Both animals and plants lay down energy reserves for periods of dormancy (hibernation), germination, growth and propagation, and for survival under adverse conditions.
In man, for example, the short term reserve energy source is glycogen, a short chain glucose polymer which is stored in the liver and the long term energy source is fat which is stored in the adipose tissue. In plants the reserve energy source may be either carbohydrate or oil (or fat). Examples include oil in olives and sucrose in sugar beet and cane. Most commonly however, the energy reserve in plants is starch.
Glucose (dextrose) is synthesized in the chloroplasts of cells from carbon dioxide and water in the presence of chlorophyll and sunlight. The process is commonly referred to as photosynthesis. The sugar is then transported, mainly at night, to the reserve organs of the plant where it is laid down in the polymeric form. The process is very sophisticated and may also play a secondary role in, for example, potatoes where adverse temperature conditions are counteracted by starch breaking down to the commensurate glucose. Such a breakdown decreases the average molecular weight of the cell solutes and as a consequence lowers the freezing point, thus allowing for an inherent protection mechanism to exist. Such systems have evolved where the natural habitat of the plant may have been subject to wide day and night temperature variations.
Starch is present in most tissues. It is laid down in both the granular and amorphous forms in defined cells. The starch, its makeup and where it is stored will be plant specific and is as diverse as seeds (maize, wheat, sorghum, rice), tubers (potato), roots (tapioca, sweet potato, arrowroot), pith (sago palm) and fruits (banana) themselves.
As mentioned above, sources of starch are numerous and in theory any of these sources could be utilized in an industrial process. In practice, however, only a limited number are used commercially for starch production owing to the viability of the processes involved. The most important of these starch sources are maize (corn), wheat, potato and tapioca (cassava, manioc) although reference will be made in this chapter to others as appropriate.
Whatever the starch source, its use will be dependent on the economics of production. This all embracing objective will be subject to geographical location, climate, sophistication of plant, whether production is seasonal or continuous (raw material storage and transport), political considerations (financial incentives) and other factors.
Various starches have specific properties in terms of their own functionality and end use capabilities. Under ‘other factors’ what must be considered is whether the starch source is for the sole purpose of glucose syrup manufacture or whether this is a parallel process to starch manufacture for other purposes. These end purposes may include the specific use of the starch in food, paper and board, textiles or its ability to be modified chemically and physically to give materials with a defined functionality and use, again in the various industries mentioned above. Such needs will often take priority and will determine the starch source. Wheat starch, it could be argued, is a waste product from the manufacture of gluten. Its use, especially when the gluten price is high is therefore very desirable compared with dent maize which may be specifically grown for glucose syrup production.
Starch from some sources may additionally be considered too valuable for conversion to glucose syrup owing to the combination of its unique functional properties in native or modified form and demand exceeding supply, e.g. waxy maize starch.