development of new shelf stable foods. Novel humectants such as glyc-
erol, sorbitol and propylene glycol were often used to adjustawin these
products in addition to the solutes salt and sugar. They were not however
well received in the market for human food because of acceptability
problems, although a number of successful pet food products were
developed. One interesting observation made during this work is that
products with the same water activity differ in their keeping quality
depending on how they are made. Traditional IMFs are generally made
by a process of desorption whereby water is lost from the product during
processing but a number of the new IMFs used an adsorption process in
which the product is first dried and its moisture content readjusted to
give the desiredaw. The hysteresis effect in water sorption isotherms (see
Section 3.2.5 and Figure 3.11) means that although products made using
the two techniques will have the same initialawthey will have different
moisture contents and so will eventually equilibrate to different aw
values. It was found that products made by desorption and having the
higher water content were also more susceptible to microbial spoilage.
Solar drying is still widely practised in hot climates for products such
as fruits, fish, coffee and grain. The traditional technique of spreading the
product out in the sun with occasional turning often gives only rudi-
mentary or, sometimes, no protection from contamination by birds,
rodents, insects and dust. Rapid drying is essential to halt incipient
spoilage; this is usually achievable in hot dry climates, though in tropical
countries with high humidity drying is usually slower so that products
such as fish are often pre-salted to inhibit microbial growth during
drying.
There are a number of procedures for mechanical drying which are
quicker, more reliable, albeit more expensive than solar drying. The
drying regime must be as rapid as possible commensurate with a high-
quality product so factors such as reconstitution quality must also be
taken into account. With the exception of freeze-drying where the
product is frozen and moisture sublimed from the product under vac-
uum, these techniques employ high temperatures. During drying a
proportion of the microbial population will be killed and sub-lethally
injured to an extent which depends on the drying technique and the
temperature regime used. It is however no substitute for bactericidal
treatments such as pasteurization. Although the air temperature em-
ployed in a drier may be very high, the temperature experienced by the
organisms in the wet product is reduced due to evaporative cooling. As
drying proceeds and the product temperature increases, so too does the
heat resistance of the organisms due to the low water content. This can
be seen for example in the differences between spray dried milk and drum
dried milk. In spray drying, the milk is pre-concentrated to about
40–45% solids before being sprayed into a stream of air heated to
114 The Microbiology of Food Preservation