Milk is known as the closest food to perfection, known as "white blood", and is a vital nutrient that is inherent to any mammal, including humans. In recent years, with the continuous improvement of cold chain configuration and the increasing awareness of health and nutrition among consumers, pasteurized milk has become a hot consumption topic. The process of pasteurized milk includes centrifugation, standardization, homogenization, sterilization, cooling, and filling, and is a commercial milk directly supplied to consumers for consumption. Compared with room temperature milk, pasteurized milk generally uses a constant temperature short-term sterilization of 72~85 ℃ for 15 seconds, which can kill the majority of harmful bacteria in milk, while maximizing the preservation of heat sensitive active ingredients and pure taste in raw milk. Many factors such as cow feeding, season, and environment have a significant impact on the physical and chemical indicators of raw milk. In order to obtain products with stable and consistent quality, it is often necessary to adjust the physical and chemical indicators of raw milk through certain technological processes in dairy processing, that is, to standardize milk.
Membrane technology has the characteristic of selective separation, which can adjust the mass ratio of each component in milk by controlling the appropriate concentration ratio, thereby achieving the goal of milk standardization. In recent years, reverse osmosis (RO) membrane technology has been introduced in dairy processing. Due to the characteristic of RO membrane that retains solutes in milk and only allows water to pass through the membrane, concentration of RO membrane at low temperatures can maximize the nutritional and heat sensitive active substance content of milk. In recent years, with the maturity of membrane system technology, the production of concentrated pasteurized milk through RO membrane concentration technology has become an emerging production technology for dairy production enterprises to improve product quality.
According to GB 19645-2010, the acidity of pasteurized milk should comply with 12-18 T. Therefore, acidity is a necessary indicator for dairy processing factories to check whether milk is qualified. The total acidity of milk includes intrinsic acidity and fermentation acidity. Intrinsic acidity refers to the acidity of freshly squeezed fresh milk, mainly derived from acidic substances such as casein, albumin, phosphate, citrate, and carbon dioxide in milk. When using RO membrane concentration to produce concentrated pasteurized milk, due to the increased content of various components in the milk during the processing, it may cause changes in acidity. Fermentation acidity refers to the change in milk acidity caused by the production of lactic acid during the storage and processing of milk due to microbial growth, lactose decomposition, and other factors. 3-4 T of total acidity comes from proteins (mainly casein and albumin), while 2 T comes from CO2. Phosphates and citrate provide the highest proportion of acidity (10-12 T). If expressed as lactic acid, CO2 in milk accounts for 0.01-0.02%, casein accounts for 0.05-0.08%, citrate accounts for 0.01%, albumin accounts for 0.01%, and phosphate accounts for the rest.
In recent years, a large-scale, rational, and scientific feeding method has been formed in the production of dairy cows. Technical personnel have continuously improved feed raw materials, dietary formulas, and feeding management in various aspects, maintaining the stability of raw milk acidity. Many links in the production process can have an impact on the acidity of milk. This article mainly focuses on the comparison of acidity detection methods and the analysis of acidity changes during the production and storage of RO membrane concentrated pasteurized milk, in order to propose acidity control measures for each link of production management of RO membrane concentrated pasteurized milk, providing technical reference for dairy production enterprises.
1 Materials and Methods
1.1 Raw Materials and Equipment
1.1.1 Raw materials
Pasteurized milk production line, including production equipment such as milk collection cooling system, raw milk storage tank, net milk separator, RO membrane concentration system, concentrated milk storage tank, milking quantitative system, quantitative tank, pasteurization system, degassing tank, homogenizer, waiting tank, filling machine, etc.
1.2.1 RO membrane concentrated pasteurized milk production process
Raw milk → cooling → net milk → storage (2-6 ℃) → RO concentration → quantification → heating (65-70 ℃) → degassing (-0.7-0.8bar) → homogenization (200bar) → sterilization (75 ℃, 15s) → cooling (2-6 ℃) → waiting for filling → filling → refrigeration (2-6 ℃) → factory transportation (2-6 ℃).
1.2.2 Acidity detection
To examine the impact of different detection methods on acidity results, the first and third methods in the "Determination of Food Acidity" (GB 5009.239-2016) were used for comparative testing.
1.2.3 Protein content detection
Use the Kjeldahl method in the "Determination of Protein in Food" (GB 5009.5-2016) for protein detection.
1.2.4 Detection of total bacterial count
The method of "Food Microbiological Examination - Determination of Total Colony Count" (GB4789.2-2016) was used for colony count detection.
1.2.5 Effect of storage time on acidity of raw milk
By conducting acidity testing on 10 batches of raw milk stored for 0, 2, 4, 6, and 8 hours, and combining the total bacterial count of the raw milk upon arrival at the factory, the trend of acidity change and the relationship between acidity change and the total bacterial count were analyzed.
