and fully pasteurized juices in barrier cartons exhibited lower microbial counts, better ascorbic acid retention, and slowing of cloud loss by the third week of storage. During the first 22 days storage, microbial, cloud, and ascorbic acid values for lightly pasteurized juice were not different from those of fully pasteurized juice. The storage behavior of chilled grapefruit juice treated with UV radiation was investigated by La Cava and Sgroppo (2015). Uysal Pala and Kırca Toklucu (2013) also studied the changes in UV-treated orange juice during refrigerated storage and found that UV treatment enhanced the storage stability of the juice. The microbial preserving of citrus juices by silver and titanium compounds incorporated into the packaging films has been recently studied by Peter et al. (2015). The effect of processing variables and packaging materials on the shelf life of aseptically filled single strength orange juice was investigated by Graumlich et al. (1986) and by Ros-Chumillas et al. (2007). Glass, multilayer PET (polyethylene terephtalate), and monolayer PET bottles were tried. Monolayer PET showed the lowest retention of ascorbic acid. However, if additional protective measures such as oxygen scavenger, liquid nitrogen drop addition in headspace during filling, aluminum foil seal in screw-cap, and refrigerated storage were applied, the shelf life in monolayer bottles could be extended to values found with glass and multilayer PET bottles. Glass has the disadvantage of weight and fragility. The most popular package for both NFC and reconstituted juice is the
254 CHAPTER 12 Shelf life of citrus products: packaging and storage
multilayer carton. The packaging material usually consists of four layers, namely: an internal layer of polyethylene for sealability, a layer of aluminum foil for gas and light impermeability, a layer of paper for mechanical strength and printability, and a layer of polyethylene for external protection. The rectangular crosssection of the carton allows considerable savings in storage and display area, compared to round bottles. Laminated and printed carton is supplied in rolls and the containers are formed in situ. The storage behavior of reconstituted orange juice, processed by conventional thermal pasteurization (80°C, 30 s) or high hydrostatic pressure treatment (500 MPa, 35°C, 5 min), was studied by Polydera et al. (2003). Polypropylene bottles and laminated flexible pouches were used for packaging. Storage temperatures were 0 and15°C. Ascorbic acid loss rates were lower for juices treated by high pressure, meaning extended shelf life compared to conventionally pasteurised juice. The kinetic model revealed stronger temperature dependence of ascorbic acid loss in high pressure treated juice. The calculated activation energies were 61.1 kJ mol−1 for high pressure treated juice versus 43.8 kJ mol−1 for thermally pasteurized juice. The increase in the length of shelf life based on ascorbic acid retention was 11% and 65% for storage temperatures of 15 and 0°C respectively. Respective values of shelf life increase for juices in pouches were 24% and 57%. Color was not significantly affected by the method of processing. In the framework of the efforts to produce a pasteurized juice with less thermal damage to the flavor, Naim et al. (1988) studied the effect of storage on moderately pasteurized orange juice with added thiols (glutathione, L-cysteine, N-acetyl-L-cysteine) as aroma protectants. Fortification with thiols was found to reduce formation of p-vinyl guaiacol (the compound most detrimental to the taste of stored orange juice, see Chapter 2), ascorbic acid degradation and browning during storage. The most important perceptible damage to quality upon storage is nonenzymatic browning, which is particularly rapid in lemon and grapefruit juices. Flavor deterioration and induction of off-flavor occurs with browning. Roig et al. (1999) investigated nonenzymatic browning in citrus juice stored in TetraBrick cartons. They found that browning was due to carbonyl compounds formed from ascorbic acid degradation and not to carbonyl-amino Maillard reaction. Nagy et al. (1990) monitored nonenzymatic browning in canned and bottled single-strength grapefruit juice stored at 10–50° C for 18 weeks. Browning was faster and more intense in bottled than in canned juices. No browning occurred in canned juices stored at 10 and 20°C, apparently due to the reducing action of tin in acidic medium. This finding has little relevance today, since canning of juice has become practically obsolete.
