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When milk quality goes wrong.

Poor milk quality will cause increased fouling in heat exchangers, reducing production time. It also makes it harder to clean, and it must be done more often during prolonged periods. Poor quality can also affect the UHT product shelf life, resulting in fat separation, sedimentation, gelation, off flavours and smell.

Handling and processing at the plant

If milk cannot be cooled quickly and stored at refrigeration temperatures until processed, the microorganisms in milk will start growing rapidly. Even if milk is cooled to refrigerator temperature, bacterial contamination on the farm should be avoided. Some microorganisms can even grow at refrigeration temperatures, forming heat resistant enzymes that may challenge the quality and shelf life of the filled product.

Tetra Pak recommends the thorough resting of incoming milk before beginning UHT processing. Some milk may not be appropriate for UHT, but for other uses such as dairy food.

The fastest-growing microorganisms are bacteria that feed on milk sugar (lactose) and ferment the sugar to lactic acid. Milk has a good buffering system and can handle some acidification without causing quality problems. If a lot of lactic acid forms, the acidity of the milk increases and it will reduce the milks ability to be heat treated, especially at UHT temperatures. The low heat stability can cause deposits in the heat exchanger, shortening production time. When pH changes from 6.70 to 6.55, it can render processing the milk at UHT temperatures impossible since all the milk might precipitate in the heat exchanger or in the package.

There are a group of microorganisms, Gram-negative psychotrophs that grow at refrigerated temperatures, forming heat-resistant enzymes. These are substances which can attack fat or proteins in milk, changing the taste and stability of the UHT product during storage. These are produced and released by the bacteria into the milk and are extremely heat-stable, surviving heat treatments that kill all other microorganisms. After a few months of storage, the activities of the enzymes will cause either a bitter or rancid taste, fat separation, sediment in the package or gelation of the UHT product. The higher the number of Gram-negative psychrotopic bacteria in the raw milk, and the longer they have been present in milk, the shorter the shelf life of the product.

How to measure bad bacteria

Bacteria are counted in terms of colony forming units, or CFUs per millilitre. Top quality cows milk will have less than 100000 cfu/ml and a pH of 6.7 to 6.9. Problems in producing UHT milk with a long shelf life start when the bacterial count reaches one million cfu/ml. If the milk contains more than five million cfu/ml, even if the pH is within the normal range, there is a large risk that the milk contains too high numbers of bacteria, producing heat-resistant enzymes, making the milk unsuitable for UHT production, due to the risk of shorter shelf life.

Lactic acid bacteria are present on the skin of the cow, on bedding, material and feed. They do not grow at low temperature and do not survive the pasteurisation of milk, or produce heat-stable toxins or enzymes. Selected strains of lactic acid bacteria are added to milk on purpose to produce fermented milk products and cheese. This type of bacteria will ferment lactose to lactic acid. This will cause acidification of the milk and a decrease in pH value. Even a small change in pH (0.1 to 0.2 pH units) can cause problems with fouling in the heat exchanger and shortened production time. A larger drop in pH will make the milk impossible to process at UHT temperatures since the milk can coagulate during the heat treatment or in the package.

Gram-negative psychrotrophic bacteria can grow at refrigerator temperatures, but optimally at a higher temperature. These microorganisms are primarily present in dirty water. Since the bacteria can grow at low temperature, it contributes heavily to the total count in refrigerated raw milk. The longer the milk is stored at low temperature, the larger the fraction of psychrotrophic bacteria will be. Gram-negative psychrotrophic bacteria may attach to the surface of milking equipment and form very thin layers, called bio films. The presence of bio films elevates the number of these bacteria in the raw material, as they probably entered the raw material during milking. Most of them do not ferment lactose to lactic acid, and thus do not lower the pH of the milk. They do not survive the pasteurisation process but they can, however, produce heat-stable enzymes that are not inactivated in UHT processes, causing off-flavours, fat separation and early gelation formation. To minimise the problem with psychrotrophic bacteria, it is important to have a good cleaning routine for milking equipment and storage tanks/containers, as well as the use of clean water. In order to maintain good raw milk quality, keep the milk properly refrigerated but limit the time at refrigerated temperature before processing at the dairy

Spore forming bacteria

Some microorganisms, when placed in what should be inhospitable environments, can gather the most important parts of their cell into a nucleus or spore and in doing so survive in exceptionally uninhabitable conditions for extremely long periods of time. Spores are very heat-resistant and their count and resistance are critical to choosing the right UHT conditions in which to eliminate them. To avoid a high numbers of spores in the milk, it is important to have a good teat cleaning routine at milking. For anaerobic spores spores that grow in the product without the presence of oxygen the quality of feed such as silage is the most important factor for the amount of spores in the milk. There are also some psychrotrophic Gram-positive spore forming bacteria that survive pasteurisation and can limit the shelf life of pasteurised milk. But none of these will survive UHT processing conditions. The bacteria are mainly present in soil.

A thermoduric bacterium is part of the skin flora of the cow and can also be found in bedding material and feed. They may cause bio films in milking equipment. Thermoduric bacteria do not influence the total CFU count very much, but since they survive pasteurisation it can influence the count after pasteurisation and are important for the total count in milk powder. Some thermoduric bacteria are psychrotrophic, which means they can survive pasteurisation and grow in pasteurised milk. They can also form bio films in the processing equipment if cleaning is not adequate.

Pathogenic bacteria may cause illness in humans. All known pathogenic bacteria are killed at pasteurisation temperature, but some can produce toxins that are extremely heat-resistant and that are not inactivated in UHT processes. Most pathogens cannot grow at low temperature. An exception is Listeria monocytogenes , but this bacterium grows at a slower rate than psychrotrophic Gram-negatives due to competition. Some human pathogens may also cause mastitis. To minimise the risk of pathogenic bacteria in milk, the cows should be as clean as possible at milking and the milking equipment and storage tank/containers should be cleaned properly. The milk should be kept at low temperature until processed.

Mastitis bacteria cause udder infections in cows, leading to an increase of somatic cells or white blood cells in the milk. The somatic cells ally themselves with heat-resistant enzymes in the milk called plasmin, which can degrade proteins in a similar way to bacterial enzymes. This can cause bitterness, gelation and sedimentation of UHT milk. Milk from cows with mastitis also has an altered composition of proteins and salts. More whey proteins and higher salt concentration make the milk more prone to fouling. Some of the bacteria causing mastitis can also be human pathogens and form heat-resistant toxins, for example Staphylococcus aureus . The maximum amounts of somatic cells allowed are: 400 000 cells/ml in the EU and 750 000 cells/ml in the US.

Yeast and moulds can be present in raw milk. They grow slowly at low temperatures and relevant species are inactivated by pasteurisation at 72C/15s. Presence of mould toxins in milk is a feed-related problem. Aflatoxin B1 is a liver toxic and carcinogenic mould toxin that can be present in high amounts in feed. The cow metabolises the aflatoxin B1 into aflatoxin M1 and secretes it into the milk. Aflatoxin M1 is also liver toxic and carcinogenic and many countries have legal limits for the presence of aflatoxin M1 in milk. Aflatoxin M1 is not inactivated by UHT processing conditions.

Source: The role of raw milk quality in UHT production by Tetra Pak

Milk is a highly complex biological system, containing microorganisms, fats, sugars, proteins, minerals, vitamins, and other elements. FOOD REVIEW looks at how composition and microbiological quality determine the milks suitability for processing and human consumption, and how types of microorganisms present and the temperature conditions, will determine the final quality of milk.

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Publication:South African Food Review
Date:May 11, 2015
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