Causes of Milk Spoilage and Home Detection Methods

Milk is one of the most essential and nutritious foods in our daily diet, rich in protein, calcium, and essential vitamins. This vital substance, due to its rich composition, provides a susceptible environment for the growth of microorganisms and, if not stored properly, spoils quickly. Milk spoilage not only affects its appearance, smell, and taste but also its consumption can harm human health.

Causes of Milk Spoilage

Milk, due to containing nutrients such as fat, protein, and lactose sugar, is a suitable environment for the growth and reproduction of microorganisms. In the pasteurization process, many pathogenic bacteria are eliminated, but some spore-forming and heat-resistant bacteria can survive and eventually cause milk spoilage.

• Spore-forming bacteria, such as Clostridium and Bacillus species, can form spores (a resistant form of bacteria) in pasteurization conditions (which are harsh conditions for bacteria), resist pasteurization, and not be eliminated. After pasteurization, when these spores are placed in suitable growth conditions, for example, when milk is left at room temperature, these spores become active and begin to grow, and by producing enzymes, they break down the protein and sugar in the milk, leading to milk spoilage. This activity in milk can cause unpleasant tastes and odors and even milk clotting.

• Heat-resistant bacteria, such as Micrococcus and Streptococcus species, can somewhat tolerate pasteurization temperatures and therefore may not be eliminated during this process. These bacteria can also begin to grow under suitable conditions and, by acting in milk, produce lactic acid and thus cause milk spoilage. For example, they can lead to a sour taste in milk.

In general, the growth rate of these bacteria is related to storage conditions and duration. Bacterial growth is usually accelerated under favorable conditions, including storing milk at temperatures above 4 degrees Celsius. Under these conditions, bacteria become active in milk and, by producing various enzymes, break down the main components of milk, such as fats, proteins, and milk sugar (lactose). These enzymatic processes lead to irreversible changes in the characteristics of milk, such as changes in taste, odor, and texture, and ultimately milk spoilage.

• Fats: Lipase enzymes break down milk fats into fatty acids and glycerol. This process leads to the creation of pungent and unpleasant tastes and foul odors.
• Proteins: Protease enzymes break down milk proteins, including casein in milk, into peptides and related amino acids. The result of this process is a change in milk texture to a clotted state and the creation of bitter tastes.
• Milk sugar (lactose): Bacteria convert lactose sugar into lactic acid. This conversion reduces the pH of milk, sours the taste, and changes its odor to an acidic state.

Note: Difference between spoiled milk and soured milk

Spoiled milk usually refers to pasteurized milk that has spoiled due to bacterial growth, while soured milk usually refers to raw milk that has naturally fermented.

Signs of Milk Spoilage and Home Detection Methods

1. Checking Appearance and Smell

The simplest way to detect milk spoilage is to check its appearance and smell. Spoiled milk usually shows changes in color, smell, and texture.

• Color

Healthy milk has a white to cream color. Changing the color of milk to deep yellow, brown, or darker shades can be a sign of spoilage. This color change is due to bacterial growth and the breakdown of milk components.

• Smell

Spoiled milk gives off a sour smell or an unpleasant smell similar to rotten cheese. This smell is due to the production of organic acids such as lactic acid and butyric acid by bacteria.

• Texture

Healthy milk has a uniform and homogeneous texture. Spoiled milk may become lumpy or have a thick and sticky state. These changes are due to the denaturation of milk proteins (loss of the natural structure of milk proteins) and clot formation.

2. Boiling Test

 Another simple method for detecting milk spoilage is the boiling test. To perform this test, pour some milk into a pot and heat it until it reaches boiling point. Spoiled milk curdles when boiled, meaning that cheese-like lumps form in it. This phenomenon is because in spoiled milk, lactic acid is produced by bacterial activity and its pH is low, meaning it has become acidic, and milk proteins are unstable in an acidic environment. Therefore, when spoiled milk is boiled, heat causes the natural structure of milk proteins to be destroyed, resulting in structural changes and the formation of solid lumps, and it is said that the milk has curdled, while in healthy milk, these proteins are suspended and their structure does not change when heated.

3. Baking Soda

Test The baking soda test is a quick method for detecting milk spoilage. To perform this test, dissolve a teaspoon of baking soda in half a cup of milk. If the milk is spoiled, it will react with the baking soda and foam. This foam is due to the production of carbon dioxide gas from the reaction between lactic acid in spoiled milk and baking soda (sodium bicarbonate). Of course, foam may also form in healthy milk, but in smaller amounts.

