Mechanisms of Food Additives, Treatments, and Preservation Technology 327
10.2.3.3 Indirect Preservation Methods: Anabiosis
With indirect methods of preservation the microorganisms present in food products
are not inactivated, but the conditions in food products are changed or preservatives
are added to inhibit their growth and development. Anabiotic treatment prolongs the
lag phase of microbial growth.
10.2.3.3.1 Osmoanabiosis
Preserving food by osmoanabiosis involves the reduction of its free water content.
Free water is the water available for spoilage microflora. It is related to water activity
aw and can be removed from fruits or vegetables by dehydration using drying or
concentration methods or reduced by addition of solutes such as sugar, salt, and
honey. The general effect of water activity on the stability of various food products
is summarized in Table 10.13. Fellows^ (2000a,b)^ and Vega-Mercado et al. (2001)
reviewed the development of drying equipment.
Vega-Mercado et al. (2001) divided the development of drying equipment into
four generations:
- Dryers for the processing of solid materials such as grains, sliced fruits,
and vegetables or chunked products. These are cabinet and bed-type
dryers such as kilns, trays, truck trays, rotary flow conveyors, and tunnels
that involve hot air flowing over the produce to remove water from the
surface. - Dryers intended for dehydration of slurries and purees. These are spray
and drum dryers. During spray drying the fluid state of the food is trans-
formed to droplets and into dried particles by spraying a hot drying
medium. In drum dryers the surface of a slowly rotating, internally heated
steel drum is covered with a uniform layer of food by dipping, spraying,
or spreading. Before completing one revolution (20 sec to 30 min) the
dried food is removed by blades in contact with the drum surface. - Freeze dehydration and osmotic dehydration that minimize losses of flavor
and aroma compounds. The freeze drying process consists of two steps.
The products are initially frozen, and this is followed by removal of water
by sublimation under reduced pressure. Osmotic dehydration is based on
water removal by means of osmotic pressure. Fruits or vegetables are
dipped in hypertonic solutions such as sugar, salt, sorbitol, or glycerol.
The efficiency of the process can be increased by the use of vacuum. - The fourth generation of drying equipment involves the latest develop-
ments in this technology, such as high vacuum, fluidization, heating by
microwaves, radio frequency (RD), and refractance window. Microwave
or RD allows heat to be brought directly to excess water in partially dried
products, hence reducing the potential of overheating relative to the more
traditional techniques. Refractance windows heating is a new technology
that is used to transmit heat to the products. It is applied to the surface
of a plastic conveyor belt floating on hot water.