Method for measuring the acidity of milk

Milk is a nutrient rich food, so it is naturally an excellent natural culture medium for various microorganisms. If microorganisms are contaminated during storage, improper storage conditions, and stored for too long, it can cause rapid proliferation of microorganisms. The various metabolic products produced by these microorganisms during the metabolic process are mainly acidic substances, some of which are harmless, some are slightly harmful, and some are very harmful. Therefore, the acidity of milk can be used to calculate the freshness of milk.

Method for measuring the acidity of milk

The acidity of milk lactic acid is usually expressed by titration acidity (t) or lactic acid content (lactic acid%). The acidity of milk is 18. Those within t are high-quality fresh milk; Milk with an acidity of less than 20 ° T is considered qualified fresh milk. So, by measuring these two acidity points of milk, it can be inferred that there are two commonly used methods for measuring acidity: titration acidity method and alcohol test method.

(1) Titration acidity method: Using phenolphthalein as an indicator, the required milliliters of 0.1mo1/1 sodium hydroxide standard solution to neutralize the acid in 100ml of milk are generally determined using a 10ml sample measurement method. The 10ml milk sample is taken and added to a 150ml triangular flask with 20ml of boiled and cooled water (the purpose of adding water is to dissolve the broken tricalcium phosphate in milk and react with the indicator to generate calcium (oh) 2, reducing the titration acidity by about 2t), and then 0.5% phenolphthalein indicator is added by 0.5m13-5 drops to mix well, Titrate with 0.1mol/I NaOH standard solution until a slight red color does not fade for 30 seconds. Multiply the number of milliliters of NaOH standard solution consumed by 10 to obtain the titration acidity (. t). Determination of acidity limit for milk: Add 5m1 of milk and an equal amount of water to a test tube, add 0.25ml of 10.5% phenolphthalein indicator solution dropwise, and then add 5m10.018mo1/1 or 0.02mo1/1 sodium hydroxide standard solution. Shake and mix, and observe the results. In a test tube containing 0.018mo1/1 sodium hydroxide standard solution, if phenolphthalein does not fade, the acidity of milk is below 18 ° T; In a 0.02 sodium oxide standard solution, the milk does not fade, and the acidity of the milk is less than 20 ° T

(2) Alcohol test method: Alcohol has a dehydration effect. When alcohol is added to milk, the hydration layer around the milk gel particles is removed, and the gel particles become unstable with only negative charges. When the acidity of milk increases, h+reacts with negative charges, and the gel particles become electrically neutral and precipitate. Therefore, there is a certain relationship between the acidity of milk and the alcohol concentration that causes casein precipitation. Therefore, different concentrations of alcohol can be used to detect milk samples and determine the acidity of milk based on whether they are flocculent. The method is to take 2-3 ml of milk sample and inject it into a test tube, add an equal amount of neutral alcohol (68 °, 70 °, 72 °), quickly and thoroughly mix, and observe the results. If there are no flocs in 68 ° C alcohol, it indicates that the acidity is below 20 ° T, indicating that qualified milk does not have flocs in 70 ° C alcohol, indicating that the acidity is below 19 ° T, indicating that it is relatively new fresh milk; Those who do not have flocs in 72 ° C alcohol and have an acidity below 18 ° T are considered high-quality fresh milk.