Milk is rich in nutrients and is loved by the public. However, various brands and packaging of milk in the market are diverse and complex. The general sterilization methods for commercially available milk are divided into two types: pasteurization and ultra-high temperature instantaneous sterilization. Different packaging and sterilization methods affect the storage conditions and shelf life of milk. When milk is opened, it is prone to rancidity, but the changes in acidity of milk with different packaging and sterilization methods are not yet clear. Therefore, studying the differences in storage conditions, shelf life, and acidity changes of milk with different packaging and sterilization methods can provide suggestions for people in purchasing and drinking milk, and provide reference for testing the freshness of milk.
1、 Materials and Methods
Three brands, Sanyuan, Yili, and Mengniu, were selected for comparative analysis of five different packaging of whole milk pure milk. At the same time, four types of Sanyuan milk with the same production date and different packaging were selected for acidity change experiments, all of which were purchased from supermarkets.
1.2.1 Comparison of packaging, sterilization methods, and shelf life of commercially available milk
Compare and analyze the sterilization methods, storage conditions, shelf life, nutritional composition, and other information of different packaged pure milk.
1.2.2 Experiment on acidity changes of different packaging milk
Open the packaging of four types of ternary milk from the same production date, put them into conical bottles sterilized with high temperature and pressure, cover them with sealing film, and place them under the same environmental conditions such as temperature, humidity, and light. Take out 10mL of milk every day and use titration to measure its acidity. The steps and calculation method for determining acidity are as follows:
Instruments used: conical flask, burette, rubber tipped burette, measuring cylinder;
Reagents used: 0.1mol/L NaOH, phenolphthalein, milk to be tested, distilled water;
Operating steps: (1) Inject 10ml of milk into a 250ml conical flask and dilute with 20ml of distilled water; (2) Add 5 drops of phenolphthalein indicator solution into the conical flask and carefully mix well; (3) Titrate with 0.1mol/L sodium hydroxide standard solution until the reddish color does not disappear within 1 minute.
Result calculation: The acidity is determined by multiplying the volume (mL) of 0.1 mol/L NaOH solution consumed in titrating 10 ml of milk sample by 10.
Among them, the acidity of fresh milk is often 16-18 ° T.
2、 Results and Discussion
2.1 Packaging, sterilization methods, and shelf life of milk
At present, the milk sold on the market can be divided into five types based on packaging: Baili bag, roof box, plastic bottle, Tetra Pak pillow, and Tetra Pak brick. The sterilization methods, storage conditions, and shelf life of milk in different packaging are shown in Table 1. From the table, it can be seen that pasteurized milk has a shorter shelf life, with bagged Baili bags only having a shelf life of about 3 days and usually requiring refrigeration at 2 ℃ to 6 ℃. Milk that is instantly sterilized at ultra-high temperatures has a longer shelf life, and ordinary Tetra Pak pillows can be stored for about 45 days.
This is due to the different effects of the two sterilization methods. Pasteurized milk cannot kill some heat-resistant bacteria and spores, becoming one of the pollution sources of milk spoilage. On the other hand, the plastic bags commonly used in Baili bags are single-layer polypropylene materials or multi-layer black and white co extruded packaging films, which have relatively poor oxygen resistance and are difficult to achieve aseptic status before filling. Therefore, they need to be stored at low temperatures to inhibit bacterial growth. Roof boxes and plastic bottles have better performance than Baili bags, so their storage time is slightly longer. Ultra high temperature instantaneous sterilization can completely destroy the microorganisms and spores that can grow inside, and it is often packaged in paper plastic composite packaging, which has relatively good oxygen resistance and greatly increases the shelf life.
2.2 Nutritional composition of milk
Milk is rich in protein, sugars, and fats, as well as various minerals and vitamins such as calcium, phosphorus, sodium, and magnesium. For the three brands of pure milk in this study with different packaging.
2.3 Changes in milk acidity
The acidity change experiment was conducted using milk 1 expired for one day and milk 2, 3, and 4 of the same production date that had been stored for 5 days but not expired. The main information of the four types of milk is shown in Tables 3 and 4. From the table, it can be seen that the shelf life of the pasteurized Baili Bao is the shortest, only 4 days, while the other three types of milk that are also ultra-high temperature sterilized also have different shelf lives due to different packaging materials. At the same time, there is little difference in the nutritional composition of milk with different sterilization methods, which is consistent with the results described in 2.1.
Over time, the acidity of milk will increase. At the initial measurement of milk number 1, the acidity had already started to rise due to being 1 day overdue. Moreover, according to the results of No.1 milk, its acidity first increases and then decreases during the spoilage process, which is consistent with the research results.
Generally speaking, the acidity of milk is composed of proteins and acidic substances such as phosphates, citrate, and carbonates in milk. The acidity of normal milk is 16-18 ° T. The results of this experiment are consistent with this. During the storage process of milk, the lactose in the milk will be converted into lactic acid under the action of bacteria, resulting in an increase in the acidity of the milk. As time goes on, bacteria multiply in large numbers in milk, and the acidity of milk will become higher and higher. However, an acidic environment can also inhibit bacterial growth, resulting in a decrease in the production of lactic acid in milk. At this time, fungi multiply in large numbers, and the acidic environment in milk is neutralized, causing the acidity of milk to begin to decrease. Acidity is a commonly used indicator for detecting the freshness of milk. However, during the spoilage process of milk, the acidity will first increase and then decrease, which will bring inconvenience to testing. During the spoilage process, the lactose content of milk will decrease, so the combined determination of acidity and lactose content will make the results more accurate. The four types of milk selected for the experiment were sterilized using two different methods, as described in 2.1, with significant differences in shelf life. Pasteurized milk quickly sours after being opened and has already deteriorated excessively by 72 hours. The rate of rancidity of ultra-high temperature sterilized milk is relatively slow. Due to the lack of significant differences in milk nutrition among different packaging and sterilization methods, the difference in rancidity rate may be due to different sterilization methods.
3、 Conclusion and outlook
Overall, different sterilization and packaging methods affect the shelf life and storage conditions of milk. After opening, milk will experience rancidity and an increase in acidity. Pasteurized milk is usually packaged easily, has a short shelf life, requires low-temperature storage, and has a faster rate of rancidity after opening. Instantaneous ultra-high temperature sterilized milk has a long shelf life and can be stored at room temperature. After opening, the rate of rancidity is slower.
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