1.2.6 Effect of RO membrane concentration process on acidity
By detecting the protein content and acidity of 10 batches of milk before and after concentration, comparing the acidity ratio and protein content ratio before and after concentration, and analyzing the impact of the concentration process on acidity.
1.2.7 Effect of degassing process on acidity
Degassing was carried out under conditions of temperature ranging from 65 to 70 ℃ and pressure ranging from -0.7 to -0.8 bar. The acidity of 10 batches of milk before and after degassing was tested, and the impact of the degassing process on acidity was analyzed.
1.2.8 Effect of different storage conditions on the acidity of concentrated pasteurized milk
The effect of refrigeration conditions on acidity: 10 batches of concentrated pasteurized samples stored for 0 days, 8 days, 10 days, and 14 days were refrigerated at 2~6 ℃ for acidity testing, and the changes in acidity during storage were compared.
Simulate the impact of temperature changes in the circulation process on acidity: By simulating the temperature changes in the circulation process, 10 batches of concentrated pasteurization samples were stored in a 2-6 ℃ refrigerator. For the first to fifth day of storage, samples were taken out every day and placed at room temperature of about 25 ℃ for 2 hours before being returned to the refrigerator. The same batch of samples were tested for acidity on the 0th, 8th, 10th, and 14th days of storage, and the acidity changes during storage were compared.
Simulate the impact of store conditions on acidity: By simulating the storage conditions of the store, 10 batches of concentrated pasteurized milk samples were stored at 10-14 ℃. The acidity of the same batch of samples was tested on the 0th, 8th, 10th, and 14th days of storage, and the changes in acidity during storage were compared.
1.2.9 Data Analysis
Use Microsoft Excel worksheet for data statistics and SPSS 26.0 for data analysis.
2 Results and Discussion
2.1 Impact of different detection methods on acidity results
The acidity of the same concentrated pasteurization sample was tested using the first and third methods in GB 5009.239-2016, and a total of 10 batches were tested.
When using the first and third methods in GB5009.239-2016 for acidity detection of the same sample, the detection value of the third method is about 1 T lower than that of the first method, and the standard deviation of the third method is smaller than that of the first method.
The first method uses phenolphthalein as an indicator for titration analysis of total acidity, which needs to be determined by comparing the color after titration with the reference solution, which is prone to subjective human error and therefore has a slightly higher standard deviation. During the testing process, it was found that the reference solution had a darker color, which affected the determination of the titration endpoint based on color changes. At the titration endpoint, the pH value was greater than 8.30. The third method is the potentiometric titrator method, which titrates to pH8.30 as the endpoint. The equipment has high sensitivity and reduces the influence of human factors. The results are stable when repeated testing. Considering accuracy and ease of operation, the third method is used for subsequent acidity testing.
2.2 Effect of storage time on acidity of raw milk
The microbial indicators of raw milk can have a significant impact on the changes in acidity during storage. The higher the initial bacterial count of raw milk, the faster the reproduction rate of microorganisms during storage, resulting in more significant changes in acidity. This study measured the total bacterial count of raw milk when it entered the tank for 0 hours, and detected the acidity value of raw milk stored in the tank for different times.
The correlation between the total number of bacterial colonies stored for 0 hours and the acidity growth value stored for 8 hours
Due to the different microbial composition of each batch of raw milk, its correlation with acidity growth value is affected, but overall, the total number of colonies is directly proportional to the change in acidity. As the storage time of raw milk prolongs, the acidity of milk changes. The acidity changes from 0.2 to 0.6 T after 8 hours of storage, and the acidity after 0, 2, 4, 6, and 8 hours of storage is significantly correlated with the total bacterial count of raw milk stored for 0 hours (Pearson correlation coefficient is 0.886 to 0.931). This indicates that microorganisms reproduce or ferment in milk, breaking down lactose to produce lactic acid, thereby causing an increase in acidity. In addition, raw milk with high microbial indicators also has higher lipase activity. Milk fat in raw milk with higher lipase activity will continue to decompose, producing a large amount of free fatty acids, leading to increased acidity and abnormal odor. Due to the long detection time of microorganisms, it is impossible to predict the degree of harm to product quality during the storage process of raw milk. Therefore, the storage time of raw milk should be strictly controlled during the production process. The "Technical Specification for High Quality Pasteurized Milk Processing Technology" requires that the temporary storage temperature of the milk warehouse should be controlled below 6 ℃, and the temporary storage time should be controlled within 8 hours.
