By reducing the microbial load of the milk, heat treatment is applied in order to obtain a product with long shelf life and beneficial to the health of the consumer. There are many types of heat treatment. One of these, UHT, heat treatment at 135–150 ° C for 2-6 sec (Çakmakçı S. et al.).
UHT sterilization method is made directly and indirectly in two ways. In the direct method, either steam injection or steam infusion is performed and sterilization process is performed in 5 seconds at 140C. In indirect method, sterilization is done in 2 seconds at 140C without any contact with plate or tube heat exchangers. The taste and odor produced in the sterilization by direct method gives the closest taste and smell to pasteurized milk (Çağlar A., and Çağlar M.Y.)
Defects in UHT milk:
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High temperature lactose decomposes causing undesirable odor, taste and color (it is not resistant to above 100 ° C).
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Fat globules membrane breaks down and denaturing proteins surrounding membrane,
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Loss of some water-soluble vitamins,
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Decrease in nutritional value of milk by causing damage to high temperature sensitive amino acids,
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Decrease in solubility of serum proteins and prolonged coagulation with casein at isoelectric point,
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SH groups are exposed to cause cooked taste,
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Reduces solubility of calcium and phosphate in water phase,
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Maillard reaction. Maillard reaction causes the loss of lysine and arginine, one of the basic amino acids, the nutritional value of milk decreases and also creates an undesired cooked taste in the taste and smell,
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When packaging is not done well, oxygen enters from the packaging, UHT milk contains more than soluble oxygen, causing vitamin losses,
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Leaving too much of the air gap during milk filling causes microbial contamination and causes many losses such as vitamin losses,
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The color browning due to storage for a long time,
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In direct UHT sterilization, if the milk is first homogenized and then sterilized, it will cause gelation in the milk.
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Low homogenization pressure causes gelling,
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Lipase and protease enzymes remain after sterilization when using raw milk with psychrotroph bacteria number more than 106 cfu / ml. The protease enzyme causes gelation in milk,
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In UHT milk stored at high temperature, SH groups decrease within two weeks and stale occurs. Accelerating unwanted reactions such as high temperature caramelization, gelling and browning,
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In the absence of excess air in the milk after sterilization, fat oxidation occurs and consequently the formation of ransid taste.
Causes of these defects and ways of prevention:
-The temperature process should be completed in a shorter time instead of keeping it at very high temperature for a long time,
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Storage should be done at low temperature. For example, in UHT milk stored in 5-6C, SH groups decrease in longer time so that byte time is prolonged,
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Be very careful about packaging. The soluble oxygen content of the milk should not be too high. For this; the joints of the packaging should be glued well, the packaging with aluminum foil layer should be used, and the packaging material should be airtight.
-The head space should not be left too much while filling the boxes,
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Direct sterilization systems can be used instead of indirect since the taste and aroma in the direct system is closer to the pasteurized milk.
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In order to prevent oil globules from aggregating after pecting, the milk should be sterilized first and then homogenized in direct sterilization.
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Homogenization can be done twice to prevent oil separation (Chenk and Gelda, 1975),
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Raw milk should not be stored at 5 ° C for more than 3 days to prevent gelation, and UHT milk should be stored at low temperature,
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Substances such as potassium iodate, L-cystine and immobilized sulfhydryl oxidase may be added to reduce the cooked taste,
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In order to prevent brown color in the milk, direct sterilization should be performed instead of indirect sterilization. Generally, the color of the milk sterilized by direct method is whiter than the indirect sterilized milk.
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Sedimentation rate is significantly reduced by adding substances such as citrate, bicarbate and hydrogen phosphate to prevent sedimentation. The amount of sedimentation in direct sterilization is higher than the index sterilization.
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After sterilization, the air in the milk should be removed so that no oil oxidation occurs and a rasid taste occurs.
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Homogenization should be done at high pressure to prevent the formation of the seed, storage time should not be long, storage temperature should be low, raw milk quality should be high, the number of psychrotroph bacteria should not be more than 106 cfu / ml, psychrorof bacteria are transmitted to raw milk after milking because they do not exist in natural flora. In order to produce high quality and perfect UHT milk, first of all milking equipment sanitation should be done very well, hygiene rules should be followed in milk transportation and storage, and milk producers should be informed for all of them.
RESULT
We have seen that some defects and negativity may be encountered in UHT milk sterilization when the application conditions are not observed. However, the milk produced by the direct UHT sterilization method has the same taste-odor and nutritional value as the pasteurized milk and has a shelf life of 4-6 months. This is the more preferred milk in terms of both consumer health and producer.
RESOURCES:
-Çakmakçı S., ve ark., Süt Bileşenleri Üzerine Isıl İşlemin Etkileri ve Besin Değeri Konusunda Değerlendirmeler,2012, Tarım Bilimleri Araştırma Dergisi 5 (1): 162-165, 2012
- Çağlar A., ve Çağlar M.Y., Süt ve Süt Ürünleri, 2013, Afyon Kocatepe Üniversitesi, Mühendislik Fakültesi, Afyon, 2013
-Akan E., ve ark., Psikrotrof Bakterilerin Çiğ Süt ve Süt Ürünleri Kalitesine Etkisi, 2014, Akademik Gıda 12(4) (2014) 68-78
-Konar A., ve Şahan N., UHT Süt Üretim Tekniği ve Araştırılan Bazı Sorunlar, 1989, Ç.Ü., Ziraat Fakültesi TÜT Bölümü, Adana, Gıda(1989)14(5) 289-293
-Rolls B.A., and Porter J.W., Some effects of processing and storage on the nutritive value of milk and milk products, 1973, National Institute for Research in Dairying *,* Proc Nutr Soc(1973)32,9
