Food Preservation: Drying Methods

Food drying is categorised as a preservation method that has been used worldwide since ancient times. It is mainly based on the removal of water present in the food, namely, evaporation. The application of food drying primarily facilitates extending the shelf-life of the dried foods. In other words, drying helps protect the quality and nutritional attributes by curtailing the growth of microorganisms as well as minimizing the activity of intrinsic enzymes and hence reducing the respiration and over-ripening of foods.

A wide range of food varieties is currently subjected to drying operation in the food industry. Among the most commonly used food commodities, fruits, vegetables and meats are extensively preserved through drying technology. Dried fruits, including raisins, prunes, banana chips, pineapples, apricots have significant level of sugar that highly contributes to longer shelf-life after drying application. Vegetables such as onions, garlics, carrots, peppers and peas are increasingly consumed internationally. The texture of the dried vegetables becomes brittle and the moisture content reaches up to 10% that helps restrain the growth of microorganisms. Dried meats are commonly made from various species of livestock, game and poultry. Examples include, beef jerky, biltong, bresaola, ham, pemmican and sausages. A variety of seafood species have been long preserved by means of drying techniques for centuries. These include cod, mackerel, herring, pollock, haddock, cuttlefish, shrimp and scallop.

In advance of drying operation, pre-treatments are mostly applied to protect the organoleptic and nutritional value of the final dried products. The protective effects of pre-treating may vary depending on the nature of the foods and the techniques of the drying operation.

Some of the most widely used drying methods employed in the industry are outlined hereunder:

Sun drying
Sun drying has been traditionally used for preserving numerous food products. It is a simple and inexpensive method that relies on direct sun radiation and air circulation. The optimal weather conditions consist of hot temperature, low humidity and breezy weather. It is suitable for drying fruits that are rich in sugar and acid components. However, this method is unsuitable for meat products due to their susceptibility to microbial growth/contamination that may result in spoilage and food poisoning. The occurrence of spoilage is mainly attributable to high protein content of meats and unmanageability of drying conditions (i.e., humidity and temperature). Furthermore, sun drying is unsuitable for drying vegetables due to the risk of food spoilage that stems from low percentage of sugar and acid in most vegetables.

Solar drying
This method is an improved method of sun drying. There are various designs of solar dryers available in the industry. In principle, the solar power as energy source is applied to either directly enables the heating of food (uses a combined system of food chamber and energy collector) or indirectly (uses a separate structure of heat collector and food chamber).

The drying is enabled by means of a ventilated dehydrator equipped with an aluminum surface positioned on the floor of the dryer (to absorb solar radiation). The ventilation helps accelerate the time of drying that has the advantage of mitigating microbial growth and food spoilage. The efficacy of solar dryer relies on the regulated temperature and plenty of air flow.

Freeze drying
Freeze drying, namely, lyophilisation or cryodesiccation, refers to a dehydration process that involves sublimation (conversion of solid phase directly to gas phase) using a freeze drying equipment. The main stages of freeze drying consist of pre-treatment, freezing, primary drying and secondary drying. The freezing stage is the crucial part of the operation that necessitates adequate attention to prevent the risk of spoilage. After the stage of freezing, the food is subjected to the primary drying that exerts vacuum with low temperature to allow sublimation of frozen water. After completion of primary drying, the secondary drying is processed to create desorption (of bound water) in response to the reduced atmosphere pressure and increased temperature. The time of freeze drying is highly critical. Quick freezing helps better preservation mainly with respect to the organoleptic quality and nutritional value. In other words, the formation of ice crystals, that are responsible for degradation of the texture and nutrition of the food, is prevented by decreasing the time duration of freezing.

Freeze drying offers some benefits in terms of food quality and health attributes of the end products. These include, maintaining the sensory profile, nutritional value and bioactivity of the intrinsic health-promoting compounds (such as phytochemicals). Furthermore, in terms of shelf-life, freeze dried foods remain stable during a long period of storage.

Other types of drying methods include convection drying, drum drying, dehumidification drying and spray drying.
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