12.2 Shelf life of single strength juices 255
Wibowo et al. (2015) investigated the effect of storage on the color of pasteurized single strength orange juice and the relationship between color change and degradation of carotenoids. The juice was stored during 32 weeks at 20, 28, 35, and 42°C. Changes in color were determined by colorimetry, using the CIELAB system and were kinetically described as a zero-order reaction. Calculated activation energies for all color parameters were 64–73 kJ mol−1. Different carotenoids appeared to have different susceptibilities to storage. Changes in carotenoids, however, have only a minor importance in color deterioration during storage, which is much more strongly affected by nonenzymatic browning. Color stability during storage was studied also with blood orange juice (Remini et al., 2015). The stability of ascorbic acid and color intensity in pasteurized blood orange juice during 1 month of storage at 4–37°C was investigated. Following Arrhenius’ Law, activation energies ranging from 51 to 135 kJ mol−1 and from 49 to 99 kJ mol−1 were found for ascorbic acid loss and color degradation, respectively. The effect of ascorbic acid fortification at the level of 100 and 200 mg L−1 on the kinetics of ascorbic acid loss and color degradation was negligible. Storage temperature and deaeration had the most significant influence on storage damage to quality. Nonenzymatic browning and ascorbic acid loss are interrelated (see Chapter 2). The rate of ascorbic acid loss in commercial single-strength orange juice aseptically filled in TetraBrik cartons was evaluated by Kennedy et al. (1992) at different storage temperatures. The level of dissolved oxygen present in the sample after packaging affected significantly the l-ascorbic acid content and so did storage temperature. Inversely, the rate of consumption of dissolved oxygen depended on the concentration of l-ascorbic acid. The authors concluded that both aerobic and anaerobic degradation of l-ascorbic acid occurs in the same system (see Chapter 2). The aerobic process predominates and the anaerobic process takes place after the level of dissolved oxygen has reached equilibrium. Soares and Hotchkiss (1999) stored deaerated and nondeaerated orange juice at 7°C in packages with different oxygen permeability. In both deaerated and nondeaerated samples the rate of ascorbic acid loss was found to be inversely correlated with oxygen permeability, regardless of initial dissolved oxygen concentration. These findings seem to indicate the ineffectiveness of deaeration and are in contradiction with other reports. Actually in industry, juices are deaerated before aseptic bulk storage in tank farms. The risk of aroma loss during storage due to adsorption by the packaging material is of concern. Pieper et al. (1992) packaged orange juice in cartons lined with low density polyethylene and monitored the absorption
256 CHAPTER 12 Shelf life of citrus products: packaging and storage
of 19 aroma component in the polymer during storage. A reduction in d-limonene of up to 50% was observed but an experienced sensory panel did not distinguish between juice stored in laminated cartons and glass bottles. Adsorption of 10 citrus aroma components by polypropylene packaging film was studied by Lebossé et al. (1997). The practical significance of this phenomenon known as “flavor scalping” is a matter of controversy among researchers. In contrast to other previously published reports (Pieper et al., 1992), sensory evaluation by means of difference tests done by Siegmund et al. (2004) showed that juice filled in the laminated carton package changed much faster than product stored in glass bottles.
