A. Factors Limiting Microbial Growth in Butter
A variety of extrinsic (e.g., temperature) and intrinsic (e.g., salt in the moisture phase) factors combine to control the microflora of butter. Most important among these are (a) fine and uniform dispersion of moisture phase, (b) addition and uniform dispersion of salt, (c) low-temperature storage, and (d) use of lactic cultures (in ripened cream butter) (Hammer and Babel, 1957; Olsen et al., 1988). Microbial growth is proportional to availability of nutrients and related to size of water droplets in butter (Verrips, 1989). Thus, the smaller and more uniform the droplets, the lower the potential for microbial growth. Salt must also be distributed evenly in the moisture phase of the product effectively to inhibit micro-bial growth in contaminated water droplets. The approximate salinity of moisture in butter with 1.5% salt is 9%; this will inhibit growth of many bacteria. However, working may not result in a homogeneous distribution of salt in the water droplets (Milner, 1995; Hammer and Babel, 1957). Data suggest that dispersion of water droplets, salt, and bacteria in butter made by continuous churns may be more uniform than in butter made with batch churns. Aerobic plate counts revealed a steady decrease in microbial contaminants in butter made in continuous churns compared with counts obtained on butter made from batch churns (O'Toole, 1978). Salt-free droplets were found in freshly worked salted butter made with a batch churn (Hammer and Babel, 1957). Technological developments that allow for uniform dispersion of moisture, salt, and bacteria enhance both safety and shelf-life of butter.
Storage of salted butter at freezing temperatures is not adequate to guarantee complete cessation of microbial growth because of the depressed freezing point in the moisture phase of the product resulting from elevated salt content and presence of other dissolved solutes. However, freezing is an effective means of storage for unsalted butter. O'Toole (1978) provided data that suggested that the lowest temperature limit for microbial metabolic activity in salted butter was -9°C. As a result of sensory evaluation, the flavor of butter held at -6°C was marginally less after 12 weeks; however, butter stored 8 weeks at 4 or 10°C dropped about one point in flavor score (O'Toole, 1978).
Some countries allow the use of potassium sorbate and sodium benzoate as preservatives in butter. However, countries such as the United States, United Kingdom, France, and Luxembourg prohibit preservatives in butter. Addition of 0.1% potassium sorbate inhibited growth of coliforms and molds in naturally contaminated butter (Kaul et al., 1979). The inhibitory effect was enhanced when 2% salt was added along with 0.1% potassium sorbate. This inhibition occurred in all samples stored 4 weeks at -18 and 5°C.
Caution should be exercised in selection of any additives blended into butter products for flavor (e.g., honey, garlic, chopped herbs, and fruits), because they may contribute additional enzymes and microflora to the product. For example, unpasteurized honey added to butter will cause hydrolytic rancidity within 2 weeks because of lipase in the honey. Butter colorants that have not been mishandled have rarely contributed to the microflora of cream or butter (Foster et al., 1957).
Any quality assurance program should incorporate maintenance and documentation of good manufacturing practices (GMPs) and hazard analysis critical control points (HACCP).
C. Hazard Analysis Critical Control Points (HACCP)
An obvious critical control point for butter manufacturers is pasteurization or repasteurization of cream received at the manufacturing site. Control of the microflora in the manufacturing environment is also critical. Each plant must evaluate its individual process and develop its own risk assessment and HACCP plan (Smittle, 1992). An environment sampling protocol should be aimed at monitoring for L. monocytogenes, S. aureus, and Salmonella. Recalls of butter because of L. monocytogenes contamination were reported as recently as 1994 (Ryser, 1999). Faust and Gabis (1988) have recommended areas of food plant environments that can be targeted for sampling for pathogens. Discovery of Salmonella or Listeria in the environment requires immediate corrective action with docu mentation of the success of that action. Irbe (1993) has recommended that manufacturers of whipped butter develop in-plant guidelines for aerobic plate count and S. aureus at critical control points of manufacture. Finished products must be free of Salmonella, and L. monocytogenes and should be free of Escherichia coli (Irbe, 1993).
