Introduction
Starch constitutes the main form of carbohydrate stores in the green plant and is found in seeds and underground organs. Starch occurs in the form of granules (starch grains), the shape and size of which are characteristic of the species, as is also the ratio of amylose and amylopectin which are the principal constituents.
Some important starches recognized for pharmaceutical use include, maize (Zea mays L), rice (Oryza sativa L), wheat (Triticum aestivum L) and potatoe (Solanum tuberosum L). Cassava starch or Tapioca (Manihot utilissima) may be used in place of the above in tropical countries because of its relative abundance and low cost.
Preparation of Starches
Many patented processes are in use for particular starches and as such the procedure adopted depends on the degree of purity desired and the nature of compounds from which the starch has to be freed. Hence, different processes may be used for different types or sources of starches. For instance cereal starches have to be freed from cell debris, oil soluble protein matter and the abundant insoluble proteins known as “gluten”. On the other hand potato starch is associated with vegetable Tissue, mineral salts and soluble proteins.
Identification of starches
Macroscopical Characters
Starches occur in irregular, angular masses as a white powder. It is insoluble in water but forms a colloidal solution on boiling with about 15 times its weight of water, the solution forming a translucent jelly on cooling. A starch mucilage is coloured deep blue with solution of iodine, the colour disappears on heating to 93ºC but reappears on cooling. Heating of starches causes the granules to first swell and then undergo gelatinization. The temperature at which the changes commerce and complete vary with different starches. Starch granules also undergo gelatinization when treated with caustic potash (KOH), concentrated solutions of calcium or zinc chlorides and concentrated solution of chloral hydrate. The PH of maize starch is neutral but the commercial starches frequently show an acid (wheat and potato) or alkaline (rice) reaction.
Microscopical Characters
Starches can also be identified by microscopial examination. They should be mounted in water or Smith’sstarch reagent (equal parts of water, glycerin and 50% acetic acid). The size, shape and structure of the starch granules from any particular plant only vary within definite limits. So that it is possible to distinguish starches derived from different species and sources. Starch granules may be simple or compound and the description of a starch granule as 2, 3, 4 or 5 compound refers to the number of component granules present in the compound granule. In some cases, the compound granule may be formed by the aggregation of a large number of simple granules (e.g, rice and cardamom.)
The starting point of formation of the granule in the amyloplast is marked by the hilum, which may be central or eccentric. Granules with an eccentric hilum are usually longer than broad. On drying, fissures often appear in the granule and are seen to originate from the hilum. On microscopial examination, the hilum takes the form of a rounded dot or a simple, curved or multiple cleft. The starch granule is built up by the deposition of successive layers around the hilum, and concentric rings or striations are often clearly visible in larger granules e.g. potato. The striations probably arise from the diurnal deposition of the starch giving variations in the refractive index, density and crystallinity. Therefore the position and form of the hilum and the presence or absence of well defined striations are of importance in the characterization of starches.
Note: Starch is required to comply with specified limits for viable microbial contamination including E. coli.
Chemical composition of starch
Starch granules usually contain two carbohydrates amylopectin (α- amylose) and amylose (β- amylose). Amylopectin constitutes over 80% of most starches while amylose constitutes only 20%. Amylose (β-amylose) constitutes essentially of linear chains while amylopectin (α- amylose) has a branched structure. It is these differences in structure and variations in proportion of these two constituents (amylopectin and amylose) in starches from different sources that contributes towards the distinctive characteristics of a starch from a particular biological source (distinguishing rice starch from maize starch).
Note: Amylose is mainly responsible for the deep blue colouration (λ max C.660nm) given by starch and iodine. Amylose has very strong affinity for iodine and as such can take up to 19% of its weight of iodine.
Biosynthesis of starch
Starches are synthesized from amyloplasts. (Look up the synthesis in Trease and Evans and other sources).
Applications of starch in the Pharmaceutical industries
Starch finds extensive use in the pharmaceutical industries as:
- Bulking agents in tablets and capsules.
- Used in making of dusting powders in which the absorbent properties are important.
- In mucilage form it is used as a skin emollient in creams
- As a base for enemas.
- As an antidote in the treatment of iodine poisoning.
- Starch is also used as a disintegrant in tablets.
- Sterilizable maize starch BP is used as a lubricant for surgeons gloves.