Effects of different temperature delivery by centrifugal pumps and screw pumps on cream quality

Usually, the cream derived from fresh milk is separated from hot milk and directly sterilized using a cream sterilizer. However, due to the capacity matching problem of dairy factories, the separated cream in actual production is cooled and sent to the raw material first. The cream vat is temporarily stored until the sterilizer is ready and then pumped to the cream sterilizer for sterilization. When producing cream, shear has a significant impact on its texture, whipping properties and flavour.

Dairy production typically uses centrifugal, rotary and screw pumps.

The centrifugal pump rotates through the impeller, throwing the inhaled material to the outer edge of the impeller accelerating it, and transporting it out through the pump pipeline. This transport method has high transport efficiency, but it produces a large shear force on the material.

The rotor pump is a form of positive displacement pump. The rotating rotor creates a low pressure on one side, allowing liquid to be continuously sucked in and discharged on the other side. It is suitable for transporting small-flow viscous liquids and harms the material. The shear force is low.

The screw pump rotates through the pump shaft, and the spiral blades push the material out step by step until the outlet. The flow rate is large but the lift is low, the shear force is small, and particles can even be transported.

Chen Zhijie of the Dairy Research Institute of Guangming Dairy Co., Ltd. uses different pumps to transport ultra-high temperature (UHT) sterilized cream at different temperatures. The viscosity, acidity, pH value, whipping properties, hardness, particle size, fatty acid composition microscopic influence of structure, etc. provide a theoretical reference for factories to improve the production process of cream derived from natural milk fat.

Cream pumping pre-treatment

The cream is fed into the tubular UHT sterilizer at 10 and 60°C, respectively, and centrifugal pumps and screw pumps are used to transport the cream. The UHT sterilization parameters are 120°C and 4 s. The sterilized cream is filled aseptically on a clean bench. The processed samples were marked as follows: 10 ℃ using centrifugal pump (10L group), 10 ℃ using screw pump (10R group), 60 ℃ using centrifugal pump (60L group) and 60 ℃ using screw pump (60R group), in order The sample before pumping was the control group.

1. Cream shows viscosity, acidity and pH value

Observing the appearance of each group of cream obtained after sterilization under different experimental conditions, there is precipitated fat on the upper layer of the 10L group of cream. This may be because the crystallized fat in the cream is transported by a centrifugal pump at 10°C. Too high a shear force cannot maintain a stable emulsion system, causing the fat globules to partially aggregate and float. There is no obvious difference in the appearance of the other groups. It can be seen from Table 1 that the cream is an oil-in-water emulsion before being whipped, and its apparent viscosity is low. The cream in the 10L group is significantly lower. This may be because the fat part accumulates and floats, resulting in a significant reduction in viscosity. The difference in pump delivery and temperature had little effect on the acidity and pH value of the cream, and there were no significant changes in the 4 groups. This shows that after being transported by the pump, the floating of fat will not cause its acidity to decrease, nor will it cause a large amount of fat to be decomposed to produce fatty acids, and the content of titratable acid and total acid will change very little.

Table 1: Viscosity, acidity and pH value of whipped cream pumped by centrifugal and screw pumps at different temperatures

Group Viscosity/(mPa▪s) Acidity/°T pH
10L 31.7±1.7 13.3±0.5 6.73±0.01
10R 58.7±3.4 12.8±0.5 6.77±0.01
60L 61.0±5.7 13.3±0.5 6.74±0.00
60R 61.0±6.5 13.1±0.5 6.78±0.02

 

2. Cream particle size analysis

It can be seen from Figure 1 and Table 2 that the particle size of cream is concentrated in the range of 1 to 8 μm. After pumping sterilization, the particle size distribution of the cream in the 60L group is the most concentrated, followed by the 10R and 60R groups, while the particle size distribution of the cream in the 10L group is the most dispersed, which is also consistent with the particle size span results. According to Dv (10), Dv (50), Dv (90) and D[4,3], the particle size of the cream in the 10L group is significantly larger than that of other groups, and D[4,3] is about 7 μm, indicating that at low temperatures After the cream is transported by a centrifugal pump, it cannot withstand excessive shear force and fat coalescence occurs. The interface film between the crystallized fat droplets at low temperatures is broken, resulting in partial coalescence of fat and uneven particle shape. , uneven size. The particle size distribution of the cream in the 60L group is the most concentrated, with D[4,3] being only 3.562 μm and the smallest span. This means that using a centrifugal pump to deliver cream at 60°C not only has no negative impact on the particle size but also makes the cream more homogeneous, probably because at high temperatures the cream fluid is more resistant to shear, unlike the fat coalescence that occurs at lower temperatures.

cream pumped by screw pump, cream transport

Fig.1 Particle size volume distribution and volume accumulation of whipped cream pumped by centrifugal and screw pumps at different temperatures

 

Table 2: Particle size of whipped cream pumped by centrifugal and screw pumps at different temperatures

