Do you know the relationship between water activity (Aw) and food preservation?

Ancient people knew that drying some fresh food could extend its storage period. Later, it was discovered that the water content in food was not directly related to the stability and safety of the food, but rather to the "state" of the waer. In other words, the "availability" of water in food was related to the stability and safety of the food, thus introducing the concept of water activity.

Water activity is the ratio of the vapor pressure of water in food to the vapor pressure of pure water at the same temperature. Expressed as Aw, it does not indicate the strength of water molecule bonding in food, but rather represents the average properties of a system.

Water activity is an important indicator in food quality control, and it is extremely useful when considering the relationship between the preservation quality of a food and microbial spoilage.

It ust be emphasized that water activity is different from relative equilibrium humidity. Food water activity indicates the inherent properties of food and is related to its composition and structure, while relative equilibrium humidity is related to the properties of the atmosphere when food is in equilibrium.

01

The relationship between water activity and microorganisms

The water activity in food can affect the reproduction, metabolism (including toxin production), resistance, and survival of microorganisms in food. Therefore, water activity is not only related to harmful microorganisms that cause food spoilage, but also has an impact on beneficial microorganisms required for fermented food.

① The impact on microbial reproduction

The Aw value not only affects the spore germination of microorganisms, but also has an impact on the growth and reproduction of bacteria. Most food related microorganisms grow well under high Aw values, with only a few requiring growth under lower Aw values.

Therefore, if the Aw value is reduced, the types of microorganisms that can reproduce in food will decrease. Different types of microorganisms have different requirements for Aw values, and bacteria have the highest requirements for water activity. Only when Aw>0.9 can they grow and reproduce; Next is yeast, which requires an Aw>0.87, followed by mold, which begins to reproduce at an Aw of 0.8. In addition, microorganisms of the same genus but different species do not have completely the same requirements for Aw.

Table 1 Minimum Water Activity Required for Microbial Growth

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Although people know the minimum Aw values for the reproduction of different microbial species, most of these values are obtained under laboratory conditions, that is, under conditions where only the Aw value is controlled and all other growth conditions are close to the optimal value. Such conditions are difficult to achieve for food, so literature data is a limit value for reference.

Compared with the food produced by adsorption (rehydration) method, the microorganisms in the former have a stronger tolerance to low Aw, indicating that the adsorption process can improve the stability of the food. However, the food produced in this way is relatively expensive.

In addition, when glycerol is present, certain microorganisms can grow, twin, and form spores at lower Aw values, which are much lower than using NaCl or sucrose as wetting agents. This is significant for moderate humidity foods.

If salt and sucrose are used as wetting agents, the maximum Aw value for most Gram negative bacteria that cause food spoilage is 0.95, and for Gram positive bacteria it is 0.90; For Staphylococcus aureus, the threshold is 0.9 under anaerobic conditions and 0.86 in air. When the Aw value is below 0.60, all microorganisms, including high osmolarity tolerant yeast and early-growth mold, will be inhibited.

② Reduced impact on microbial metabolic activity

The Aw value can reduce the growth rate of microorganisms, leading to a decrease in food spoilage rate, food toxins, and microbial metabolic activity. However, the water activity required to halt different metabolic processes varies. For example, the Aw value required for bacteria to form spores is higher than the value they grow. In fact, the twin process of spores can occur below their growth threshold.

The production of toxins is the most closely related microbial metabolic activity to human health. When Clostridium grows, toxins are formed. The minimum Aw values that can maintain its growth and toxin formation are 0.95 for Class A, 0.94 for Class B, and 0.97 for Class E. These values were measured using NaCl as the wetting agent, and if glycerol is used as the wetting agent, the values are even lower.

The growth of Staphylococcus aureus and the formation of enterotoxins are also related to Aw. Increasing the concentration of NaCl can inhibit bacterial growth and the formation of enterotoxins, resulting in a stronger inhibitory effect on enterotoxins. Reducing the Aw value has a greater impact on enterotoxin B than on enterotoxin A.

