Major quality changes in cheeses affected by packaging include: Physicochemical changes due to oxidative reactions
Physical changes such as water loss and subsequent rheological alterations Microbial changes, including survival and growth of both desirable and undesirable microbes
Figure 1 Interplay between external factors within the food packaging sphere and quality changes taking place in the product.
The above-mentioned changes all affect sensory aspects and thus influence how consumers perceive the products.
The interplay between internal and external factors within the food packaging sphere and quality changes occurring in the product is outlined in Fig. 1.
The major physicochemical quality-determining factor in cheese is oxidation, induced either thermally, enzymatically, by the presence of transition metals, or by light exposure. Irrespective of the induction mechanisms, these changes result in off-flavor formation as the primary reaction products from lipid oxidation, the lipid hydroperoxides, are decomposed into volatile secondary oxidation products such as alkanes, alkenes, aldehydes, alcohols, ketones, esters, and acids (1,2). Furthermore, oxidative reactions involving proteins, vitamins, and pigments occur, which result in discoloration, nutritional loss, formation of carcinogenic compounds, and changes in physical characteristics (3,4).
Light-induced oxidation plays a key role in relation to packaged cheeses. Cheeses may be exposed to light from natural and/or artificial sources during processing, storage, distribution, and marketing, resulting in oxidation of lipids, proteins, and sterols. Additionally, many vitamins (e.g., ascorbic acid, riboflavin, vitamin A, h-carotene, and tocopherols) may be destroyed when cheeses are exposed to light. Some of these are degraded by the direct effect of light; others indirectly, by reaction with active oxygen species formed during light-induced oxidation.
Photooxidation may take place by photolytic autoxidation or photosensitized oxidation. Photolytic autoxidation is a direct formation of free radicals initiated by high-energy light such as sunlight (10,000-100,000 1x) (5). However, visible light may also trigger oxidative changes due to the presence of photosensitizers (e.g., riboflavin in cheeses) and oxygen (photosensitized oxidation) (6). Riboflavin is an efficient photosensitizer: it readily absorbs visible light energy, thereby exciting the sensitizer to a higher energy level, which makes for reactions with (for example) unsaturated free fatty acids or produces the very reactive singlet oxygen (1O2). Subsequently, the 1O2 reacts with the unsaturated free fatty acids, producing free radicals, lipid hydroperoxides, and finally volatile carbonyl compounds. Many of these photochemical reactions result in autocatalytical oxidative processes, implying that even short light exposure time may have detrimental effects on the stored product and that oxidation, initiated by light, may continue even when the cheeses are protected from light.
Cheeses are exposed to different light sources during processing and distribution and at the retailers. Light with high quantum energy (i.e., lower-wavelength light in the visible/ UV spectrum) has the potential for the most severe quality deteriorative effects (4,7) because this light can be absorbed by a variety of molecules. Packaging materials absorb most of the energy-rich UV light, and thus UV light is generally not so harmful to the packaged dairy product as is light in the blue-violet region (400-500 nm) of the spectrum. Light in this wavelength range is absorbed by the two major colorants of milk—the prooxidant, riboflavin, and the antioxidant, h-carotene—and is thus critical with respect to photooxidation. The balance between the concentrations of these two compounds, which is largely determined by feeding (8) and dairy unit operations, determines to which degree light in this wavelength range impairs product quality during retail storage. A recent study on the effect of specific wavelengths present in commercial fluorescent light (366 nm, 405 nm, 436 nm, and storage in the dark) revealed that visible light was more detrimental than was near-UV light and dark storage (9). Furthermore, the study indicated that 405 nm exposure was more detrimental to semihard Havarti cheese than was 436 nm. Further studies evaluating the effects of specific wavelengths are called for.
Increasing light intensity (i.e., the photon flux) affects light-induced oxidation negatively (10,11). Intensities found in the literature vary from approximately 40 to 12,000 1x in dairy displays, with average intensities of 1000-3000 1x (5,12,13). Intensity results from different studies can most frequently be compared on a qualitative basis only because the light sources seldom have identical spectral distribution of radiation.
Limited temperature dependence is expected for photochemical processes because these reactions have low activation energies (14). Very recent studies substantiated the limited temperature dependence for photooxidation of semihard cheeses at 3°C and 10°C (15). However, autoxidation is temperature dependent and may prevail when the initial free radicals are formed through photooxidative processes. Photooxidative changes increase with storage time progression (9,15-20).
Important packaging parameters for prevention of photooxidation encompass the following:
Oxygen is required for oxidation to occur, and hence, minimizing oxygen levels is effective in preventing oxidation. Factors influencing the amount of O2 include initial gas composition (both dissolved in product and present in headspace), product-to-headspace volume ratio, product respiration (i.e., microbial and enzymatic conversion of O2 to CO2), and oxygen transmission rate of the packaging material (4).