4. Finger Test

The finger test is a simple method for evaluating milk quality that focuses on changes in surface tension and milk structure. Surface tension is the ability of a liquid to resist spreading, which is due to the attractive forces between surface molecules and keeps the liquid cohesive. In this method, a drop of milk is placed on a finger. Healthy milk, due to its higher surface tension and stable structure of protein and fat components, moves slowly and does not tend to spread quickly. In spoiled milk, microbial activity breaks down proteins and fats, resulting in a decrease in surface tension, so spoiled milk spreads faster on the finger.

Note: This method may not provide accurate results due to its simplicity. Other factors such as the temperature of the milk or finger can also affect the result.

5. Microwave Test

The microwave test is a quick method for detecting milk spoilage using heat. In this method, some milk is poured into a suitable glass or container and the glass is placed in the microwave for 60 seconds (may vary depending on the microwave's power). When spoiled milk is exposed to microwave heat, curds in the form of cheese lumps are formed. These changes are due to the production of lactic acid in spoiled milk, which lowers the pH of the milk. The decrease in pH causes the natural structure of milk proteins, especially casein, to be destroyed. The heat applied in the microwave accelerates this process and causes the proteins to separate from the liquid phase and form solid lumps. Microbial and enzymatic activity also breaks down fats and proteins and produces compounds that cause unpleasant odors and color changes (such as yellow).

6. Alcohol Test

The alcohol test is a method for detecting milk spoilage using alcohol. In this method, equal amounts of milk and medical alcohol (70%) are poured into a small container. If the milk is spoiled, it quickly curdles and solid pieces appear in it. Milk curdling in the presence of 70% alcohol is due to the effect of alcohol on milk proteins, especially casein. In spoiled milk, due to bacterial activity and lactic acid production, the pH decreases and the proteins are in an unstable state. This instability causes the alcohol to quickly destroy the natural structure of milk proteins and turn them into solid lumps. In contrast, in healthy milk, the proteins are more stable and the pH remains close to neutral, so the mixture remains more homogeneous or may only form very small curds.

7. Taste Test

For testing suspicious milk, taste testing is generally not recommended, but if no other method is available, a small amount of milk can be tested, but by no means should a large amount be consumed. If the milk is spoiled, it will taste sour or bitter, which is due to the production of lactic acid by bacteria.

Risks of Consuming Spoiled Milk

Consuming spoiled milk can lead to food poisoning. Symptoms of this poisoning include nausea, vomiting, diarrhea, and abdominal pain. The severity of these symptoms depends on the individual's health status, immune system, and the amount of milk consumed.

Milk Expiration Date

The expiration date is determined based on optimal storage conditions, i.e., at refrigerator temperature. After this date, the likelihood of bacterial multiplication and their reaching a level that can lead to milk spoilage increases. However, if milk is stored in the refrigerator, it may still be safe for a short period after the expiration date (e.g., one day). If you wish to consume it under these conditions, spoilage signs such as changes in odor, texture, etc., should be checked before consumption, and if there are no signs of spoilage, consuming it is not a problem, but to prevent any health risks, it is recommended that milk not be consumed after the expiration date.

In some cases, milk may show spoilage signs before reaching the expiration date. This situation is usually due to storing milk under improper conditions such as temperatures higher than recommended or exposure to environmental contamination. In addition, problems in the production process, including incorrect pasteurization, can also lead to reduced shelf life and premature spoilage of milk.

Tips for Proper Milk Storage

• Milk should be stored in the refrigerator at a temperature of 4 degrees Celsius or lower to prevent bacterial growth and premature spoilage.
• Milk should not be left out of the refrigerator, especially in direct sunlight or heat sources. Light and heat can increase the temperature and accelerate the fat oxidation process of milk, leading to reduced quality and shelf life of milk.
• The milk container lid should be completely closed to prevent environmental contamination and direct contact with air. This contact can increase the risk of microbial contamination and the absorption of foreign odors, which affects the taste and quality of milk.
• It is better to store milk in the inner part of the refrigerator, as this section usually has a more constant temperature and protects against sudden temperature changes caused by opening and closing the refrigerator door. The refrigerator door and nearby sections may be exposed to temperature fluctuations, which can reduce the quality of milk and increase the risk of premature spoilage. Storing milk in the inner part of the refrigerator, where the temperature is constantly below 4 degrees Celsius, can increase the shelf life of milk.