Hot air drying

The term drying refers generally to the removal of moisture from a substance. It is one of the oldest, most commonly used and most energy consuming unit operation in the process industries.

The hot air drying technique is the most frequently used technique to produce dehydrated vegetables and fruits, in which heat is transferred from the hot air to the product by convection, and evaporated water is transported to the air also by convection.

Air that has a low moisture concentration is blown through the material bed so that it absorbs water molecules from the product away. In some dryers the air flow is also what is used to heat the material being dried.

Hot air driers are classified as parallel flow, counter flow, direct flow, or cross flow, pending on the direction in the product moves in direction to the direction of flow of the heated air.

Hot-air drying is efficient at removing free water as long as it’s present as continuous liquid phase (high water content) preventing thermal deterioration of the products; this removal is governed by Fick’s Law. The water elimination generates shrinkage decreasing the internal water diffusivity and increasing the thermal conductivity of the product and thus the internal heat transfer.
Hot air drying

Analysis of moisture contents of food

Moisture determination is one of the most important and most widely used measurements in the processing and testing of foods. It is an important factor in food quality preservation and resistance to deterioration.

The amount of moisture in food products is a measure of yield and quantity and as such is of economic importance.

Moisture content must be known for optimum processing of foods, for example, in the milling of cereals, mixing of dough to optimum consistency, and production of bread with the best grain.

Moisture removal or dehydration has long been used as a technique for improving food storage stability. Small increases in the moisture content of low and intermediate moisture foods can significantly reduce their shelf life.

Determination of moisture content is necessary to calculate the content of other food constituents on uniform basis. The dry matter that remains after moisture analysis is commonly referred to as total solids.

Of the oven methods, microwave drying, infrared drying and the moisture analyzer technique are fastest. 

Some force draft oven procedures require less than 1 hour drying, but most forced draft oven and vacuum oven procedures a much longer time.

Hygroscopic foods with low moisture contents may require the use of more sophisticated techniques, such as the Karl-Fischer procedure.

Oven drying procedures are official methods for a variety of food products. Reflux distillation is an AOAC method for chocolate, dried vegetables, dried milk and oils and fats.
Analysis of moisture contents of food

Moisture in fish

Moisture in fish
Water is the principle component (up to 80%) of the edible portions of seafood.

Usually the oil and water content together total about 80%.

The method of storage as well as further processing, such as freezing determines the final moisture content of the fish flesh.

Considerable moisture, as well as soluble nutrients, may be lost in thaw drip. Water retention is highest in fresh fish.

Finfish moisture contents generally show an inverse relationship to the lipid content.

The average percentage of moisture in raw edible flesh, summarized from various sources, is 77.2 with a range of 64.3 – 82.8%.

Raw shellfish moisture contents fall in the same range as finfish, but average is slightly higher, 80.1%.

About one fourth of the moisture can be lost during cooking, which results in concentration of other components.
Moisture in fish