Effects of Pasteurization on the Microorganisms in Raw Milk

Bacterial spoilage is one of the major factors in extending the shelf life of conventional pasteurized milk. Microbial growth and metabolism have subsequently shorten the shelf life of milk by producing undesirable changes in the aroma and taste attributes that influence consumer acceptability of the products (Frommand and Boor, 2004). The main bacteria of raw milk are (Lactococcus lactis, Staphylococcus, E. coli& psychrotrophic bacteria) (Cronjé, 2003). The pathogens of the raw milk were almost exterminated after pasteurization. However, some heat resistant bacteria still remained in the milk medium. The thermoduric bacteria are organisms capable of surviving the industrial pasteurization processing, and can be transferred into products causing quality defects, or creating health hazards. When pathogenic bacteria are relatively low in raw milk (less than 500 cfu/mL), some bacterial species did not only survive pasteurization, but grew in very large numbers during the food manufacturing processes. The heat resistant bacteria included (lactic acid

micro coli, streptococcus thermophilus) and heat resistant micro (aureus & endorspores of bacillus). The survival of these bacteria‟s caused huge

problems for food manufacturers. Much time and efforts have been expended on the studying methods for controlling the large numbers of these bacteria in milk and milk products.

Many researches were focused on the effect of heat treatment condition to the resistance of bacteria. (Cronjé 2003) isolated and identified microbes in pasteurized and in double -pasteurized milk. The “milk isolates” included strains of (Acinetobacter sp., Candida lipolytica, Chryseobacterium

meningosepticum, Pseudomonas putida) and four isolates which is related to

the (Bacillus cereus) group. The presence of these microorganisms in pasteurized milk can cause spoilage before the expiration date of the product .Their survival of the pasteurization is determined not only by their survival ability, but the pasteurization conditions. (Dumalisile et al. 2005) has investigated the different pasteurization conditions to the survival ability of these bacteria. It reported that different pasteurization methods (LTLT, HTST and pot pasteurization) placed different impacts on the sterilization of these milk bacteria. The research indicated that Bacterial strains of (E. coli, A.

baumannii, B. cereus, Chr. meningosepticum, P. putida,) yeast (Can. Lipolytica) and a reference strain (B. coagulans) were pasteurized by different

pasteurization methods. Only the (B. cereus) strain could survive pasteurization in the LTLT and the HTST pasteurization treatments, whereas the other bacterial and yeast strains did not survive. By contrast, the same bacterial strains when treated with the „pot‟ pasteurizer survived pasteurization, with the exception of the yeast. In short, different pasteurization methods showed different efficiency for the elimination of microorganisms. Furthermore, the microbiological quality of the raw milk before processing would place an impact on the final milk quality after pasteurization. Thus, there are different pasteurization standards for different dairy products, which depend on the bacteria quality of raw milk, fat content and the intended usage. (e.g.), the pasteurization standards for cream differs from the standards for fluid milk and the standards for pasteurizing cheese are designed to preserve the phosphatase, which aids in cutting. The HTST pasteurization standard was designed to achieve a 5-log reduction, killing 99.999% of the number of viable micro-organisms in milk. This is considered adequate for destroying almost all yeasts, mold and common spoilage bacteria and to also ensure adequate extermination of common pathogenic heat- resistant organisms (including Mycobacterium tuberculosis, which causes tuberculosis but not Coxiella burnetii, which causes Q-fever). HTST pasteurization processes must be designed appropriately so that the milk is heated evenly, and no part of the milk is subject to a shorter time or a lower temperature. (Champagne et al. 1994) has reviewed the growth and activity of psychrotrophs in milk. The psychrotrophic bacteria in milk do not cause the serious problems related to the spoiling of the milk (Čanigová, et al., 2002). It is well known that, Gram-negative bacteria, such as the (Pseudomonas,