There are at least 200 types of mycotoxin produced by many molds, so the relationship between Aw and mold growth and toxin production is complex. It is generally believed that the water activity required for the growth of toxic molds is lower than that required for toxin formation. In addition, due to the production of metabolic water, the growing mold can increase the Aw value of the growth environment. Therefore, in foods containing toxic bacteria or molds, the presence of toxins is highly likely,

③ The impact on microbial heat resistance

Heating is a commonly used and effective method for inhibiting or killing microorganisms in food, and the heat resistance of different microorganisms and their spores varies.

Among the various factors that determine the heat resistance of bacteria, the physical properties, chemical composition, and Aw value of the hot solvent are all important. Generally speaking, the heat resistance of bacterial spores increases with the decrease of Aw value, and is strongest in the range of Aw 0.2~0.4. For example, in nutritional microorganisms, reducing the Aw value of suspended media and significantly increasing thermal resistance are prominent when using NaCl as a wetting agent. Sometimes the thermal resistance of bacteria in high concentration solutions is lower than in dilute solutions, because the solute itself can exacerbate the thermal damage of cells during the heating process.

④ The impact on the survival ability of microorganisms

Microorganisms that cannot grow will gradually die. Therefore, if the Aw value of food is lower than the minimum value for microbial growth, the number of microorganisms will gradually decrease. In low Aw conditions, Gram positive bacteria have a stronger survival ability than Gram negative bacteria.

In frozen foods (Aw<0.10), the number of microorganisms increases as the Aw value decreases, which is rare in foods with certain humidity. Protective substances such as proteins, non reducing sugars, or glycerol can increase the number of bacteria during food storage. Non esterified fatty acids or preservatives can accelerate bacterial death. In addition, low pH, low oxygen and dark light, high humidity, etc., lead to a large number of microbial deaths in dry food.

The study on the relationship between the survival and Aw of food toxic microorganisms such as Salmonella, Staphylococcus aureus, and Cl.Sotulinum proves that the germination of bacterial spores requires a lower Aw value than the reproduction of nutritional bacteria. Therefore, in foods with low Aw values (allowing spore germination but preventing nutrient bacteria from growing), the spore count will decrease, and such foods may even become sterile during storage.

The survival of parasites in food is also affected by low Aw values, which can be killed during freezing or drying processes. When studying the survival of Trichinella spiralis in meat during the drying process, it was observed that in fermented sausages, these parasites become inactive when the Aw value is 0.949~0.931, while in raw ham, the Aw should be 0.948~0.904.

From the above, it can be concluded that by selecting appropriate conditions (Aw value, pH value, humidity, preservatives, etc.), microorganisms can be reduced or killed, thereby improving food stability and safety.

02

Water activity of food

Reducing the Aw value can improve the stability and safety of low moisture foods (Aw=0.0~0.6) and medium moisture foods (Aw=0.6~0.9), but this phenomenon is not significant in high moisture foods (Aw=0.9~1.0). The Aw value not only helps with the preservation of traditional foods, but also contributes to the preservation of modern new foods.

To reduce the Aw value in food, methods such as drying, dehydration, concentration, or adding wetting agents are generally used. After food is dried or concentrated, solutes will accumulate in the residual water, which can reduce the Aw value. The wetting agents used in traditional foods are mainly NaCl and sucrose, which are added to the food in the same proportion. Among them, NaCl has three times the ability to reduce the Aw value compared to sucrose.

The main wetting agents used in new types of food include glycerol, sorbitol, lactate, propylene glycol, etc. Adding low water content components such as starch, milk powder, or soy protein to food can also reduce the Aw value. In meat products, increasing the amount of fat added can also reduce the Aw value because the water content in fat is very low, only 5-10%, while the water content in meat is as high as 70-75%.

Aw has a promoting effect on the shelf life, stability, and safety of dry foods or foods with added wetting agents, but the degree of impact varies for different foods.

High moisture foods, such as fresh milk, meat, fish, vegetables, fruits, etc., are easily infected with Gram negative and Gram positive bacteria or rapidly growing molds. Similar situations also occur in foods containing trace amounts of salt, such as beef sausages. The preventive measures are refrigeration or heating treatment.

