A high-quality processed cheese should have a smooth, homogeneous structure; shiny surfaces; uniform color; and no gas holes due to fermentation. However, there are numerous possible defects, ofphysicochemical or microbial origin, caused either by (1) unsuitable blend, or (2) inadequate processing.
An unsuitable blend comprises poor-quality or contaminated natural cheese; a bad relationship of blend components; poor-quality proteins; an improper protein-to-fat ratio; irregular quality or quantity of emulsifying agent; and incorrect values for pH, moisture content, and quantity of reworked cheese.
Inadequate processing means unsuitable time-temperature regimes, inadequate agitation, improper cooling, or unsuitable storage. Fortunately, most of the problems in processed cheese technology, when once properly detected, can be corrected. So the first task is to identify the cause responsible for the defect. As soon as the cause is determined and eliminated, the defect showed disappears in further processing.
The most common quality defects of physicochemical and microbial origins in processed cheese, their causes, and suggestions for their correction are presented in Table 5. A number of the cited defects originate in the natural cheese used in the blend, some of which can be avoided by proper processing. Processed cheese with certain minor defects can be recovered by reworking small quantities of it into subsequent batches. More serious defects cannot be corrected and render the product unsuitable for human consumption (eg, microbial changes, Clostridium botuli-num toxin, presence of metal ions, and excessive Maillard browning).
Crystal formation, sometimes visible, is a serious defect in processed cheese. Most often, the reason for crystal formation is a low solubility of the emulsifying agent used. The situation is accentuated with excessive amounts of emulsifying agent, high calcium content in the natural cheese, high pH of the finished product, and storage of the processed cheese at low temperatures. Some emulsifying agents react with calcium in the cheese, producing insoluble calcium salts. Using sophisticated instrumental methods, such as electron microscopy, energy dispersive spectrometric analysis, and Debye-Scherrer X-ray analysis, crystals in processed cheese have been characterized. They have been chemically identified as calcium phosphate, calcium citrate, sodium calcium citrate, and disodium phosphate (3-5,3436). Apparently, the most frequently formed crystals are calcium phosphate. During processing, calcium from the natural cheese reacts with phosphates of the emulsifying agents, thus resulting in insoluble calcium phosphate crystals (1). The growth of calcium phosphate crystals in processed cheese, when sodium diphosphate is used as an emulsifying agent, has been shown (28). In Figure 6, calcium diphosphate crystals are visualized (a) in processed cheese, (b) as isolated from cheese, and (c) as a chemically pure form (spray dried). Sheetlike citrate
Table 5. Most Common Quality Defects in Processed Cheese
Burned with browning
Hard, with water separation
Sticky (adhering to lid foil)
Air contamination, moldy raw cheese
Salty raw cheese or other components, too much emulsifier
Maillard reaction (lactose and amino acids); usually when very young cheese or whey products are present
Use hermetically sealed foils, eliminate all moldy cheeses from the blend Reduce emulsifier
Add young, unsalted cheese or fresh curd to blend, decrease the quantity of emulsifier Add younger cheese (with lower pH value), use emulsifying agent with lower pH value Use processing temperatures <90°C, cool processed cheese immediately after packaging, avoid large containers, store <30°C, avoid high pH values in final product
Texture (body, consistency)
High moisture, improper emulsifier, insufficient emulsifier, high pH, fast cooling, excess ripe cheese in blend, prolonged processing, slow agitation
Low moisture, improper or excess emulsifier, low pH, slow cooling, improper blend, excess creamed or overcreamed, reworked cheese
Colloidal change in cheese structure (overcreaming), bacteriological action leading to reduced pH
Unsuitable blend, improper emulsifier, insufficient or excess emulsifier, low pH, short processing time, low processing temperature, improper amount of added water, inadequate agitation, colloidal or bacteriological changes caused by improper storage
Sticky foil, insufficiently impregnated, excessively high pH, processed mass left hot too long without agitation
Bacteriological changes (growth of Clostridia, coliform or propionic bacteria); physical changes (occluded air, C02 from emulsifier mixture (citrates), holes filled up with fluid from emulsifying agent having low solubility); chemical changes (hydrogen from reaction between processed cheese and aluminum foil)
Calcium diphosphate and calcium monophosphate crystals (when phosphates are used in emulsifying agent), calcium crystals (when citrates are used in emulsifying agent), crystals due to undissolved emulsifying salt, large crystals due to excess emulsifier, lactose crystal formation, caused by excess whey concentrates or low water content, light coloured, grainy precipitate of tyrosine (very mature cheese in blend)
