Rapid Inspection Technology for Fresh Milk Adulteration
Han Qiwen¹, Qin Lihu², Sun Wubin³
(1. Baoji Huimin Dairy Group Co., Ltd., Baoji 721305, Shaanxi, China; 2. Xi'an Dongfang Dairy Product Factory, Xi'an 710065, Shaanxi, China; 3. Shanghai Heshaoyu Chemical Co., Ltd., Shanghai 210000, China)
Abstract: This article introduces several rapid technical methods used to detect adulteration in fresh milk, including the detection of alkaline substances, nitrite, glucose, starch, urea, sucrose, hydrolyzed protein, powdered oils, antibiotics, preservatives, and mastitis milk. These techniques have been applied in real-world production scenarios to ensure milk quality and safety.
Keywords: Fresh cow (goat) milk; Adulteration; Inspection
Rapid Methods for Detecting Adulterants in Milk
Han Qiwen¹, Qin Lihu², Sun Wubin³
(1. Baoji Huiming Dairy Group Corp, Baoji 721305, China; 2. Xi'an Oriental Dairy Factory, Xi'an 710065, China; 3. Shanghai Hesibi Chemistry Corp, Shanghai 210000, China)
Abstract: This paper presents rapid technological methods for detecting basic substances, nitrite, starch, bactericides, preservatives, urea, and sugar in adulterated milk. These tests are crucial for maintaining the integrity and safety of dairy products.
Keywords: Milk; Adulteration; Detection
0 INTRODUCTION
The competition among major dairy companies in China has increasingly focused on securing high-quality milk supplies. Unfortunately, some dairy farmers adulterate fresh milk for economic gain, which negatively affects both the quality of dairy products and consumer health. Over the years, I have developed and refined various testing techniques to identify adulterated and abnormal milk. This article aims to share practical insights and methods that can help improve the quality control process during milk procurement.
1 Measuring Specific Gravity
1.1 Purpose
The main purpose of measuring specific gravity is to determine whether fresh milk has been diluted with water. The specific gravity of normal milk typically ranges from 1.028 to 1.032. If the specific gravity falls below 1.028, it may indicate adulteration.
1.2 Detection Methods
1.2.1 Equipment
- Hydrometer (20°C/4°C)
- Thermometer (100°C, mercury rod type)
- Glass graduated cylinder (250 mL)
1.2.2 Procedure
Mix the fresh milk thoroughly, take a sample of 400–500 mL, and carefully pour it into the cylinder. Insert the hydrometer gently and wait until it stabilizes. Measure the temperature and calculate the specific gravity.
1.3 Explanation
While this method does not directly measure the freezing point, it is simple and effective for identifying water dilution. It is widely used in the industry due to its ease of use and reliability.
2 Alkaline Substances
2.1 Purpose
Common alkaline substances like soda or lime are often added to milk to mask spoilage, especially during hot seasons. Testing for alkalinity helps identify such adulterated milk.
2.2 Detection Method
2.2.1 Principle
Bromothymol blue changes color from yellow to blue in an alkaline solution (pH 6.0–7.6).
2.2.2 Reagents
- 0.04% bromothymol blue ethanol solution
2.2.3 Procedure
Take 2 mL of milk in a test tube, add 0.6 mL of the indicator along the wall, mix gently, and observe after 2 minutes.
2.2.4 Result Interpretation
| Ring Color | Alkali Content | Conclusion |
|------------|----------------|------------|
| Yellow | No alkali | Qualified |
| Yellow-Green | 0.03% alkali | Abnormal |
| Light Green | 0.05% alkali | Abnormal |
| Green | ≥0.1% alkali | Severe Abnormal |
2.3 Alternative Test (Rhodamine Acid Color Reaction)
2.3.1 Principle
Rhodamine acid changes from brownish yellow to rose red in alkaline conditions.
2.3.2 Reagents
- 0.2% rhodamine alcohol solution
2.3.3 Procedure
Add 1 mL of milk and 1 mL of reagent to two tubes. Shake and observe. Rose-red indicates the presence of alkali.
3 Determination of Nitrite and Nitrate
3.1 Purpose
Nitrite and nitrate are common preservatives but can be harmful in excess. Monitoring their levels ensures milk safety.
3.2 Nitrite Detection
3.2.1 Principle
In weak acidic conditions, nitrite reacts with sulfanilic acid and α-naphthylamine to form a purple dye.
3.2.2 Reagent Preparation
- 0.1 g α-naphthol, 0.2 g α-naphthylamine, 0.6 g sulfanilic acid in 400 mL 50% glacial acetic acid.
3.2.3 Procedure
Add 1.5 mL of reagent to 2 mL of milk and observe the color change.
3.2.4 Result Interpretation
| Milk Color | Nitrite Amount | Conclusion |
|------------|----------------|------------|
| White | No nitrite | Qualified |
| Pink | 0.2 mg/kg | Abnormal |
| Pink Water | 0.3 mg/kg | Abnormal |
| Deep Pink | ≥0.4 mg/kg | Severe Abnormal |
3.3 Nitrate Detection
3.3.1 Principle
Nitrates are reduced to nitrites and then react with sulfanilic acid and methamphetamine to form a red azo compound.
3.3.2 Reagents
- Reducing agent: 44 g barium nitrate, 5 g manganese sulfate, 2 g cadmium acetate, 2 g zinc powder.
- Color reagent: Same as for nitrite detection.
3.3.3 Procedure
Add 0.3 g reducing agent to 2 mL milk, shake, add 2 mL color reagent, and observe after 3 minutes.
3.3.4 Result Interpretation
| Milk Color | Nitrite/Nitrate Content | Conclusion |
|------------|--------------------------|------------|
| Milky | No nitrite/nitrate | Qualified |
| Pink | 0.2 mg/kg | Abnormal |
| Pink Water | 0.3 mg/kg | Abnormal |
| Red | ≥0.5 mg/kg | Severe Abnormal |
3.3.5 Notes
This method is convenient, fast, and accurate for controlling both nitrite and nitrate levels. However, low-desalination whey powder may affect results.
By implementing these methods, dairy producers can effectively monitor milk quality, ensuring safer and higher-quality products for consumers.
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