milk science (assignment)

Summarize of Journal “Microbiological Quality of Heat-Treated Milk during Storage”

Milk is very easily contaminated by microbes, this occur when the time of milking and processing which making a relatively short shelf life of milk. This is the responsibility in each side of producers, processors, retailers, and consumers to extend the milk shelf life from hours to months and even years to keep the quality when distributed. So, needed an effort to make it happen by ultra pasteurization process, but the consumer do not like the flavors and would prefer high temperature short time treated milk. In addition, the treatment in post-milking such as cold storage after milk collection is able to minimalize the psychrotrophic bacterial that can make the proteases and lipases which contributes to the milk spoilage.

       Then there are several pathogens that involved within milk consumption such as Salmonella, Listeria, Monocytogens, Campylobacter, Staphylococcus aureus, and Bacillus cereus. All of those bacteria affecting the milk shelf life, so needed a heat treatment. The conditions of heat treatment used for pasteurization depend on the final product, lower temperatures are used for refrigerated product and higher temperatures are used for products stored at room temperature. The main objective of milk heat treatment is to eliminate inborn pathogenic organism or reduce them to increase the milk safety for consumer during extended-shelf life.

         In this research, used raw milk samples were obtained from Khartoum University farm and two private farms, one in Omdurman and the second in Khartoum North. Milk samples were heat treated in glass containers, using temperature adjusted water bath. Heat treatment using combination between 85°C for 40 minutes (Low Pasteurization=LP) and 98°C for 1.87 minutes (High Pasteurization=HP). Milk stored at 4°C and microbiological examination begun at day 1, 10, 20, and 30.  Milk samples were microbiologically examined for total bacteria count and lactic acid bacteria count.

     Then the result from this research shows that heat treatment had significant effect in reducing the bacterial load. In the early storage there is no significant difference, but in late storage there is a significant difference toward bacterial load. After drying process such as pasteurization and sterilization indeed the several microbial activity are still hibernates and begin activated at few time. The heat sterilization of milk is essential to ensure total microbial safety and stability of enzymic activity. Standard pasteurization is effective for the destruction of pathogens in raw milk, however microbial population counted significant beyond day 20 indicating that injury and recovery time to growth again. In other hand also revealed that changes in total bacterial count in milk were not observed before 7-14 days of storage at 4°C but significant change was observed on day 21 of storage at 4°C.

          This is followed by lower of pH or higher acidity when the end of storage, because increase the lactic acid producing bacteria especially Streptococci and Lactobacilli which ferment lactose into lactic acid. The antimicrobial effect of lactic acid bacteria was mainly due to their lactic acid production, causing the pH of the growth environment to decrease. The other bacteria that can be found in heat treated milk during storage such as Bacillus, Staphylococcus, Micrococcus, Enterobacter, Pseudomonas, Streptococcus, Pediococcus, and Lactobacillus. Thus bacteria can control their activity and affect the quality of post pasteurized products. The Micrococci are generally present in the greatest proportion followed by Streptococci and rods. Psychrothropic bacteria from numerous genera in milk such as Pseudomonas, Bacillus, Micrococcus, and Lactobacillus.

           This is indicated that the drying process such as pasteurization does not destroy all of pathogen microorganism, but only reduce the number of them in appropriate level for consumer health. Finally, this research conclude that the heat treatment of milk did not decrease toward the number of total bacteria as well as lactic acid bacteria, but at few time later the count is increased, while the pathogenic bacteria that still found indicating the condition during production, processing, and storage is unhygienic.

What kind of microbe influences the milk condition?

Based on the sources, microbial contamination can be divided into:

Microbial Contamination from within the Udder: Raw milk as it leaves the udder of healthy cows normally contains very low numbers of microorganisms and generally will contain less than 1000 total bacteria per ml. In healthy cows, the teat cistern, teat canal, and the teat apex may be colonized by a variety of microorganisms though microbial contamination from within the udder of healthy animals is not considered to contribute significantly to the total numbers of microorganisms in the bulk milk, nor to the potential increase in bacterial numbers during refrigerated storage. The influence of mastitis on the total bacteria count of bulk milk depends on the strain of infecting microorganism(s), the stage of infection, and the percentage of the herd infected. Mastitis organisms found to most often influence the total bulk milk count are Streptococcus spp., most notably S. agalactiae and S. uberis.

Microbial Contamination from the Exterior of the Udder: The exterior of the cows udder and teats can contribute microorganisms that are naturally associated with the skin of the animal as well as microorganisms that are derived from the environment in which the cow is housed and milked. Of more importance is the contribution of microorganisms from teats soiled with manure, mud, feeds or bedding. Organisms associated with bedding materials that contaminate the surface of teats and udders include streptococci, staphylococci, spore-formers, coliforms, and other Gram-negative bacteria. Both thermoduric (bacteria that survive pasteurization) and psychrotrophic (bacteria that grow under refrigeration) strains of bacteria are commonly found on teat surfaces.

            Influence of Equipment Cleaning and Sanitizing Procedures: The degree of cleanliness of the milking system probably influences the total bulk milk bacteria count as much, if not more than any other factor. Milk residue left on equipment contact surfaces supports the growth of a variety of microorganisms. Less efficient cleaning, using lower temperatures and/or the absence of sanitizers tends to select for the faster growing, less resistant organisms, principally Gram-negative rods (coliforms and pseudomonas) and lactic streptococci. Psychrotrophic bacteria tend to be present in higher count milk and are often associated with occasional neglect of proper cleaning or sanitizing procedures and/or poorly cleaned refrigerated bulk tanks.

  Milk Storage Temperature and Time: Refrigeration storage, while preventing the growth of non-psychrotroph bacteria, will select for psychrotrophic microorganisms that enter the milk from soiled cows, dirty equipment and the environment. Under conditions of poor cooling with temperatures greater than 7.2°C (45°F), bacteria other than psychrotrophs are able to grow rapidly and can become predominant in raw milk. Streptococci have historically been associated with poor cooling of milk, appearing as pairs or chains of cocci (spherical bacteria) on microscopic examination of milk smears. These bacteria will increase the acidity of milk. Certain strains are also responsible for a “malty defect” that is easily detected by its distinct odor.

REFERENCE

Hassan, N.B.A., Mohamed, O.M.A. and Abdel, A.A.M.N. 2009. Microbiological Quality of Heat-Treated During Storage. Pakistan Journal of Nutrition 8(12): 1845-1848.