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Effect of Freezing Processing on Milk Properties
Author: admin Date: 2018-12-06 Views: 75

Effect of Freezing Processing on Milk Properties

冷冻对蛋白质的影响 (I) Effect of freezing on protein

The frozen processing of cow's milk mainly refers to the processing methods of freezing sublimation drying and freezing preservation. -5 温度下保存 5 周以上或在 -10 的温度下保存 10 周以上,解冻后酪蛋白产生凝固沉淀。 When the milk is stored frozen, if stored at -5 ° C for more than 5 weeks or at -10 ° C for more than 10 weeks, casein will coagulate and precipitate after thawing. At this time, the instability of casein is mainly affected by the concentration of salt in the milk (especially colloidal calcium), the crystallization of lactose, the heating and thawing speed of the milk before freezing. Insoluble casein in which calcium and phosphorus are almost the same as before freezing. Therefore, it can be considered that the casein colloid changed from its original state to an insoluble state.

The protein in milk becomes unstable during freezing. In the initial stage of freezing, after the milk is melted, a fragile feather-like precipitate appears, and its composition is calcium caseinate. This precipitate is easily dispersed by mechanical stirring or heating. As the instability deepens, a precipitate is formed that no longer disperses after mechanical stirring or heating.

The stability of casein colloidal solution in milk is closely related to the calcium content. The higher the calcium content, the worse the stability. 2% )或四磷酸钠,或其他和钙有螯合作用的物质。 In order to improve the stability of dairy eggs when the milk is frozen, part of the calcium in the milk can be removed, and sodium metaphosphate ( 2% ) or sodium tetraphosphate, or other substances that chelate with calcium can be added .

Protein instability during cryopreservation is also closely related to lactose. When frozen milk is frozen, the effect of lactose crystals can promote protein instability, and the addition of sucrose can increase the stability of casein complex. Among the sugars, the effect of sucrose is the best. This effect is due to the increase in viscosity that affects the drop in freezing point and at the same time prevents the lactose from crystallizing.

Freeze-sublimation drying is often used in the processing of colostrum products and casein phosphopeptides, which need to be frozen in advance during processing. This requires a thin-layer quick-freezing method, which can completely avoid the instability of casein.

冷冻对脂肪的影响 (B) the impact of freezing on fat

When the milk is frozen, due to the change in the structure of the fat globule membrane, the fat emulsification is unstable, so that the emulsification ability is lost, and the fat masses of different sizes float on the surface. When the milk is frozen in a stationary state, the upper layer has a high fat concentration due to the rising of the cream, so the milk can be seen as a thick and light layer. 25%~30% 的稀奶油,由于脂肪浓度高,黏度也高,脂肪球分布均匀,因此,各层之间没有差别。 However , the cream with a fat content of 25% to 30% has a high fat concentration, a high viscosity, and a uniform distribution of fat balls, so there is no difference between the layers. 1 μ m 以下,同时黏度也稍有增加,脂肪不容易上浮。 In addition, after homogenizing the milk, the diameter of the fat globules is below 1 μm , and at the same time, the viscosity is slightly increased, and the fat is not easy to float.

Freezing breaks down the emulsified state of milk fat. The process is first to produce ice crystals due to freezing. When these fragments are assembled into large pieces, the fat balls are pressed by the mechanical action of ice crystals and collided to form triangles. In addition, as the fat globule film loses moisture as it thaws, its physical properties change and it loses its elasticity. In addition, due to the formation of crystals in the fat ring, squeezing occurs, and the liquid film is released from the fat to destroy the spherical membrane, so the emulsified state is also destroyed. 60 22.54~24.50MPa )。 There are many ways to prevent the instability of the emulsified state. The best method is to perform homogenization before freezing ( 60 , 22.54 ~ 24.50MPa ).

Co-occurrence of bad flavors and changes in bacteria

Frozen milk is often oxidized, metallic and fishy. This is mainly due to the mixing of metal ions during the treatment, which promotes the oxidation of unsaturated fatty acids and generates unsaturated hydroxyl compounds. When this happens, antioxidants can be added to prevent this.

When the milk was stored frozen, the bacteria hardly increased, similar to that before freezing.

name

Number of bacteria / ( CFU / mL )

name

CFU/mL Number of bacteria / ( CFU / mL )

个月后 6 months before freezing

个月后 6 months before freezing

Antiseptic milk

3600 1500

Sterilized, homogenized milk

200 400

 

name

Number of bacteria / (number / mL )

name

(个 /mL Number of bacteria / (a / mL )

后( 4.4 ℃) 6 个月后 Immediately after melting for 24 hours ( 4.4 ℃) After 6 months

后( 4.4 6 个月后 Immediately after melting for 24 hours ( 4.4 ) After 6 months

Antiseptic milk

1200 1200 8000

Sterilized, homogenized milk

400 400 450

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