2. Surface Area and Product-to-Headspace Volume Ratio
Photooxidation takes place mainly on the surface of the cheese (21). Thus, increasing the surface area leads to more severe photooxidative effects (10,15). Factors such as mobility and diffusion of oxygen into the product have not yet been researched; consequently, such evaluations are essential.
Light protection offered by the packaging material depends on numerous factors, which may be altered in order to provide the necessary protection against photooxidation of specific dairy products:
Type of material (oxygen transmission, light transmission) (7,20) Wall thickness/grammage (7,22,23)
Processing (including orientation of the polymer, crystallinity, and incorporation of additives) (7) Inks/pigmentation/cavitation (20) Metallization (7) Attachment of labels (23)
Recent reviews on photooxidation in dairy products include Bosset et al. (24), Skibsted (3), Thron (25), and Mortensen et al. (4).
Packaging may affect physical changes such as water loss at the surface (26,27) resulting in color changes (a more intense color) and rheological alterations in the cheeses (28-30).
Important packaging parameters affecting water loss and the texture include the following:
1. Composition, Volume, and Humidity of the Initial Gas Atmosphere
The gas composition may affect water loss. Gonzales-Fandos et al. (31) found that the water loss in fresh goat cheese (Cameros cheese) packed in 100% CO2 was 10.7% compared to a water loss of approximately 3% for Cameros cheese packed in either 20% CO2/ 80% N2, or 40% C02/60% N2. The differences may be attributable to reduced pH in the cheese packed in 100% C02 and/or a lower water content of the C02 gas compared to N2. Furthermore, a gas composition consisting of 100% C02 may affect the texture negatively, probably due to increased water loss (32).
In addition, the headspace volume and humidity of the gases are presumed to affect the rate of water loss. However, literature in this area is nonexistent.
Water vapor permeability is essential for water loss. The water vapor permeability depends on numerous factors, including humidity, the type of material, and the wall thickness/ grammage. Desobry and Hardy (33) noted that a 2.5-5% weight loss of cheese due to insufficient barrier properties is normal. They also found that dehydration of fresh cheeses should be avoided, because a dehydrated surface is a major quality defect in these products. Topal (34) noted a 8.5% weight loss of Kashar cheese during traditional curing, which could be prevented by maturing the product in Cryovac films, thereby increasing profitability. British Standard (35) recommended water vapor permeabilities of less than 30 g m-2 day-1 for consumer packages and 4.0 g m-2 day-1 for gas and vacuum packaged cheeses (25°C, 75% RH). Obviously, the required permeability may vary from product to product. Weight loss of unpackaged versus packaged Camembert cheeses was also reported to be different (36).
Even though cheese was originally developed as an efficient way to store the nutritional part of fresh milk for an extensive period, the product is still susceptible to microbial degradation. The types of organisms capable of surviving in cheese are, however, quite different from the ones present in milk, due to the low pH (4.5-5.2) and the moderate to high salt content. The specific group of organisms growing on a cheese is termed the associated microbiota of the cheese. The composition of this microbiota is strongly affected by the chemical and structural composition of the cheese, which again depends on type of milk, production, maturation, and storage conditions. In order to avoid microbial changes through proper packaging, it is of great importance to know how the cheese is produced, its microbiota, and to have some general knowledge about its chemical composition. It is an established fact that the primary spoilage organisms in cheese are molds. Furthermore, yeasts may cause problems in products such as Feta and decorated cream cheese (i.e. cream cheese covered with herbs, nuts, spices, etc.); in fact, lactic acid bacteria may spoil decorated cream cheeses.
All cheese production starts with mechanical pretreatment of the milk, from simple filtration to remove larger particles (e.g., dirt), to adjustment of fat content, to homogeni-zation or ultrafiltration. These processes have only limited direct effect on the microbiological status of the milk unless the milk is sterile-filtrated. The subsequent heat treatment is important in controlling the number of microorganisms transferred from the raw milk to the cheese. The microbiota associated with the raw milk includes nonstarter lactic acid bacteria, yeasts, and molds. Nonstarter lactic acid bacteria and some yeasts exhibit beneficial impact on maturation and the sensory quality of cheese (37,38), whereas other yeasts and bacteria may lead to spoilage.
Hygiene in the dairy plant is also an important issue because most microbial problems arise from insufficient sanitation. This is, however, a very complex issue. Several authors address the cleaning and disinfection regimens applied in dairy plants in order to minimize the load of bacteria, yeasts, and molds (39). Others have tried to track down and map distribution of spoilage organisms within the dairy plants and how these organisms actually entered the plant facilities (40,41).
Finally, decoration and coating of cheese pose a great risk of contamination because the spices, herbs, or nuts used may contain yeasts and molds (42).
2. Cheese as a Substrate for Spoilage
The following includes an overview of different microbial changes likely to occur in cheese.
