Thermisation (also known as thermalisation) is a mild continuous heat treatment, immediately followed by cooling, such that the properties of the raw milk are almost unchanged while its bacterial flora, especially the psychrotrophic flora is considerably reduced.
Thermisation is applied to cheesemilk usually to prolong its storage life prior to pasteurisation and/or cheese manufacture, while having a minimum effect on milk constituents and flavour. Thermisation involves sub-pasteurisation heat treatments which range from 57 to 68 0C with holding times of 15±30 s.
However, treatments of 63±65 0C for 15 s are usual. An upper limit of 65 0C for 20 s for thermisation means that >50% of alkaline phosphatase activity survives. This facilitates the determination of alkaline phosphatase activity as an indicator for subsequent pasteurisation of milk. Cooling and storage of the milk at 4±7 0C after thermisation is important to maintain microbial quality.
Spoilage of raw milk stored at <7 0C is predominantly due to its psychrotrophic microflora, e.g. Pseudomonas spp., most of which are heat labile but which produce heat stable proteinase, lipase and phospholipase enzymes, which are not inactivated by thermisation or pasteurisation and which cause reduction in yield and flavour and texture defects in cheese. Therefore thermisation is used to reduce the bacterial load by minimising growth particularly of psychrotrophic bacteria, and to prolong the keeping quality of raw milk under cold storage for a further 24±72 h after receipt at a processing plant.
Extension of storage time without a deleterious effect on milk quality allows greater flexibility to plant processing schedules. The extension of storage time is dependent on the age and microbial quality of the raw milk prior to thermisation, the temperature and time of the thermisation treatment, avoidance of recontamination after treatment, and maintenance at temperatures of 4±7 0C.
Thermisation inactivates only some pathogenic microorganisms and does not fulfil public health requirements as does pasteurisation. In certain countries, cheeses manufactured from raw or thermised milks are required to be stored at 20C for 60 days to allow pathogens to die. Thermisation may result in germination
of spores (e.g. Bacillus cereus) present in milk during subsequent cold storage, but subsequent pasteurisation of the milk will inactivate these vegetative cells. Thermisation of cheesemilk has little effect on renneting properties during cheese manufacture
Cheeses manufactured with milk that has been thermised rather than pasteurised may develop a more intense flavour profile, possibly because of a lower inactivation of enzymes and non-starter lactic acid bacteria.
As PLATECH; we beliave that thermisation process has quite huge incluence both on the overall milk quality and also that of the final product. Many common defects such as bitterness, eye-formation, sour taste and off-flavour formation can be minimised by placing a thermisation unit prior to cold milk storage tank. The changes on the composition of milk during the entire year due to seasonal changes, animal health issues, feed and etc, is a big matter of unconsistent product quality and production. As the processor, what we need is to minimise these effects and provide the consumers same taste and aroma as well as healthy consumption.
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