How important is moisture activity to shelf life?

Definition of Moisture Activity

Moisture activity is an important factor in the shelf life of food products, and we must take this characteristic into account in the design of product formulations. Moisture activity affects the shelf life of food products to a greater extent, so it is a necessary consideration in food preservation control. So, what is moisture activity?
Simply put: Moisture activity refers to the state of moisture in a sample, i.e. the degree of water binding.
Strictly speaking: moisture activity Aw is the ratio of the water vapor pressure P displayed by the food at a certain temperature to the pure water vapor pressure Po at the same temperature, i.e.: Aw=P/Po.
The difficulty of removing water from food is related to the form in which it exists in the food. There are actually two forms in which water exists in food: (1) Free water – this part of the water maintains the physical properties of water itself, and can act as a dispersant and solvent for colloids.
The most free water is pure water or distilled water, and the content of free water in food can be measured directly by drying method.
(2) Bound or bound water – this part of water is tightly bound, such as honey, water in plant and animal cells, present in cell walls or protoplasm, and firmly bound to proteins; chemically bound water, such as lactose monohydrate, sodium dehydroacetate, and so on. It is not possible to measure the water content inside by drying.
Data on product moisture and water activity can provide some judgment, but there is no necessary proportionality.
Relationship between water activity and microorganisms

Water that can be utilized by microorganisms is free water. The tight binding of water to other ingredients in the food reduces microbial growth and chemical reactions that lead to decomposition and spoilage. Water activity Aw is also an important quality indicator for sensory evaluation, such as hard or soft, crunchy or sticky, and other sensory properties.
A high water activity and a low degree of water binding in the food means that there is more water available for microbial growth and reproduction, and conversely, there is less water available for microbial growth and reproduction, which inhibits the growth of microorganisms. This is one of the most important reasons why water activity can affect the shelf life of food.
Table 1 Range of minimum Aw values for growth of major microbial taxa in foods

Foods with a moisture activity of 0.60 to 0.85 are medium moisture foods, which do not require refrigeration to control pathogens, but have a limited shelf life due to spoilage caused primarily by yeasts and molds. For most of the moisture activity of 0.6 below the food, there is a longer shelf life, also do not need refrigeration, these foods are called low-moisture food.

Since moisture activity has a significant impact on microorganisms, it is also a very important guide for our daily microbiological testing, because most species of microorganisms have a minimum threshold of moisture activity for growth and reproduction.
In general, the moisture activity of the food is lower than this threshold, the microorganisms in the food will be difficult to grow. And once we know this threshold, we can also judge the scientific validity of our test results. Methods to reduce moisture activity

1. Extend the heating time. For example, bakery products, due to the temperature in the oven will have differences, resulting in products in the baking process in different parts of the moisture activity has a large difference, the stability of the oven temperature, the stability of the food to play an important role.
2. Increase the osmotic pressure, generally for the method of increasing the sugar level. Since ancient times, sugar and salt are the most traditional and effective method of preservation, but with the market demand for low-sugar products, resulting in insufficient sugar, food microbial growth accelerated. It is a very effective method to increase the sugar level by using low sweetness sugar under necessary conditions. Generally microbial growth is inhibited at sugar concentrations above 50%. But some of the permeability of yeast and molds, in the sugar concentration of up to 70% or more can still grow.
3. Syrup or liquid sugar alcohol to replace part of the water. Many varieties of syrup on the market, each syrup taste, water activity, physical and chemical properties are very different, according to the taste of the product, the state to choose the right syrup.
4. Increase water-absorbing substances such as colloid and modified starch in the formula to lock up the free water. Under the premise of executing according to GB 2760, through the reasonable combination of one or several thickeners, it can effectively reduce the water activity, enhance the taste, improve the organization and slow down the aging problem of the noodle products.
Determination of water activity
At a certain temperature, using the sensor in the Aw tester device, according to the change of vapor pressure of water in the foodstuff, the water activity shown by the pointer can be read from the display of the instrument.
A word of caution: the instrument can sometimes be inaccurate, so please remember to calibrate the machine using standard reagents.
In conclusion, water activity (Aw) is a very important fence factor, we can judge the water activity of the product to determine the risk of microbial growth during the shelf life, so as to develop a reasonable preservation program.