(1) Sensory identification
Color: Fresh milk should be light blue, milky white or light yellow. Milk is milky white because the fat globules, calcium butyrate colloid, calcium phosphate, etc., have refraction and reflection effects on light. The opalescence occurs due to the reflection of the colloid and calcium phosphate on the light, causing the milk to have a light blue color. Because of the presence of lutein in the feed taken by cattle, part of it is transferred to milk fat, the milk produced by cows fed with grass and carrots may be yellowish, while the milk produced by cows fed with hay and grains will be light blue.
Odor: Fresh milk contains sugars and volatile fatty acids, so it has a slightly sweet taste and a pure lactic acid taste. In addition, the milk produced by dairy cows during long-term milk production or during the dry period often has a bitter taste; the milk often has an unpleasant smell when the afterbirths are retained after an abortion; fish meal, moldy feed, garlic, and the milk also has an unpleasant smell; disinfection The temperature is too high and the stirring is not uniform, the milk often has a burnt smell.
Condition: The fresh milk curd is dense, firm, uniform, without bubbles, allowing a small amount of whey on the surface. If the microorganisms are immersed in the milk due to improper storage or too long storage time, clear liquid will appear in the upper and upper part of the milk bottle, and tofu brain-like deposits appear in the lower layer.
(2) Dissolved water test
Put a few drops of milk in a bowl with water. If the milk has condensed and sinks to the bottom of the bowl, it is fresh milk; if it melts, it is not fresh milk. In the case of bottled milk, as long as thinning is observed on the upper part of the milk or precipitation at the bottom of the bottle, it is not fresh milk.
(3) Boiling test method
After the milk is boiled, the milk with milk skin (milk fat) on the surface is good milk, and the milk with tofu flower-like surface is bad milk. Any milk with abnormal color, smell and condition cannot be eaten. In addition, you can take about 10 ml of milk sample in a test tube (or a transparent glass) and place it in boiling water for 5 minutes to observe. If there is condensation or flocculation, it means that the milk is not fresh or has deteriorated.
(4) Acidity inspection
The pH of normal milk is generally between 6.3 and 6.9, which is weakly acidic. It is composed of weakly acidic substances such as acidic protein, citrate, phosphate and carbon dioxide inherent in fresh milk. During the storage of milk, the activity of microorganisms decomposes lactose into lactic acid, which increases the acidity of milk. The higher the acidity of the milk, the more serious the degree of microbial contamination of the milk. Therefore, the freshness of milk can be evaluated by measuring the acidity of milk.
The higher the acidity of the milk, the worse the stability to heat. Milk that exceeds 25°T will coagulate by itself when it is boiled, making it difficult to reprocess and use. Milk powder made from milk with too high acidity has poor solubility and poor quality. Therefore, the acidity of milk is an indicator that dairy processing companies must inspect when purchasing raw milk.
Titration acidity (°T) refers to the number of milliliters of 0.1 mol/L sodium hydroxide standard solution required to neutralize the acid in 100 milliliters of milk with phenolphthalein as an indicator. The method is to draw 10 ml of milk sample into a 150 ml Erlenmeyer flask, add 20 ml of boiling and cooling water and 3 to 5 drops of phenolphthalein indicator solution, mix well, and titrate with 0.1 mol/L sodium hydroxide standard solution until pink , And does not fade within 0.5 minutes. Multiply the number of milliliters of the 0.1 mol/L sodium hydroxide standard solution consumed by 10, which is the titer acidity (°T).
Alcohol has a dehydration effect. When alcohol is added to milk, the hydration layer around the cow cheese protein colloids is removed, and the colloids become an unstable state with only negative charges. When the acidity of the milk increases, H+ interacts with negative charges, and the colloidal particles become electrically neutral and precipitate. There is a certain relationship between the acidity of milk and the concentration of alcohol that causes casein precipitation. Therefore, different concentrations of alcohol can be used to test milk samples to determine the acidity of milk by whether it is flocculent.
The method is to take 2 to 3 ml of milk sample into a test tube, add an equal amount of neutral alcohol of a certain concentration (68°, 70°, 72°), mix quickly and thoroughly and observe the result. If there are no flakes in 68° alcohol, it means that the acidity is lower than 20°T, and it is qualified milk; if there are no flakes in 70° alcohol, it means that the acidity is lower than 19°T, which is relatively fresh milk; °If there is no flakes in the alcohol, the acidity is lower than 18°T, which is good quality fresh milk.
③Determination of milk boundary acidity
Milk whose acidity is within 18°T is considered as good-quality fresh milk; milk whose acidity is within 20°T is considered as qualified fresh milk. Therefore, by measuring the two acidity points of the milk, the freshness of the milk can be known.
The method is to prepare in advance 0.018 mol/L and 0.02 mol/L sodium hydroxide standard solutions and 0.5% phenolphthalein ethanol solution. During inspection, add 5 ml of water, 0.25 ml of phenolphthalein indicator solution to the test tube, and then add 5 ml of 0.018 mol/L or 0.02 mol/L sodium hydroxide standard solution, shake to mix, and observe the results.
In the test tube of 0.018 mol/L sodium hydroxide standard solution, the phenolphthalein does not fade, the milk acidity is lower than 18°T; the 0.02 mol/L sodium hydroxide standard solution does not fade, the milk acidity is lower than 20°T.