Reduce water content, use suitable emulsifier, increase emulsifier content, decrease pH, slow down cooling, increase proportion of young cheese in blend, reduce processing time, increase agitation speed Increase water content, use proper emulsifier, decrease emulsifier content, increase pH, speed up cooling, change blend composition, avoid addition of creamed or overcreamed reworked cheese Remove all factors that affect excess creaming, choose blend components carefully, keep processing temperatures >85°C Add younger cheese, use suitable emulsifier, correct emulsifier quantity, correct pH, prolong processing time, increase processing temperature >85°C, increase the ammount of added water, continue agitation during processing and filling; proper cold storage
Change aluminum foil, decrease water addition and add in two portions, increase proportion of ripe cheese or cause better creaming, keep pH <6.0, continue agitation until packaging
Select cheese blend components carefully, keep processing temperatures >95°C, use proper vacuum, preheat citrate emulsifier before processing, extend processing time, test porosity of aluminum foil and if necessary change it
Avoid monophosphates and diphosphates as emulsifying agents, or combinations with higher phosphates and polyphosphates; exclude citrates from emulsifying agent; exclude sandy reworked cheese from blend; distribute emulsifying agent better; increase processing time; add emulsifying agent in solution; use prescribed quantity of emulsifier, reduce level of whey products, and increase water content; exclude raw cheese that contains tyrosine crystals
crystals are shown in commercial processed Gruyère cheese in Fig. 7. Quite recently crystalline monoclinic calcium pyrophosphate dihydrate was also found in processed cheese when emulsifying agents containing pyro-and polyphosphates were used (37). Authors assume that this crystalline product either comes from migration in the protein matrix of calcium ions and pyrophosphate anions, which are either directly introduced, or results from the hydrolysis of the polyphosphates. In addition to the emulsifying agents, lactose and free tyrosine may de-
velop crystals in processed cheese, if present at excessively high concentrations (1,2,5).
Discoloration or browning is a defect in processed cheese caused by the Maillard reactions (nonenzymic browning), when the product develops a dark brown or pink color. The exact mechanism and interrelations during all stages of Maillard reactions have been discussed in detail (30,38). Maillard reactions commence at elevated temperatures and continue autocatalytically. Because the main reactants in Maillard browning are amino acids and reducing sugars, the products most susceptible to these changes are blends containing high levels of young cheese; that is, high lactose concentration and other lactose-containing ingredients (particularly whey powder). Browning is more prevalent in processed cheese spreads because of higher processing temperatures, longer processing times, higher water and lactose contents, and higher pH. It has been shown that the intensity of the browning reaction can be reduced by using a galactose-fermenting strain of Streptococcus salivarius subsp. thermophilus, together with a mesophilic lactic starter culture, in curd production (39). A high NaCl content had the opposite effect, possibly by suppressing the activity of the lactic acid starter culture. Processed cheese that contains a high level of reworked cheese is exposed to more severe thermal treatment than usual and, consequently, the Maillard reactions are accentuated. Most noticeable are the levels of melanoidins, the main products of Maillard reactions, in sterilized processed cheese, even if cooled immediately after production, packed, and stored at low temperature (30).
Microbial defects in processed cheese are caused by spore-forming bacteria, which usually originate in the cheese milk and enter the process through the natural cheese used for blending. Other sources of microbial contamination include water supply, equipment, and additives (1-5). The normal thermal treatment during blend processing (Tables 1 and 3) does not eliminate viable spores from the product.
The spore-forming bacteria, which cause defects in processed cheese by producing gas, belong to the genera Clostridium (C. butyricum, C. tyrobutyricum, C. histolyticum, C. sporogenes, and C. perfringens) and Bacillus (B. lich-eniformis and B. polymixa). Especially hazardous is C. bot-ulinum, producing a toxin that causes botulism. Whether produced continuously or in a batch cooker, processed cheese is not sterile. Germination of spores after processing is influenced by various factors; for example, blend composition, sodium chloride concentration, type and concentration of emulsifying agent, water level, pH, and the presence or absence of natural inhibitors. However, spore outgrowth can be prevented in a number of possible ways: preservatives in the cheese blend, sterilization of the processed cheese, or an increased redox potential of the blend. One of the most widely used methods of preventing spore outgrowth is the addition of preservative into the blend. There is, for example, a procedure patented in the United States where the outgrowth of C. botulinum spores and subsequent toxin formation is completely prevented in pasteurized processed cheese spread inoculated with 1000 spores per gram and incubated 48 weeks at 30°C, through the incorporation of nisin at the level of 250 ppm in the blend (40,41).
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