Jangam, Mujumdar - Basic Concepts and Definition
pilot scales coupled with field experience and know-how is essential to the development
of a new dryer application. Dryer vendors are necessarily specialized and normally offer
only a narrow range of drying equipment. The buyer must therefore be reasonably con-
versant with the basic knowledge of the wide assortment of dryers and be able to come
up with an informal preliminary selection before going to the vendors with notable ex-
ceptions.
1 .2. MICROBIAL DAMAGE
During harvesting and subsequent steps involved in processing, food products are
prone to different kinds of damage involving mechanical, physical, chemical and micro-
bial damage. Mechanical and physical damage can contribute to enhance the chemical
and microbial damages (Rahman, 1999; Mujumdar 2004). Amongst these drying mainly
concentrates on reducing or completely overcoming potential microbial damages. Mi-
crobial growth can occur during post harvest processing and storage and the main con-
taminant are soil, water, air and animals. The main microorganisms contributing to such
damages are bacteria, fungi (molds and yeast), protozoa, however, insects can also con-
tribute to the microbial damage (Mujumdar 2004). Each microorganism has an optimum
temperature for its growth; however, increasing temperature above certain value sup-
presses the growth and microorganisms have absolute demand for water which en-
hances the growth of microorganism. Each microorganism needs minimum quantity of
water to grow which will be discussed in the next section in terms of so called water ac-
tivity. Microbial growth can result into loss of sensory characteristics of the food items
(fruits and vegetables) and in many cases the damages food will be of unacceptable
quality (various fish products, fruits). Several chemical reactions occur during storage of
food products such as enzymatic browning, lipid oxidation. For example, fruits when cut
become brown, such as, apple. However, reduction in moisture content below certain
level can reduce the microbial damage of food materials and drying accompanied by
proper pre treatment can help reduce the chemical damages. The rate of deterioration
by different reactions will be discussed later in this chapter based on the water activity
values.
1. 3. BASIC TERMINOLOGIES IN DRYING
Drying occurs by effecting vaporization of the liquid by supplying heat to the wet
feedstock. As noted earlier, heat may be supplied by convection (direct dryers), by con-
duction (contact or indirect dryers), radiation or volumetrically by placing the wet ma-
terial in a microwave or radio frequency electromagnetic field. Over 85 percent of indus-
trial dryers are of the convective type with hot air or direct combustion gases as the dry-
ing medium. Over 99 percent of the applications in food drying involve removal of water.
All modes except the dielectric (microwave and radio frequency) supply heat at the
boundaries of the drying object so that the heat must diffuse into the solid primarily by
conduction. The liquid must travel to the boundary of the material before it is trans-
ported away by the carrier gas (or by application of vacuum for non-convective dryers).
Transport of moisture within the solid may occur by any one or more of the following
mechanisms of mass transfer: