Investigating carotenoid loss after drying and storage of

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1. Literature review
1.2 DRYING AND STORAGE TECHNOLOGIES ADAPTED TO LOCAL
CONDITIONS
1.2.1 Introduction
Sweet potato roots are highly perishable under tropical conditions and have short shelf-
life (Fowler and Stabrawa 1992). Preservation of food by drying is one of the most
common methods used in developing countries. Drying and storage of sweet potato are
traditionally practiced in Asia (e.g. China, Philippines, Bangladesh, India), South
America (e.g. Peru) and Africa (e.g. Uganda, Kenya) (Woolfe 1992; van Hal 2000).
Other methods of preservation include canning, freezing, pickling and adding sugar
(Woolfe 1992), but these methods are not widely practiced in sub-Saharan Africa
because of lack of awareness along with lack of resources and infrastructure. Drying has
the advantage of being low cost because no extra ingredients are involved and it reduces
the weight of the product which facilitates transport and trade and allows storage in
granaries. In-ground fresh storage and piece meal harvesting is another method of
preservation of sweet potato that is common in sub-Saharan Africa (Hall et al. 1998),
however, drying permits storage in low season and so enables consumption throughout
the year in the regions where crop production is seasonal.
Drying is a mass transfer process that consists of water moisture evaporation from
foodstuffs. Moisture (or dry matter) content is defined as the quantity of moisture (or dry
matter) contained in the product and water activity describes the amount of water
available for hydration of foods and is defined as the vapour pressure of water in the
food divided by that of pure water at the same temperature. The initial moisture content
of the product influences on the drying rate. During drying, the moisture contained in the
product is vaporised under the effect of heat and transferred to the ambient air. Air flow
helps heat application through the product and removal of humidity. Relative humidity is
defined as the ratio of water vapour in air to water vapor. The lower the relative
humidity in the air, the more capacity to remove moisture from the product it would
have. There is also a relationship between temperature and relative humidity. The
temperature of the air affects the relative humidity (as temperature increases, relative
humidity decreases in adiabatic conditions) and this is described on a psychrometric
chart (ITDG 1988). At atmospheric pressure, the efficiency of drying therefore depends
of the temperature/relative humidity and air flow through the product.