In addition to being the most abundant element in the world, water is the universal solvent. Water is the single most important component in ensuring food is kept fresh and safe. How? What is it? Let's find out.
There are two types of water in food: those that are free to move about and those that are held in place by other substances. Water that is unable to flow freely is of no use to living things. Most of the time, it is found in a bonded state with other nutrients like carbs and proteins. When water is bound, it cannot participate in metabolic activities or support microbial development.
Moisture content is a prominent word that comes up in discussions regarding the storage and transport of perishable foods. Indicative of the overall quantity of water present in the meal, moisture content comprises both free and bound water. On the other hand, a product's shelf life may be prolonged by careful management of its water activity.
Water Activity
Water activity, in layman's terms, is the percentage of total water in a meal that is unbound to any other components and hence accessible for microbial development and chemical and biological processes. Specifically, it may be anywhere from 0 to 1. (pure water). As defined by science, it is the ratio of the vapor pressure of water in the food product to the vapor pressure of pure water at the same temperature.
Scientists in the 19th century made the connection between a food's moisture content and its shelf life. Through time, they learned that the availability of clean water is more crucial to maintaining the freshness of food than the quantity of water available. Hence, knowing the water activity of a medium was useful for limiting spoiling caused by microorganisms.
The water activity of a food product is a key factor in determining its microbial growth and the stability of its chemical, biochemical, and physical qualities. In the event of perishable or high-moisture foods, microbial deterioration is a major cause for alarm when it comes to food safety.
Microorganisms would not be able to thrive without water's help. Water is essential for microorganisms since it is used for digesting food, flushing out waste, and keeping the cells themselves in good form. Hence, microbial deterioration might occur, and food-borne diseases could be spread when germs are given enough quantity of water.
The next question is, "How might water action aid us in preventing the development of microbes?" Water activity may be used as a proxy for the minimal quantity of free water accessible for microbial development, such as that required by yeasts, molds, and bacteria. We refer to this as the "limiting water activity level," or the point at which further microbial growth is inhibited. Hence, optimal moisture is necessary for the development of all microorganisms. So, understanding the water's activity may aid in pinpointing the origin of any deterioration. The HACCP plan in the food industry uses this as a crucial component.
Foods may be divided into low moisture (water activity 0.60), intermediate moisture (water activity 0.60-0.85), and high moisture (water activity > 0.85) categories based on their water content. The water activity level correlates with the likelihood of deterioration. Water activity levels greater than those needed for growth by fungus are required by bacteria. Zero growth of microbes is seen below a pH of 0.60.
Managing the water activity of a product may also aid in maintaining its chemical stability. Several biological processes need water as a solvent or a reactant. This means that water activity influences the non-enzymatic browning (Millard reaction), lipid oxidation, protein denaturation, starch gelatinization, and retrogradation processes. The pace of these reactions slows down when water activity drops.
The water activity of a food product also modifies its structural attributes. As compared to meals with a lower water activity, which tend to be hard and dry in texture, those with a higher water activity may be soft and juicy. Changing the water activity of these goods up or down might alter their feelings.
Since different parts of a multi-part product have different water activities, moisture migration is possible. When the water moves from the component with the higher water activity to the one with the lower water activity, the final product undergoes physical changes. Take bhel, for instance, where the puffed rice, or murmura, eventually turns mushy because of the other wet components, such as onion. Water activity may vary even when two components have the same moisture level. Moisture transference between two components is prevented if their water activities are equal. Powdered goods are more susceptible to caking and losing their rehydration capabilities if their water activity is enhanced.
In a similar vein, moisture may transfer from the air to the meal. This occurs because of the correlation between water activity and relative humidity. Air with a relative humidity of 80% has a water activity of 0.80, which is perfectly balanced. Biscuits go soggy if left out in the open. To what end does that occur? Because biscuits absorb moisture from the air. If the relative humidity is 85%, and the water activity of a biscuit is 0.75, the biscuits will absorb water from the air and get soggy. When food is dried, the opposite effect occurs.
The shelf life of a product is directly related to its water activity. The stability of an item is affected by the rate of change in its water activity over time. Hence, a high-quality, shelf-stable product is achieved by the regulation of water activity, which inhibits the development of vegetative cells, spore germination, toxin generation, and physicochemical changes.