2.3 Effect of RO membrane concentration process on acidity
RO membrane technology can achieve milk concentration under the condition of maximizing the retention of active substances. The concentration process can achieve changes in milk concentration, which may lead to an increase in acidity. This study analyzed the changes in protein content and acidity during the concentration process of milk RO membrane.
Changes in protein concentration ratio and acidity concentration ratio during milk RO membrane concentration process
During the RO membrane concentration process of milk, as the protein content increases, the acidity also increases accordingly. The acidity concentration ratio is similar to the protein concentration ratio, with a protein concentration ratio of 1.17 ± 0.02 and an acidity concentration ratio of 1.16 ± 0.02. The RO membrane concentration process itself is a physical change and does not cause an increase in acidity. However, due to the concentration process only removing water, the proportion of acidic substances in milk increases, resulting in a proportional increase in acidity during the RO membrane concentration process. Due to the requirement in GB19645-2010 that the acidity of pasteurized milk must meet 12-18 T, the concentration ratio should be strictly controlled when producing RO membrane concentrated pasteurized milk.
2.4 Effect of Degassing on Acidity
CO2 is one of the factors that contribute to the acidity of milk, and the degassing process after vacuum treatment can cause some CO2 in the milk to be lost, resulting in a decrease in acidity. This study analyzed the acidity changes of milk before and after degassing. The CO2 content in freshly extruded milk is approximately 200mg/L. After storage, heating, stirring, and vacuum treatment, some CO2 is lost, resulting in a decrease of approximately 0.02% in titration acidity. Milk undergoes a degassing process at 65-70 ℃ and -0.7-0.8bar, resulting in a decrease in acidity of 0.4 ± 0.11 T. Therefore, in order to control the acidity of the concentrated product, adding degassing technology to the production process of concentrated pasteurized milk is a better treatment method.
2.5 Effect of Different Storage Conditions on the Acidity of Concentrated Pasteur Milk
75 ℃ and 15 seconds cannot kill all microorganisms in milk. The total number of bacterial colonies in 10 batches of test samples is 263 ± 55CFU/mL, so it is necessary to store them in an environment of 2-6 ℃ to fully weaken the metabolic activity of microorganisms and maintain the stability of sample acidity and quality. In the actual circulation process, the product may experience cooling off during factory warehouse loading, first level warehouse unloading and loading, second level warehouse unloading and loading, terminal store unloading and loading, and consumer purchasing processes. At the same time, during the temperature monitoring process of terminal stores, it was found that the actual temperature range of some terminal containers in some stores was between 10 and 14 ℃.
RO membrane concentrated pasteurized milk samples may exhibit different acidity changes under different storage conditions due to their characteristic of nutrient enrichment. This study tested and analyzed the acidity changes of 10 batches of concentrated pasteurized milk samples under three different storage conditions for 14 days.
In a cold storage environment of 2-6 ℃, microbial growth activity significantly weakened, and the acidity of the sample did not increase significantly within 14 days. This indicates that maintaining the low-temperature storage conditions of the sample at 2-6 ℃ can control the stability of acidity.
Simulating the temperature changes in the circulation process, the sample was subjected to a total of 10 hours of cold and hot shock for 5 days under refrigeration conditions of 2-6 ℃ to simulate the 5 cooling stages in the product circulation process. It was found that there was a slight increase in the acidity of the sample within 14 days, with an increase value of 0.26 ± 0.05 T, which was significantly higher than the acidity change under refrigeration conditions. This indicates that temperature changes can enhance the reproductive and metabolic activities of microorganisms in milk. Under this condition, the acidity of RO concentrated pasteurized milk increased by 0.12 ± 0.09 T in the first 8 days, and 0.26 ± 0.05 T in the 14th day, indicating that the acidity continued to increase after stopping the cold and hot shock.
When simulating partial store conditions and storing RO concentrated pasteurized milk samples at 10-14 ℃, the acidity of 10 batches of products increased by 0.27 ± 0.05 T at 8 days, 0.37 ± 0.07 T at 10 days, and 0.63 ± 0.12 T at 14 days. The increase was higher than that of products with 5 days of cold and hot shock under 2-6 ℃ refrigeration conditions. It is speculated that microorganisms in milk continue to metabolize slowly to produce acid under elevated temperatures.
Based on the above research results, in order to ensure that the acidity of concentrated pasteurized milk samples meets the requirements of GB19645-2010 throughout the shelf life, and to ensure product quality, dairy companies must strictly comply with operating standards during the processing process. By controlling the microbial content and freshness of raw milk and appropriate concentration ratio, the acidity and microbial content of the product at the time of delivery are controlled, and the loading and unloading time of the product at the time of delivery and after delivery is strictly controlled, Control the temperature changes over time.
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