Detailed explanation of the most popular food spoi

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Detailed explanation of food spoilage and its control technology (I)

1 Food corruption and deterioration

microorganisms are widely distributed in nature. Food will inevitably be polluted by a certain type and number of microorganisms. When environmental conditions are suitable, they will grow and reproduce rapidly, causing food corruption and deterioration, which not only reduces the nutritional and hygienic quality of food, but also may endanger human health

food spoilage refers to the process in which food is affected by various internal and external factors, resulting in changes in its original chemical or physical properties, reducing or losing its nutritional value and commodity value. Such as the putrefaction of fish, the rancidity of oil, the decay of fruits and vegetables, and the mildew of grain

there are many reasons for food spoilage, including physical, chemical and biological factors, such as the role of enzymes in animal and plant food tissues, and the pollution of insects, parasites and microorganisms. Among them, food spoilage caused by microbial contamination is the most important and common, so the sample: 50mm wide, 300mm long, 20 pieces in radial and latitudinal directions. This chapter only discusses the problem of food spoilage caused by microorganisms

1.1 basic conditions of food deterioration caused by microorganisms

food processing 15. Automatic storage: experimental conditions, test results, test curves and data can be automatically stored before the raw materials always contain a certain number of microorganisms; During and after processing, the finished products are inevitably exposed to microorganisms in the environment. Therefore, there are many kinds and quantities of microorganisms in the food, from general-purpose fixtures to special fixtures for wire rod test pieces, as shown in the photos. However, whether microbial contamination of food can lead to food corruption and deterioration, and the degree and nature of deterioration are affected by many factors. Generally speaking, food spoilage is closely related to the nature of the food itself, the type and quantity of polluting microorganisms and the environment in which the food is located, and the three factors interact and influence each other

1.1.1 matrix characteristics of food

⑴ nutritional components of food

food contains rich nutritional components such as protein, sugar, fat, inorganic salt, vitamins and water, which is a good culture medium for microorganisms. Therefore, microorganisms can easily grow and reproduce rapidly after contaminating food, resulting in food deterioration. However, in different foods, the proportion of the above ingredients varies greatly, and the ability of various microorganisms to decompose various nutrients is different, which leads to different microbial groups causing different food corruption, such as meat, fish and other protein rich foods, which are prone to be contaminated by Proteus, Penicillium and other microorganisms with strong ability to decompose protein and cause corruption; Rice and other foods with high sugar content are easy to be polluted by Aspergillus, Rhizopus, lactic acid bacteria, beer yeast and other microorganisms with strong ability to decompose carbohydrates; However, foods with high fat content are prone to rancidity and deterioration due to contamination by microorganisms with strong fat decomposition ability such as Aspergillus flavus and Pseudomonas

⑵ hydrogen ion concentration of food

all kinds of food have a certain hydrogen ion concentration. According to the characteristics of the pH range of food, food can be divided into two categories: acidic food and non acidic food. Generally, those whose pH value is above 4.5 belong to non acidic food; Those with pH value below 4.5 are acidic foods. For example, the pH value of animal food is generally between 5 and 7, and the pH value of vegetables is between 5 and 6. They are generally non acidic food; The pH value of fruit is between 2 and 5, which is generally acidic food

all kinds of microorganisms have their most suitable pH range. The concentration of hydrogen ions in food can affect the nature of the charge on the cell membrane. When the charge properties on the microbial cell membrane are changed by the concentration of hydrogen ions in food, the absorption mechanism of some substances by microorganisms will change, which will affect the normal metabolic activities of cells and the role of enzymes. Therefore, the pH value of food is one of the important factors that restrict the growth of microorganisms and affect the spoilage of food

the most suitable pH for the growth of most bacteria is about 7.0, and the pH range for the growth of yeast and mold is wide, so non acidic food is suitable for the growth of most bacteria, yeast and mold; The lower limit of bacterial growth is generally about 4.5, and only a few acid tolerant bacteria, such as Lactobacillus, can grow when the pH value is below 3.3 ~ 4.0. Therefore, the spoilage of acidic food is mainly the growth of yeast and mold

in addition, the pH value of food will also change due to the growth and reproduction of microorganisms. When microorganisms grow in the food matrix containing sugar and protein, microorganisms first decompose sugar to produce acid, which will reduce the pH value of food; When the sugar is insufficient, the protein is broken down and the pH value rises again. Due to the activities of microorganisms, the pH value of food substrate changes greatly. When the acid or alkali accumulates to a certain amount, it will in turn inhibit the continued activities of microorganisms

⑶ water in food

water is a necessary condition for microbial life activities. Microbial cells are indispensable to water. Water is the solvent for all kinds of biochemical reactions in cells. In the environment of water shortage, the metabolism of microorganisms is blocked, and even die. However, the water content required by the growth and reproduction of various microorganisms is different. Therefore, the water content in food determines the types of growing microorganisms. Generally speaking, food containing more water is easy for bacteria to multiply; Molds and yeasts are easy to reproduce in foods with less water

water in food exists in two forms: free water and bound water. When microorganisms grow and propagate on food, the available water is free water. Therefore, the water required for the growth and propagation of microorganisms in food does not depend on the total water content (%), but on the water activity (AW, also known as water activity). Because part of the water is combined with protein, carbohydrate and some soluble substances, such as amino acids, sugar, salt, etc., this kind of combined water is useless for microorganisms. Therefore, water activity is usually used to express the water that can be used by microorganisms in food

water activity (AW) refers to the ratio of water vapor pressure (P) and pure water vapor pressure (P0) of food in a closed container, that is, aw=p/p0. Aw of pure water = 1; The aw of anhydrous food is equal to 0, which shows that the aw value of food is between. Table 1 gives the range of the lowest aw value for the growth of different microbial groups. From the table, it can be seen that if the aw value of food is below 0.60, it is considered that microorganisms cannot grow. It is generally believed that the aw value of food is below 0.64, which is the mould proof water content for safe storage of food

table the minimum aw value range of the growth of major microbial groups in food

the minimum aw value range of microbial groups

most bacteria 0.99 ~ 0.90 halophilic bacteria 0.75

most yeast 0.94 ~ 0.88 hypertonic yeast 0.60

most mold 0.94 ~ 0.73 dry mold 0.65

fresh food raw materials, such as fish, meat, fruits, vegetables, etc., contain more water, The aw value is generally 0.98 ~ 0.99, which is suitable for the growth of most microorganisms. If it is not treated in time, it is easy to deteriorate. In order to prevent food deterioration, the most commonly used method is to reduce the water content of food and reduce the aw value below 0.70, so that it can be preserved for a long time. Many studies have reported that foods with aw values between o.80 and O.85 can only be preserved for a few days; Foods with aw value of about 0.72 can be stored for 2 to 3 months; If aw is below 0.65, it can be stored for 1 to 3 years

in practice, for convenience, the percentage of water content is often used to express the water content of food, and it is used as a measurement index to control the growth of microorganisms. For example, in order to achieve the purpose of preservation as a key link, the moisture content of milk powder should be less than 8%, that of rice should be about 13%, that of beans should be less than 15%, and that of dehydrated vegetables should be between 14 and 20%. Although the water content percentage of these substances is different, their aw value is about below 0.70

⑷ osmotic pressure of food

osmotic pressure has a certain relationship with the life activities of microorganisms. If the microorganism is placed in a low permeability solution, the bacteria will absorb water and expand, or even rupture; If placed in hypertonic solution, the bacteria will dehydrate or even die. Generally speaking, microorganisms have certain resistance in foods with low osmotic pressure and are easy to grow, while in foods with high osmotic pressure, microorganisms often die due to dehydration. Of course, different microbial species have different tolerance to osmotic pressure

most bacteria cannot grow in foods with high osmotic pressure, and only a few species can grow in hypertonic environments. For example, some species of Halobacillus can live in foods with 20 ~ 30% salt concentration; Leuconostoc mesenteroides can tolerate high concentrations of sugar. Yeasts and molds can generally tolerate high osmotic pressure, such as Hansenula anomala, Saccharomyces rouxii, Pichia membranafaciens, which can tolerate high sugar and often cause the deterioration of high sugar foods such as syrup, jam, fruit juice, etc. Among the molds, the prominent representatives are Aspergillus glaucus, Penicillium, Cladosporium, etc

salt and sugar are the main substances that form different osmotic pressures. Adding different amounts of sugar or salt to food can form different osmotic pressures. The more sugar or salt added, the higher the concentration, the greater the osmotic pressure, and the smaller the aw value of food. In order to prevent food from spoilage, salt and sugar are often used to preserve food for a long time

existing state of food

food in good condition is generally not prone to corruption, such as potatoes, apples, etc. without crushing and wounds, which can be stored for a long time. If the food tissue breaks down or the cell membrane breaks, it is easy to be contaminated by microorganisms and deteriorate

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