Common problems in yogurt production

Dairy products, as the representative of nutritious food, are being accepted and chosen by more and more people. Yogurt is an important member of dairy products and is favored by people for its unique flavor and higher nutritional value. Yogurt is a type of fermented dairy product made of high-quality pure fresh milk and sugar homogenized, sterilized by ultra-high temperature and then fermented with lactic acid bacteria. During the fermentation process, the casein in the fresh milk is coagulated by acid and becomes an elastic curd with milky white color, fragrant smell, sweet and sour taste. The seemingly simple production process, in fact, every small detail may affect the quality of the final product. The problems that often occur in the production process of yogurt are as follows:

1. No fermentation or slow fermentation

Unfermented yogurt refers to the fact that the fermented milk does not coagulate or is not fully coagulated, the acidity does not increase, and the network structure is not formed within the predetermined fermentation time, resulting in the failure of yogurt fermentation. The fermentation time of stirring yogurt is usually 5~6h, but sometimes the fermentation is delayed due to the following reasons. This phenomenon will seriously affect the taste of yogurt.

(1) The problem of raw materials

There may be residues of antibiotics, cleaning agents, disinfectants, preservatives, etc. in the milk, which will inhibit the normal growth of lactic acid bacteria; when the sterilization is insufficient, the production equipment pipeline or the environment is not up to standard, the bacteriophages in the raw milk And miscellaneous bacteria will also prolong the fermentation time of yogurt and reduce the quality of the product.

(2) The problem of starter

If the amount of bacteria added is not suitable or the vitality of the bacteria itself is not enough, the fermentation may be slow or not fermented.

2. Flavor defects

(1) Poor quality of raw materials

The feed odor, bovine body odor, oxidation odor, etc. of raw milk will cause bad odor of yogurt and affect the finished product.

(2) Improper operation of inoculation and fermentation process

The starter generally contains two kinds of bacteria, Streptococcus thermophilus and Lactobacillus bulgaricus. Studies have shown that the mixing ratio of the two bacteria is in the range of 1:1~1:2, and it is appropriate to maintain the above ratio in the normal production of yogurt. Short-term fermentation at high temperature and insufficient solid content of raw milk will also cause insufficient aroma; under normal circumstances, the peculiar smell of yogurt will also be caused by the contamination of miscellaneous bacteria during the fermentation process. For the problem of bacterial contamination, in addition to strictly controlling sanitary conditions and milk sterilization, it is also necessary to pay attention to avoid mixing air during operations such as stirring.

(3) The acidity of the finished product is too large

Because Lactobacillus bulgaricus continues to produce acid under acidic conditions, the post-acidification of yogurt results in the deterioration of the flavor of yogurt and the high acidity, which is difficult to be accepted by consumers. At this time, the fermentation time should be controlled according to the fermentation process, storage and transportation conditions. , temperature and suitable sugar to acid ratio.

(4) Other flavor changes

The burnt taste of yogurt is caused by excessive heat treatment or the addition of condensed milk or milk powder with poor flavor; the bitter taste comes from the contamination of protein-decomposing bacteria or the long-term storage of yogurt; the unpleasant and non-slippery taste is due to the excessive mucus produced by fermentation. , and the second is insufficient production of lactic acid.

3. There are bubbles or sandy texture on the surface of the finished product

There are many sand-like particles in the appearance of yogurt tissue. Although these particles do not affect the taste and quality, their appearance is not smooth, which affects consumers' purchasing psychology and product sales.

(1) Too much starter or uneven distribution

Some studies believe that the low activity of the starter and the inoculation of too much starter will lead to the formation of sand on the surface of the product. If the bacteria are not stirred in time or the stirring time is not enough after receiving the bacteria, the bacteria will not be fully dissolved, resulting in the fermentation process. Regions with high unit activity produce acid too quickly, resulting in protein aggregation, as well as particles and pseudoprotein denaturation points that often occur in production. The bacteria can also be dissolved in sterile water in the early stage and then added to the culture tank, but this will increase the probability of contamination, and aseptic protection must be done well.

(2) Floating of fat particles

In the pretreatment stage of raw milk, it is necessary to go through the step of homogenization. If the homogenization conditions are not suitable, the milk is not completely homogenized, and the fat particles will float up in the later stage.

(3) Temperature and speed of demulsification

Sometimes in order to speed up the cooling of yogurt, stirring the demulsification just after the fermentation reaches the end point acidity may cause the product to feel bad. Some studies believe that the appropriate stirring temperature should be below 18°C, and increasing the stirring speed can reduce the surface particles of the product to a certain extent. However, if you stir too vigorously, air bubbles will appear.

(4) Insufficient sterilization

Air bubbles caused by yeast or Escherichia coli metabolic activities, incomplete sterilization of processing equipment, pipelines and environments, or personnel errors and poor technical management may cause bacterial contamination. The lactic acid degree of the raw material with a higher number of miscellaneous bacteria will increase, and even in the raw milk treated at high temperature, there are still bacterial spores in it. In the process of yogurt fermentation, when the temperature is suitable, these miscellaneous bacteria will produce acid and gas, causing the product to be clotted, rough and porous, and the clot to be suspended, resulting in quality defects of the product.

4. Whey precipitation

When producing yogurt, because lactic acid bacteria decompose lactose to generate lactic acid, the casein in milk changes from the original colloidal structure to a network-like gel structure, which wraps the moisture and fat in the milk to form a clot. When the process conditions are right, the water and fat are locked in the curd network; when the curd particles are contracted or destroyed by shock and become smaller, the water flows out of the casein network structure, causing the whey to separate out. Rapid acidification of milk and higher fermentation temperature in yogurt production are the main reasons for whey precipitation.

(1) The quality of raw milk is not up to standard

The low dry matter and low fat content of raw milk will inevitably lead to low viscosity of yogurt and precipitation of whey. Therefore, when selecting raw materials, it is necessary to taste the taste and support the test indicators.

(2) Improper heat treatment of raw milk

The heat treatment of raw milk in the production of yogurt not only has a bactericidal effect, but also enables whey protein to participate in the formation of a casein network structure. After denaturation of whey protein, the hydrophilic bonds will be expanded and the water holding capacity of whey protein will be enhanced. During the coagulation process, casein and whey protein can interact, so that free whey protein can be fully utilized. If the heat treatment temperature is too low or too high, the whey protein will lose its water-holding function due to incomplete denaturation or excessive denaturation. According to research, in order to ensure that yogurt absorbs a lot of water and does not cause syneresis, at least 75% of the whey protein should be denatured.

(3) The fermentation time is too long or the temperature is too high

As the fermentation time prolongs, the lactic acid bacteria will continue to multiply, which will increase the acidity. Excessive acidity will destroy the colloidal structure that has been formed, so that the water combined with each other will be freed to form whey; similarly, if the fermentation temperature is too high, the milk will rapidly produce acid causing whey to separate out.

(4) The demulsification time is too early

Stirring the product before the fermentation has reached the end acidity will destroy the curd state of the product prematurely. At this time, the colloidal structure of the milk protein has not been fully formed, and the original water in the milk cannot be encapsulated, or the whey is precipitated due to the low water-holding capacity. Therefore, try to avoid milk breaking in the early stage of curdling.