Moisture in a food is the basic parameter controlling multifaceted quality of the food by regulating chemical and structural changes over processing and storage. Controlling and managing moisture availability in the food and components in a food system is vital. Humectant is a class of food additives used to stabilise the food products and increase the shelf life through moisture control. They have the characteristic ability to bind with water or moisture available in the food and thus prevent the moisture loss, keeping the food ideally moist.

 Humectants have a long history of widespread use in food, sugar and salt being the oldest and most commonly used in old days. Other commonly used humectants are honey, egg whites, molasses, glycerol, sorbitols, prolypropylene glycol, sugar alcohols and so on. The overall use of humectant in food industry has grown significantly in the recent years owing to the consumers’ inclination towards nutritional and functional foods and beverages along with sugarless additives and no chemical preservatives.

A humectant, when added in food binds the free water molecules, making them unavailable for the microbial cells. Moisture deprived cells experience osmotic stress. This inhibits the microbial growth and can also cause cellular damages. However, microorganisms like yeasts and moulds have their mechanisms for coping with osmotic stress and can withstand very low water activity levels. Therefore, identifying the aw of food products and choosing the right kind of humectant is very crucial for extending shelf life of foods. aw levels also decide the growth of specific microorganisms. For example, aw of 0.95 controls species of Pseudumonas, Bacillus, Shigella and some Clostridium species in highly perishable foods like fresh fruits and vegetables. aw value of .91 controls Salmonella, Lactobacillus and Clostridium botulinum  in stored meats and cheeses. aw value of 0.87 controls yeast in sausages, cakes and dry cheese. aw value of .85 with pH lower than 4.60 is considered not hazardous, therefore not requiring refrigeration to inhibit the growth of pathogenic microorganisms.

The ability of a humectant to lower water activity depends on its chemical composition and right dose. Humectant compounds have hydrophilic groups attached to their structures which create ready tendency for binding water molecules. However, excessive loss of moisture directly affects the texture of the food. The existing aw and the targeted aw are the principal controlling factors while selecting the right humectant and its dosage. The humectant should be added at a specific concentration to attain the targeted aw. Certain humectants can also cause unintended effect on the food products. For instance, in a thermally processed product containing protein, lowering the water activity to 0.7 by using reducing sugars as humectant would cause Maillard browning.

Some humectants like honey or molasses can impart sweetness and strong unintended colours to the product. Salt or sugar can be also disadvantageous if used at concentrations above saturation points, as that would cause granulation. To counteract these different challenges, many manufacturers use combinations of humectants at low concentrations. Also humectants that are chosen should be easy to blend and economical to use. Dry infusion and moist infusion are two widely used methods of adding humectants to foods. Dry infusion involves initial dehydration of the food product and then the resultant product is added to a humectant solution to reach the desired water activity. With moist infusion, the food product is directly soaked in humectant solution to reduce its water activity.

When developing a new product or extending a new product line, developers can take the following approach for deciding the type and amount of a humectant or multiple humectants to be used for attaining the desired aw.

Find out the current awof the product
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Set the targeted aw to be attained
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Identify the potential humectants
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Determine reactivity of each humectant in the food matrix
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Predict aw change through calculations based on moisture sorption isotherms

The global humectant market is segmented into synthetic and natural. A few of the most popular humectants that being used and studied for applicability in processed food industries for their microbial growth inhibiting property are sorbitol, glycerol, invert sugar, maltodextrins, maltitol, sucrose, trehalose, polydextrose and corn syrup. However it is evident that overconsumption of sugar alcohols (glycerol, sorbitol, maltitol) could relate to bowel upset syndromes like bloating, diarrhoea and flatulence as they are not absorbed in the small intestine of the human gastrointestinal tract. Controlled use of individual humectant or use in combination of two or more, considering limited alteration in physical and chemical nature of food can be recommended.

The code of Federal Regulations (title 21, 2018) of USFDA mentions the regulations related to the use of humectants in food products. Although synthetic segment is on high demand due to its cost-effectiveness but extensive research is going on in extraction and utilisation of natural humectant owing to customer preferences towards natural ingredients. Furthermore, increase in consumer’s demand and affinity towards high shelf life foods like ready-to-eat (RTE) meals has increased the use of humectants in various food products.