Intermediate moisture foods (IMF) are shelf-stable products that have water activities of 0.6-0.85, with a moisture content ranging from 15% - 40% and are edible without rehydration. These food products are below the minimum water activity for most bacteria (0.90), but are susceptible to yeast and mold growth. Historically, ancient civilizations would produce IMF using methods such as sun drying, roasting over fire and adding salt to preserve food for winter months or when preparing for travel. Currently, this form of processing is achieved by using one of four methods: partial drying, osmotic drying using a humectant, dry infusion and by formulation. A variety of products are classified as IMF, such as dried fruits, sugar added commodities, marshmallows, and pie fillings.
Shelf-life and safety The purpose of IMF foods is to achieve a water activity that the food can be stored safely without refrigeration. However, the food is not sterile. Staphylococcus aureus is a microorganism of concern as it can grow and produce specific enterotoxins in water activities of 0.83-0.86 under aerobic conditions. Because of this, proper handling, storage, hygiene and good manufacturing practices are necessary to prevent Staphylococcus aureus. Molds of Aspergillis and Penicillium species can grow and produce harmful mycotoxins at water activity 0.77-0.85. Salmonella and Bacillus cereus are the primary pathogens of concern with low-moisture foods and IMFs. Most illnesses associated with low-moisture foods or IMFs have been caused by Salmonella spp. To reduce the risk of bacterial growth, products are treated with a combination of low pH, addition of sugar, salt and preservatives, and a thermal process that can eliminate pathogens and extend shelf-life. In the case of yeasts and molds, chemical preservatives such as sorbates and propionates are used to inhibit their growth.
Processing
Partial drying To achieve 0.6-0.84 water activity in food products, partial drying is employed for raw food that naturally have a high amount of humectants such as raisins, apricots, prunes and sultanas. Humectants are solutes (such as sugar or salt) that immobilize water in food. The drying process removes free water, and the humectants in the product bind the rest of the water, not allowing it to be utilized for chemical reactions or for microbial use.
Osmotic drying using a humectant Osmotic dehydration is the process of soaking food in highly concentrated solutions of humectant. Salt and sugar are commonly used humectants for this process. Water diffusion from the food to the humectant solution is caused by osmotic pressure. The water is replaced by the humectant, which results in a lowered water activity for the food product. Osmotic dehydration process results in two way mass transfer in regards to the moisture lost and the solids gained, with moisture loss being much greater than the addition of solids. Advantages of osmotic dehydration include low processing temperatures, short drying times, and 20-30% lower energy consumption than typical dehydration processes. Sugar is used as the humectant for candied intermediate moisture fruits, and salt is used for intermediate moisture vegetables and fish. Additionally, a mixture of humectants can be formulated to manipulate the sensory properties of the food product. Osmotic drying using a humectant results in a soft texture in the final product.
Dry infusion Dry infusion is the combination of partial dehydration and osmotic dehydration using a humectant. The food product is first dehydrated and then the resultant product is added to a humectant solution to reach the desired water activity. This method is desirable because it results in a higher quality and more appealing product. However, more energy is used for this method because it is two processing steps combined. Dry infusion is primarily employed by the U.S military and NASA for production of IMF to produce safe, palatable food that can be consumed much later than it is produced.
Formulated intermediate moisture foods Many types of food are specially formulated to achieve water activity in the IMF range. Food ingredients are mixed with salt and/or sugar, and additives (such as propylene glycol and potassium sorbate) and then subjected to processing methods such as cooking, extrusion or dehydration to result in an intermediate moisture final product. Examples of formulated IMF are confectioneries and pet food.
Applications
Fruits and vegetables
Sugar is added to fruit to protect against microbial contamination and reduce water activity in the fruit. This allows the fruit to be more stable at room temperature. Some examples are strawberries, prunes, peaches, apricots, and pineapples. IMF blueberries are prepared by osmotic dehydration. They are soaked in sugar for one to two days followed by a freeze drying process until the desired moisture level is reached.
Meat Fermented meats, sausage, jerky, and corned beef can last many months without refrigeration. Pastirma is a beef product that is often eaten raw in the Middle East and Mediterranean countries. It is made from the hindquarter of beef cattle. Pastirma is a type of intermediate moisture food and can be stored for several months in humid climates. The meat is salted and dried to reduce water activity and increase microbial safety. Additionally, nitrites are added for preservation. The final product has 5% salt and a moisture content between 30-35%.
Pet food Semi-moist pet food such as chewy dog treats and soft cat treats are shelf-stable, soft and do not have a high moisture content. Ingredients added to intermediate moisture pet food to achieve lower water activity are soy flakes and wheat flour in addition to solutes such as glycerol, salt, and sugar. Processing techniques such as extrusion are employed to attain the final intermediate moisture pet food. Intermediate moisture pet food are convenient products because they leave less odor and are less messy than canned wet pet food. Additionally, they have been found to be more palatable to pets than dry pet food products.
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