Listeria is one of the most feared microorganisms within the dairy industry. Most dairies have a zero tolerance for Listeria, and based on the apparent growth domain of Listeria, this would make sense, because some strains of Listeria may grow at pH levels as low as 4.1, at a temperature of 0.5°C, at water activity levels below 0.93, and in environments containing 10% NaCl. Additionally, the species thrive on anaerobic conditions. Fortunately, growth at these extreme conditions is only possible when otherwise optimal conditions exist. Thus, Listeria has primarily been a problem in soft, low-salt, mold-ripened cheeses produced under improper pasteurization or inadequate hygienic conditions. These cheeses are more susceptible to Listeria because the fungal metabolism during ripening will make the pH rise considerably in the center of the cheese. Based on results from experiments on fermented meat products, it appears that modified atmosphere packaging with high CO2 content would serve as an effective protection (43,44).
Late blowing is another economically important spoilage process of cheese. It is attributable to the growth of Clostridium tyrobutyricum, C. butyricum, and related species, which produce CO2 by butyric acid fermentation (45). These are mesophiles, so they will not cause spoilage in chill-stored cheeses.
Nonstarter lactic acid bacteria may in some cases cause problems in modified atmosphere packaged, decorated cream cheese (42), because the bacteria are facultative anaerobic, which implies that they cannot be controlled by modified atmosphere packaging.
Yeasts constitute an important group of spoilage organisms, especially relative to products with moderate to low salt levels such as cream cheese and cottage cheese or products stored in brine, (e.g., Feta). Yeasts are nonpathogenic and do not produce mycotoxins. However, several species produce undesirable off-flavors and may cause changes in color and texture. Deformation of the package may take place due to gas production by fermentative yeasts such as Torulaspora delbrueckii, which causes swelling of Feta cheese (42). A range of yeasts spoils Feta, and several of these are restricted to a few dairies, where they dominate. The most important spoilage yeasts of Feta are Debaromyces hansenii, Klyveromyces maxianum, Yarrowia lipolytica, and Candida butyri. Other important yeasts include T. delbrueckii, K. lactis, C. sake, and C. butyric (42). In cream cheese, the most important spoilage yeasts include T. delbrueckii, C. parapsilosis, Pichia fermentans, D. hansenii, and Y. lipolytica. Total removal of oxygen by applying an oxygen scavenger does not fully preclude yeast growth. Combinations of very low oxygen levels and high carbon dioxide levels may inhibit growth of the weakly fermenting yeasts (e.g., D. hansenii), whereas the strongly fermenting species, K. lactis and T. delbrueckii, are nearly unaffected (46).
Molds are the dominating spoilage organisms of cheese. Several fungi have been isolated from cheese but only a small number are actually significant spoilers (47). Most important is Penicillium commune. Two species, P. nalgiovense and P. verrucosum, dominate in cheese with high salt content, and P. solium and P. nalgiovense appear mainly in cheese stored at temperatures below 5°C. P. roqueforti, which is used in the manufacturing of blue mold cheese, also appears to be a contaminant in inadequately vacuum packaged cheeses. Haasum and Nielsen (48) did a detailed study of the physiology of cheese-associated fungi. One of the main conclusions of this study was that fungi resistance to reduced water activity (or high salt) was in fact linked to sensitivity to high carbon dioxide levels, corresponding to the normal conditions on the surface of the cheese. Other species such as P. roqueforti and Geotrichum candidum are less affected by high carbon dioxide levels; however, they are strongly affected by reduced water activity, which corresponds with their affinity for the center of the cheese. This is important relative to optimizing the packaging conditions for cheese, because it indicates that the normal surface fungi (P. commune, P. camemberti, P. solitum, P. nalgiovense, and P. caseifulvum) are effectively controlled by high carbon dioxide levels.
Studies of growth at very low oxygen levels showed that most fungi were inhibited only when the O2 levels were less than 0.5%, and levels as low as 0.01% were required to efficiently inhibit any and all fungi (71). Most resistant to very low oxygen levels were the Fusarium species and yeasts, which all showed restricted growth (<2 mm) after 1 to 2 weeks at 25°C with oxygen absorbers (O2<0.01%). On an optimal growth medium, all cultures reached almost the same size irrespective of the initial amount of O2, whereas the combination with high CO2 levels retained growth. Another study indicated that packaging gas had a pronouced effect on survival of spores on the cheese (49). Storage at high carbon dioxide levels effectively prevented growth and preserved the conidia spores effectively, whereas packaging at lower carbon dioxide levels allowed the conidia to germinate but not grow, which eventually led to their inactivation, resulting in a longer shelf life for the product.
Modified atmosphere packaging is in some cases combined with preservatives to ensure product stability and safety even after opening. Sorbates are mainly used for cream cheeses and processed cheeses, whereas hard cheeses may be immersed in a solution of natamycin (pimaricin, E235) or covered by a plastic coating containing natamycin and sorbate. Used alone, these preservatives cannot completely hinder spoilage because a few fungal species are resistant to either sorbate or natamycin (50,51). Here, as in all other cases, optimal product safety is obtained by combining several product-specific preservation factors.
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