Testing for these organisms can be done to validate success of the manufacturer's HACCP program. All testing of pathogens must be done away from the manufacturing site. Most in-plant laboratories are not equipped with the needed accessories to prevent spread of pathogens to the plant environment. Manufacturers should also test for lipolytic and psychrotrophic spoilage organisms in the finished product and develop a three-class attribute sampling plan (Smittle, 1992). These data can be used to establish goals and measure success based on principles of continuous quality improvement (Crosby, 1984). Sanitation of equipment used to manufacture product should be assessed regularly by testing environmental swabs for selected microbes.
The authors of this chapter recommend that pasteurized cream for butter manufacture has <5000 cfu/g (APC) with <2 coliforms/g. Finished butter should contain <5000 cfu/g (APC), <2 coliforms/g, no staphylococcal entero-toxins, no Salmonella in 375 g, no L. monocytogenes in 25 g, and <10 yeasts and molds/g.
Margarine, like butter, contains approximately 80-81% fat, 15% moisture, 0.6% protein, 0.4% carbohydrate, and 2.5% ash (Irbe, 1993). In margarine, edible fats, oils, or mixtures of these with partially hydrogenated vegetable oils or rendered animal carcass fats are substituted for milkfat (Code of Federal Regulations, 1994). Eighty percent fat in butter and margarine is considered too high by many individuals concerned about their diets (Varnam and Sutherland, 1994). Consequently, numerous spreads have been manufactured with lower fat contents. In many countries, there are no legal standards or definitions for these low-fat spreads. However, a working categorization has been made based on fat content (Varnam and Sutherland, 1994). Full-fat spreads are described as those with fat contents of 72-80%; reduced-fat spreads have 50-60% fat; low-fat spreads have 39-41% fat, and very low-fat spreads have less than 30% fat. Vegetable fats, mixtures of vegetable fat and milkfat, and milkfat alone have been used to develop these spreads (Varnam and Sutherland, 1994). Another trend has been production of spreads in which fat has been replaced in part or completely by a variety of substances such as Neutrifat, Simplesse, and Stellar (Varnam and Sutherland, 1994). Olestra a sucrose polyester with fatty acids, was recently (1996)
approved by the U.S. Food and Drug Administration (FDA) as a substitute for conventional fats and may appear in products in the future.
B. Dairy Spreads: Manufacture and Microbiological Considerations
Low-fat spreads are also water in oil emulsions but contain more moisture than butter. Consequently, there is increased likelihood of microbial growth in these products unless preservatives are added. The use of preservatives is allowed in some countries but not in others. Because of combining ingredients at 45°C, in an emulsifying unit, growth of thermoduric organisms (e.g., Enterococcus fae-cium, E. faecalis) and thermophils may occur. Higher fat dairy spreads are typi-
cally made using a swept-surface heat exchanger and texturizer where the aqueous blend of ingredients is mixed in the correct ratio with oil-soluble ingredients.
Crystallization of fat during working is critical to obtain desired consistency and spreadability in the finished product. Rapid supercooling to -10° to -20°C under high sheer conditions in the scraped surface heat exchanger initiates and maintains crystallization and disperses moisture within the fat matrix (Varnam and Sutherland, 1994). Control of cross contamination during packaging is more critical than in butter manufacture because of the higher potential for microbial growth in spreads.
Microorganisms that cause spoilage in butter have been implicated in margarine spoilage. However, vegetable fats are typically more resistant to lipolytic breakdown than is milkfat (Varnam and Sutherland, 1994). Yarrowia lipolytica, Bacillus polymyxa, and E. faecium are spoilage organisms of concern in low-fat spreads (Varnam and Sutherland, 1994; Lanciotti et al., 1992). Lanciotti et al. (1992) showed that L. monocytogenes and Yersinia enterocolitica can grow in ''light'' butter at 4 and 20°C. A class I recall of 60% butter, 40% margarine product occurred in 1992 (FDA Enforcement Report, 1992). More detailed descriptions of margarines, spreads, and industrial milkfat products can be found in the report by Varnam and Sutherland (1994). An outline of margarine and spread manufacture is shown in Fig. 13.
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