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Reducing moisture content in food is a common method of preservation that has been practiced since ancient times. Across cultures around the world, the science behind this preservation technique primarily revolves around inhibiting the growth of microorganisms and enzymatic reactions that cause spoilage. Since bacteria, mould and yeasts require pretty much the same conditions as human beings – food, water and oxygen, to grow and reproduce, reducing the moisture content in food can create an environment where these microorganisms are less likely to thrive.
Mainly classified by three factors – namely microbial growth, enzyme activity, and chemical reaction, microbes present in food need water for metabolic reactions, and without it, their growth is slowed or halted. The enzymes in food can catalyse chemical reactions that lead to spoilage, such as browning, change in the way an ingredient tastes and texture changes. Proper moisture control and packaging are critical factors in successful food preservation. Lowering the water activity of food can effectively denature these enzymes, rendering them inactive.
Many chemical reactions responsible for food spoilage require water as a reactant. To illustrate this further, the oxidation of fats and oils leading to rancidity is inhibited when moisture is reduced and so, by limiting water availability, you can slow down or prevent these undesirable chemical reactions. Various methods are employed to reduce moisture content in food, including drying, dehydration, freeze-drying and canning. Each method has its unique advantages and applications, but are directed to remove or reduce the water content in food to levels where microbial growth and enzymatic activity are inhibited.
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Water activity (Aw) – a crucial concept in food preservation, measures the availability of water in a food product for microbial and chemical reactions. Foods with low Aw values (typically below 0.85) are considered shelf-stable because they are less susceptible to spoilage. In food preservation, controlling and reducing Aw is one of the key objectives along with proper packaging, to maintain the reduced moisture content in preserved foods. Hermetically sealed containers, vacuum-sealed bags, and moisture-proof packaging materials are used to prevent moisture ingress and maintain the desired low Aw levels.
By reducing moisture content and employing suitable packaging techniques, the shelf life of many foods can be significantly extended since this allows for the storage and distribution of food products over extended periods without significant quality degradation. While reducing moisture content is effective for preserving many types of food, not all foods can be preserved in this way. Some foods, like fresh fruits and vegetables, rely on high moisture content to maintain their texture and quality. In such cases, other preservation methods, such as canning, pickling or freezing, are more suited to extend the shelf life and preserve the freshness of these products.
