Dairy Microbiology PDF

Applied microbiology “ Dairy microbiology“ By Dr. Rasha M. El-Meihy Professor of Agricultural microbiology Milk It is a liquid rich nutrient-food and consists of varying amounts of nutrients secreted by special glands. The major constituents of Milk are water (87.4%) and...

Applied microbiology “ Dairy microbiology“ By Dr. Rasha M. El-Meihy Professor of Agricultural microbiology Milk It is a liquid rich nutrient-food and consists of varying amounts of nutrients secreted by special glands. The major constituents of Milk are water (87.4%) and solids (12.6%). The solids are, 5% lactose, 3% protein (such as casein and albumin), 3.5% fat as well as 1% minerals (like calcium), and some vitamins such as A, E, K, B, C and enzymes such as catalase, lipase and phosphatase. Milk is neutral in effect, with a pH of about 6.7  Sources of milk contamination Milk is sterile at secretion from the The type and number of microbes udder but is contaminated by depend on: bacteria even before it leaves the The animal health. udder. Except in the case of The milking method mastitis, the bacteria at this point (automatic - manual). are harmless and few in number. The farm environment. So, it is recommended to exclude Transportation. the first quantities from the milking Storage conditions. process. Pre-processing activities. Milk is exposed to contamination by The quality of the milk produced many microbes, including bacteria, depends on the conditions of its yeasts and fungi, from the time it is production and the number of milked until it is consumed. microbes it contains.  Tools The most sources of milk contamination  Microorganisms in milk The most important microorganisms in milk are the spoilage and pathogenic microorganisms. Spoilage microorganisms belong to one of four physiological groups of bacteria that produce: lactic acid, propionic acid, butyric acid, degradative enzymes. Some spoilage organisms, such as lactic acid bacteria, can be used to produce fermented dairy foods when they are allowed to grow under controlled conditions. Pathogenic microorganisms in milk can be either infectious or toxin- producing. Types of bacteria in milk (1) Lactic acid bacteria This group of bacteria ferment lactose to lactic acid. They are normally present in milk and can used as starter in the production of some dairy products such as yogurt. Lactococci Lactococcus lactis Lactobacilli Leuconostoc (Streptococcus lactis ) Lactobacillus casei Leuconostoc lactis (2) Coliforms: Coliforms are facultative anaerobes with an optimum growth at 37°C. Coliforms are indicator organisms; they are closely associated with the presence of pathogens but not necessarily pathogenic themselves. Coliforms also can cause rapid spoilage of milk because they are able to ferment lactose with the production of acid and gas and degrade milk proteins. Coliforms are killed by fast pasteurization. Therefore, their presence after milk treatment is indicative of contamination. Escherichia coli is an example belonging to this group. (3) Spoilage Microorganisms in Milk In milk, the microorganisms that are principally involved in spoilage are psychrotrophic organisms. Most psychrotrophs are destroyed by pasteurization, however, some like Pseudomonas fluorescens, Pseudomonas fragi can produce proteolytic and lipolytic extracellular enzymes which are heat stable and capable of causing spoilage. Some species such as Bacillus, Clostridium, Cornebacterium, Lactobacillus, Micrococcus, and Streptococcus can grow at refrigerator and cause spoilage problems. (4) Pathogenic Microorganisms in Milk Hygienic milk production practices, proper handling and storage of milk, and mandatory pasteurization has decreased the threat of milk-borne diseases such as tuberculosis, brucellosis, and typhoid fever. Some foodborne illnesses result from consuming raw milk, dairy products made from improperly pasteurized milk, or milk that has been handled poorly and contaminated after processing. The following bacterial pathogens are still of concern today in raw milk and other dairy products: Bacillus cereus , Listeria monocytogenes , Yersinia sp., Salmonella spp., Escherichia coli O157:H7 , Campylobacter jejuni.  Effect of temperature on milk microbes Microbes present in milk have different thermal needs: Some of which are Psychrophilic, which can grow near zero degrees Celsius, Some are Mesophilic, which prevail at room temperature, Some are Thermophilic, which can grow at temperatures above 65°C, Some are heat-resistant, which remain alive even after pasteurization. It is noted that: At low temperatures, the activity of acid-producing bacteria stops and protein-degrading bacteria becomes active. At medium temperatures, acid-producing bacteria become active and the activity of fat- and protein-degrading bacteria decreases. At high temperatures, i.e. when milk is boiled, acid-producing bacteria die and the bacterial spores remain, which grow, become active and degrade protein.  Methods of preserving milk After milking, milk is immediately cooled to about 4°C as it helps to maintain the quality of milk and extend its shelf life. ❑ Cooling milk reduces the growth rate of microbes, especially mesophilic and psychrotrophic bacteria (minimum temperatures are 20 and 7°C, respectively). Therefore, this temperature must be kept constant during milk transportation and handling. ❑ Cooling can reduce the activity of thermoduric bacteria (e.g., Bacillus cereus and Bacillus circulans) (20-65°C) in raw milk but does not completely eliminate them, which can grow under cooling even if at a slow rate.  It is a process in which packaged milk is treated with mild heat, usually lower than boiling, where 90-99% of the live bacteria are eliminated and extend shelf life. Pasteurization eliminats most of the spoilage microbes and all pathogenic microbes, including Mycobacterium tuberculosis (the causative agent of tuberculosis, TB), which is one of the most severe bacterial pathogenic microbes present in milk. It is heat-resistant, as it does not die unless exposed to a temperature of 61.1℃ for 10 minutes. It considered an appropriate method of Mycobacterium tuberculosis preserving milk because it preserves its nutritional components, especially vitamins and calcium, and does not lead to a significant change in its taste or appearance. There are two types for pasteurization: ❑ Slow pasteurization ❑ Fast pasteurization Low Temperature Holding Method High Temperature Short Time (LTHM) In which the milk is treated at Method (HTST) In which the milk is a temperature of 62.85°C for 30 treated at a temperature of 71.75°C minutes. for 15 seconds.  Phosphatase test ▪ Alkaline phosphatase (ALP) is an enzyme naturally present in all raw milks, which is used as an indicator of proper milk pasteurization. ▪ Complete pasteurization will inactivate the enzyme to below levels which are detectable by conventional methods. Because the heat stability of ALP is greater than that of pathogens which may be present in milk, the enzyme serves as an indicator of product safety. ▪ However, the failure to detect ALP activity does not ensure that the product is pathogen free. ▪ There are both classic and rapid tests for phosphatase. The classic test Principles: Microbes present in milk after pasteurization Thermophilic Thermophilic bacteria can survive the pasteurization process bacteria and may proliferate in pasteurized milk during refrigerated storage and cause spoilage if milk is stored improperly. Examples: Streptococcus thermophilus, Lactobacillus delbrueckii Thermoduric Some thermoduric bacteria endure the heat treatment, utilizing bacteria the milk components like proteins and fats as nutrient sources, and this led to undesirable changes in the milk's flavor and quality over time. Examples: None-spore forming bacteria Streptococcus spp., Enterococcus spp., and Lactobacillus spp. Psycrophilic The presence of these bacteria results in many defects such as bacteria fruity, rancid, or acidic tastes, color defects, and stringy milk defects. These defects do not appear in pasteurized milk until several days later. Examples: Alcaligenes, Pseudomonas and Lactococcus. Spore- Especially those belonging to the genera Bacillus and forming Clostridium. Bacteria Coliform The genera of this group are considered bacteria that can’t group tolerate pasteurization heat, so their presence in pasteurized milk is considered evidence of the inefficiency of the pasteurization process or contamination after it, but they do not have a significant effect on pasteurized milk.