Product Quality Evolution During Drying of Foods, Vegetables and Fruits


The processing of a food products involves several stages and drying is one such operation that is commonly used in a food manufacturing plant. Quality changes and food spoilage might occur during drying due to environment that is not always conducive for processing the food products. Conventionally, food products should be dried below the level susceptible to growth of microorganism, typically at water activity level of less than 0.6.

Apart from this, product quality is often related to various physical and chemical parameters, which in many cases, are decided upon by consumers whom will eventually purchase and consume it. The governing criteria could be due to taste, colour, texture, size, shape and the functionality associated with the product.

In addition to that, food manufacturers should also conform to the microbiological and nutritive aspects of quality control during manufacture.

The aim of this chapter is to provide some general aspects of product quality attributes associated with the drying of food, vegetable and fruits. The effect of various drying parameters on product quality is also discussed based on published literature.


The quality attributes of a dried food product can be classified into physical, chemical, biological and nutritional.  The specification of dried food products also largely depends on these quality attributes. In general, improper processing conditions results in higher nutritional loss and poorer product quality.

High value of hue angle indicates less browning and vice versa while chroma is a measurement of the strength of colour such as the intensity or saturation.

The constant k refers to the kinetic constant, Y is the measured colour scale (L*a*b*) and (±) indicate the formation and degradation of the any quality parameter. The purpose of modeling is to relate the kinetic of colour changes to the moisture content reduction and product temperature evolution during drying.


Structural collapse in food due to moisture removal from the food product results in significant changes in texture. This causes shrinkage and change in porosity of the dried product. Texture attributes such as hardness, fracturability, springiness, chewiness, gumminess, cohesiveness and resilience can be determined by texture profile analyses.

where F is the textural attribute, t is the time, k is the rate constant and n is the kinetic order of textural changes.


Flavour is the primary concern to consumers irrespective of the texture, shape and colour of a dried product. Flavour of food consists of various food aroma compunds that constitute the taste and odour of the food. Some flavour compounds are volatile and these are carried away during moisture removal process. The change in shape and texture influence the microstructure of the food product and controls the release of flavour during processing and consumption.

sensory evaluation plays an ultimate role in deciding the final acceptability of the food product. This is done by comparing the test product to a reference sample and rating is given during evaluation.

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Water activity

The removal of moisture content reduces the water activity of the product during drying. In general, the water activity (aw) in most dried food is relatively low, typically less than 0.6, which is the recommended level for safe storage. Low water activity inhibits the growth of various microorganisms and prevents oxidation and enzymatic reactions.

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