Foods within the Aw range of 0.95 to 0.90, such as baked goods, fermented cheese, salted butter, fermented sausages, raw ham, pickled vegetables, and concentrated dried fruit juice, can all have their shelf life extended.

These foods can maintain freshness by drying and adding sodium chloride, sucrose, and other methods. After fermentation of high moisture foods, pH value, competitive bacterial communities, and air infiltration will have a greater impact on food stability.

Medium moisture foods, such as aged cheese, hard sausages, cooked ham, pickled fish, jam, fruit juice gummies, and dried fruits, are preserved by adding an appropriate amount of salt or sugar and drying. The spoilage of medium moisture foods is mainly caused by early resistant fungi or high osmolarity resistant yeast, while spoilage caused by salt tolerant bacteria is rare. Therefore, adding sorbic acid, sulfur dioxide, and other substances to these foods can inhibit the spoilage bacterial community.

Low moisture foods, arranged in order of Aw value from high to low, mainly include chocolate, candy, honey, cocoa, cake, milk powder, flour, and dried vegetables. Bacteria in these foods are difficult to grow. These products reach very low Aw values after drying, but excessive dehydration should be avoided. Wetting agents play a certain role in the stability of the above foods, but in most cases, dehydration plays a decisive role.

03

Obstacle effect and its application in food preservation

The stability and safety of most foods do not solely depend on low Aw values, but also on other influencing factors. In practical work, Aw values should be considered in conjunction with other factors.

In the food industry, the "barrier effect" refers to the comprehensive factors that affect the quality of food storage, including heating, cooling, water activity, acidity, deoxygenation, preservatives, and competing microbial communities. The concept of "barrier technology" is derived from the "barrier effect".

In order to achieve the goal of food preservation, adopting a method to solve one of the above factors often fails to achieve the desired effect. "Barrier technology" is the comprehensive use of several methods, such as heating treatment, adjusting pH value and deoxygenation, adding preservatives, selecting appropriate water activity, etc. The degree and amount of use of each method cannot reach the limit standard for individual use, but due to the synergistic effect of various methods, the final preservation effect is relatively ideal.

The activity of microorganisms in food during fermentation or storage is not static but dynamic, so microbial prediction is also important for food preservation.

When considering the intrinsic barriers of a food product, it is necessary to understand the growth curves of spoilage bacteria (such as in vacuum packaged meat products) or food toxic bacteria (such as in sausages) during storage, as well as the residual curves of food toxic bacteria in low moisture foods (dry or frozen products). In the research and development of new foods, barrier effects and predictive microbiology can be used for product design and quality control, and to develop hazard analysis and critical control points for food.

The barrier effect is usually applied to the development of foods with medium and high moisture content, such as meat products with medium moisture content. During the processing, gentle heating is used, and sodium chloride or sucrose is added to reduce the Aw value. After appropriate drying and comprehensive treatment, the finished product only contains very few microorganisms.

Preventing reinfection after meat processing is a key issue in meat preservation. Setting the Aw below 0.69, combined with the presence of Mirad reaction products in meat, can inhibit the growth of mold during storage and rapidly kill Salmonella and Staphylococcus.

High moisture meat products that are not stored in a frozen environment can be divided into several types: F-spp (shelf stable products made by heating), Aw spp, and pH spp. For all of these products, the Aw value is equally important. Adjusting the Aw value to 0.97 or 0.96, and the pH value to<6.5, F-spp high-pressure sterilized sausages (F>0.4) can have lower oxidation-reduction electromotive force and become sterile during storage because the remaining spores, although germinating, do not grow asexually and microorganisms do not grow, resulting in death.

Aw spp only needs to be heated to 78 ℃ during the processing, with an Aw value of less than 0.95, and only a small amount of bacterial spores remain and putrefactive toxic microorganisms are blocked.

Adjust the pH of pH spp to 5.4~5.6, accompanied by a gentle heating process (80 ℃), in order to prevent damaged bacterial spores and non activated bacteria from re infecting the product. These processes are carried out after packaging. Fermented sausages also use barrier technologies such as adding nitrite, deoxygenation, competing bacterial communities, adjusting pH and Aw heating to ensure product stability and safety. Obstacle technology has been widely used in meat processing.