Group Dv(10)/ μm Dv(50)/ μm Dv(90)/ μm D[4,3]/ μm Particle diameter span
Comparison group 1.399±0.008 3.292±0.008 6.050±0.019 3.540±0.010 1.412±0.006
10L 2.204±0.007 3.943±0.039 7.501±0.271 6.998±1.963 1.343±0.054
10R 2.051±0.023 3.519±0.007 6.067±0.017 3.814±0.009 1.141±0.012
60L 2.135±0.002 3.346±0.009 5.232±0.025 3.562±0.050 0.925±0.004
60R 2.034±0.021 3.480±0.021 5.917±0.137 3.751±0.045 1.116±0.039

 

3. Cream whipping rate and hardness after whipping

When the cream is whipped, air enters the cream to form bubbles, which are then broken into small bubbles and gradually form a stable network connection structure with the fat globules. It can be seen from Figure 2 that the cream of the 10L group cannot be whipped no matter how long it is whipped. The whipping rate of the cream in group 10R is the lowest, but the hardness of the whipped cream is the highest. Comparing the creams of the 10R and 60R groups, although there is no significant difference in particle size between the two, the higher whipping rate of the 60R group shows that the fat globules in the cream are more likely to connect and wrap into bubbles, forming a good and stable cream texture. The higher hardness of the cream in the 10R group may be because, with the same fat content, the lower whipping rate and less air content make the cream harder.

 

cream Whipping rate, cream transfer

Fig.2 Whipping rate (A) and hardness (B) of whipped cream

 

4. Cream microstructure

It can be seen from Figure 3 that the fat globules stained by Nile red before whipping in the cream of the 10R group and the untreated control group were significantly less than those of the 60R and 60L groups that were pumped at 60°C. The fat globules of cream transported by the pump at 10℃ are large and sparse, while the fat globules of cream transported by the pump at 60℃ are small and dense. This is significantly related to the agglomeration and agglomeration of fat. After whipping, the fat globule diameter of the cream in the untreated control group was smaller than that of the other three groups, which shows that the delivery of the pump will cause a certain degree of fat globule agglomeration, making the fat globule diameter larger. After whipping, the size and distribution of fat globules in the 60R and 60L groups were more uniform, indicating that the cream was more conducive to subsequent whipping after being transported by the two pumps at 60°C.

Microstructure of fat, cream transfer

A1~D1. Control group, 10R group, 60R group, and 60L group before sending;
A2~D2. After sending, the control group, 10R group, 60R group, and 60L group.

Fig.3 Microstructure of fat globules in cream before and after whipping

5. Fatty acid composition of cream

The milk fat in cream contains many ingredients that are beneficial to the human body, including conjugated linoleic acid, sphingomyelin, sterols and fat-soluble vitamins. According to the fatty acid measurement results, without treatment, the total fatty acid content of the cream was 39.9 g/100 g; after pump transportation and sterilization, the fatty acid content of the cream decreased to varying degrees. The total amounts of fatty acids were (38.1±0.5), (39.2±0.4), (39.4±0.3), (39.7±0.3) g/100 g. Among them, the fat content of the 10L group was significantly lower than that of other groups, which was obviously due to the precipitation and floating of part of the fat.

It can be seen from Table 3 that the fatty acid composition of cream has the highest content of long-chain fatty acids of C14, C16 and C18. After pump delivery and sterilization, the proportion of saturated fatty acids in the cream increased to a similar extent, while the proportion of unsaturated fatty acids decreased, which was mainly reflected in the decrease in the proportion of monounsaturated fatty acids. This shows that after the cream is heat treated, Some fatty acids are oxidized, and unsaturated fatty acids are oxidized at a higher rate than saturated fatty acids. There is no obvious difference in the fatty acid composition of cream processed by different processes. Therefore, the difference in fat precipitation and whipping rate is mainly due to its physical changes, that is, the coalescence of fat globules and the precipitation of milk fat caused by temperature and shear force.

6. Conclusion

The cream fat in the 10L group has agglomerated and precipitated. The viscosity of the cream is significantly lower than that of the other groups, and it cannot be whipped. The 10L group has the largest cream particle size and is the most uneven. The other groups have similar particle sizes, while the 60L group has the largest cream particle size. The particle size of the cream is the most uniform; the 10R group has the lowest whipping rate but the highest hardness after whipping, while the whipping rate and hardness of the 60R and 60L groups are close; there is no significant difference in the fatty acid composition of the cream in different groups, and the difference in microstructure shows that after pumping at 60 ℃, the fat globules of the transported cream are small and dense, which shows that the cream at low temperature is easily sheared and coalesced, causing fat to precipitate and affecting the quality of the cream. Therefore, in production, the impact of shear force on the quality of cream at low temperatures should be avoided as much as possible. Rotor pumps, screw pumps, etc. can be used as delivery pumps; increasing temperature can make the cream more resistant to shearing, and centrifugal pump transferring may result in a more even distribution of fat globules without significant impact on whipping properties and cream quality.

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