Moraxella, Flavobacterium, Acinetobacter, & Alcaligenes) predominate over

Gram-positive bacteria in causing spoilage of pasteurized milks. These bacteria‟s are part of the micro flora of raw milk that resides in the dairy plant and contaminate the milk after it has been pasteurized because these Gram- negative bacteria are sensitive to heat and would be killed by normal pasteurization (Meer et al., 1991). In Canada, all milk produced at a processor and intended for consumption must be pasteurized, legally requiring it to be heated to at least 72 o C for at least 16 s and then cooling it to 4 o C. This ensures the elimination of any harmful bacteria and the re growth of bacteria in the shelf life of milk. There are different temperatures for the pasteurization, but the shelf life of the milk will not be influenced by the process temperatures. (Gandy et al. 2008) have investigated the effect of pasteurization

temperature on the shelf life of fluid milk. They found that varying pasteurization temperature had no effect on shelf-life. They also found that the milk could not be differentiated based on pasteurization temperature by a trained sensory descriptive panel or volatile compound composition toward the end of shelf-life. In addition, the shelf life of pasteurized milk was not only influenced by the pasteurization conditions but was affected by the packaging materials, due to post-pasteurization contamination which placed great impacts on the shelf life of milk. Meantime, (Petrus et al. 2010) have focused their research on the microbiological shelf life of pasteurized milk in bottle and pouch. They determined the Q10 and Z-value and presented that storage

temperature has a greater effect on microbiological shelf life of pasteurized milk packaged in LDPE pouch compared to HDPE bottle. Thus, the HDPE bottles were preferred for its superior performance over the LDPE pouch with regard to microbial growth at storage temperatures ranging from 2 - 16 o C .In short, the factors limiting milk stability are well established: bacterial contamination, inadequate packaging system and improper temperature control. (Cromie 1991) reported the factors that influence the shelf life of pasteurized milk include the quality of the raw material, the binomial temperature/time pasteurization, resistant microorganisms to pasteurization (particularly psycrotrophics), the presence and activity of post pasteurization contaminants, the packaging system and storage temperature post pasteurization which had the greatest impacts on the stability of the product. (Griffiths & Phillips, 1990) reported that the one of the most critical factors lowering the durability of pasteurized milk products is the storage temperature of raw milk. (Burdova‟s) research indicated that storage temperature of 10 o C reduces the shelf life of pasteurized milk to one third in comparison with storage at 4.0 o C. The average shelf life of the full cream pasteurized milk reached 31 d at 4 o C; the average shelf life of skimmed pasteurized milk was 32.57 d.

Besides, (Douglas, 2000) have also published the result that the final microbial numbers were significantly influenced by the processing plant. (Fromm & Boor 2004) have also obtained the characterization method of pasteurized milk shelf life attributes. The Gram-positive organisms can be present in raw milk, but they also may enter milk products at various points during production and processing. They showed that the variability observed among plants suggests that plant-specific strategies will be needed to identify and reduce or eliminate sources of contamination. Development of these strategies might be achieved through systematic sampling of the dairy plant environment, including areas such as milk contact surfaces; equipment‟s,

floors, and drains. Environmental sampling in place would facilitate to identify bacterial reservoirs, which must be targeted to reduce contamination at identified entry points and contribute to extended shelf life in fluid milk products. In other words, the control of the post pasteurization contaminants is as important as the pasteurization process on the microorganism quality of milk. In conclusion, the shelf life of pasteurized milk was affected by many aspects: the quality of the raw material, the binomial temperature/time pasteurization, resistant microorganisms to pasteurization, the presence and activity of post pasteurization contaminants, the packaging system and storage temperature post pasteurization. Currently, bacterial spoilage is still the most limiting factor in extending the shelf life of conventionally pasteurized high- temperature short-time (HTST) processed fluid milk products beyond 14 d (Boor 2001). However, the pasteurization still played an important role in the fluid milk processing, which provided adequate extermination of bacteria and offered full safety for human consumption.