12.3 SHELF LIFE OF CITRUS CONCENTRATES Concentrated citrus juices and comminuted bases utilize a larger number of aseptic packaging and storage options. Burdurlu et al. (2006) investigated the storage stability of vitamin C in citrus juice concentrates of orange, grapefruit, lemon, and tangerine during storage at ambient and relatively warm temperatures (28, 37, and 45°C). Ascorbic acid loss followed a firstorder kinetic model at all temperatures, with an activation energy of approximately 13–26 kCal mol−1 (54–108 kJ mol−1). Retention of ascorbic acid, after 8 weeks of storage at 28°C, was 55–84%, while at 37 and 45°C the losses were 73–76% and 80–85% respectively. Hydroxymethyl furfural accumulation correlated well with ascorbic acid loss at all the concentrates and all storage temperatures. The rate and extent of furfural accumulation in aseptically processed single strength orange juice and concentrate was studied by Kanner et al. (1981). Surprisingly, furfural accumulation in orange juice was found to be faster than in 34, 44, and 58 0Bx concentrates. The amount of furfural in orange juice (12 0Bx) was four times higher than in 58 0Bx concentrate stored at 17°C for 100 days. Insufficient enzyme inactivation of concentrated citrus juice may lead to cloud loss and even gelation during storage (Gómez et al., 2011). Nonenzymatic browning, ascorbic acid degradation, furfural accumulation, and sensory changes in aseptically packed orange juice and concentrate were followed by Kanner et al. (1982). Juice at 11 0Bx and concentrates at 34, 44, and 58 0 Bx were stored at temperatures between −18 and 36°C. Browning did not occur below 12°C. Ascorbic acid loss rate depended on the temperature between 5 and 25°C and was affected by the concentration. 58 0Bx concentrate did not exhibit flavor change after 17 months storage at 5°C or 10 months at 12°C. The practical conclusion of these and other studies is that properly processed orange juice concentrates can successfully withstand aseptic storage
12.5 Shelf life of miscellaneous citrus products 257
for 1 year or longer at 5°C or below and do not require freezing. In the big industry, concentrates stored in refrigerated tank farms comprising tanks with capacities of up to 250,000 gallons. For transport, concentrates are aseptically filled and shipped in refrigerated tankers, polymer lined steel drums, and Scholle bags. Scholle bags are pouches of different sizes, made of multilayer film and presterilized for aseptic filling. They are fitted with a special closure designed for aseptic filling nozzles. They come in standalone or bag-in-the box versions. For retail trade, concentrates are frozen, packed in small cans or plastic cups, and stored at subfreezing temperature (see Chapter 9).
12.4 SHELF LIFE OF CITRUS BY-PRODUCTS Due to their unsaturated molecular structure essential oils are fairly reactive substances susceptible to oxidation, polymerization, and racemization resulting in loss of sensory quality and pharmaceutical value. The patterns of spoilage are similar to lipid oxidation. Oxygen, light, and certain catalysts are the principal storage factors and their effect in retarded by low temperature (Turek and Stintzing, 2013). Citrus essential oils should be packed in metal (tinplate or aluminum) containers or colored glass bottles and stored under refrigeration for a shelf life of 1 year. Aqueous aroma solutions are produced and stored in large quantities. Guadagni et al. (1970) studied the influence of storage temperature on the quality of orange essence solutions. The stability of orange aroma solutions was tested at storage temperatures of 10, 20, 34, and 70°F. Aroma strength and fresh orange character in samples packed in screwcapped glass vials were rapidly lost at 34 and 70°F. Aroma strength was maintained at 0 and 20°F. Significant changes in aroma character were noted after 6 months at 20°F and 1 year at 10°F. Fresh orange aroma was retained without significant change for up to 88 weeks at 0°F. These findings indicate unequivocally the need to store essence at low temperature. Indeed, at present, aqueous essence is stored in refrigerated tank farms. Citrus pectin and fiber are shelf-stable dry powders. Storage in dry, cool locations is recommended.
12.5 SHELF LIFE OF MISCELLANEOUS CITRUS PRODUCTS Canned grapefruit segments are shelf-stable. If stored at moderate ambient temperature they gradually acquire a slight yellowish color. Usually, a shelf life of 1 year is specified, if stored at cool (not refrigerated) temperature.
258 CHAPTER 12 Shelf life of citrus products: packaging and storage
Storage at high temperature such as found in warm climate regions causes rapid browning. Plain, nonlacquered cans are the best package. This was confirmed by Miltz et al. (1995) who studied the shelf life of grapefruit segments in monolayer and multilayer plastic trays, in comparison to tinplate cans. Candied fruit and peels are slightly hygroscopic. Packaging in hermetic containers or pouches made of polymers with low permeability to water vapor and storage in a cool, dry area are recommended. At high humidity, caking, stickiness, and mold attack may occur. Jams and jellies are usually hot filled into presanitized glass jars. After filling and before cooling, the jars are inverted to sanitize the lids. The types of spoilage usually observed are severe browning and sugar crystallization.
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