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EFFECT OF STEEPING PERIOD ON YIELD AND ACCEPTABILITY OF STARCH EXTRACTED FROM SORGHUM

ABSTRACT

Starch was extracted from two varieties of sorghum bicolor grains (white and red varieties) steeped for 6, 8 and 12 hours by wet milling method, the starch samples were analysed for yield, functional properties and overall acceptability. The yield of the white variety ranged from 0.45 – 0.70kg and the red variety was 0.40 – 0.50kg. The result of their functional properties were as follows; Bulk density, white (0.700 – 0.733g/ml), red (0.723 – 0.753g/ml) Gelation temperature; white (70 – 72oc) red (73 – 74oc). Least gelation concentration; white (0.60 – 0.90g/10ml), red (0.70 – 1.00g/10ml). Metabolisable energy value, white (3.12 –3.82kcal/g), red (3.73 – 3.82 kcal.g). Glycosidic cyanide content, white (4.5 – 8.5%), red (8.5 – 10.5%). The starchy samples with shorter steeping periods gave better results in terms of functional properties, metabolisable energy value, and percentage moisture content. The starch samples steeped for longer periods had greater yield but poorer functional properties. The white variety tested cyanide free and had better results than the red variety. The result of the sensory evaluation revealed that the samples had significant difference at p(0.05) and p(0.01) in colour and consistency. But there was no significant difference at p(0.05) and p(0.01) in flavour, texture and overall acceptability

CHAPTER ONE

INTRODUCTION

Starch is non-crystalline white powder, insoluble in cold water. It can be hydrolysed into simpler molecules by either heating with an acid or a suitable enzyme. (Awan and Okaka, 1983). Starch is the major storage form of carbohydrate in sorghum and millets. It consists of amylopectin, a branched – chain polymer of glucose, and amylose, a straight chain polymer (FAO, 1992). The digestibility of the starch, which depends on hydrolysis by pancreatic enzymes, determines the available energy content of cereal grain. Processing of the grain by methods such as steaming, pressure cooking, flaking, puffing or Micronesian of the starch increases the digestibility of sorghum starch. This has been attributed to a release of starch granules from the protein matrix rendering t hem more susceptible to enzymatic digestion (FAO, 1992). When starch is heated in water (moist heat) it will produce a gel – an important property needed in the thickening of gravies, sauces, in the baking of bread and production of custards (Awan and Okaka, 1983).

The physico-chemical properties of the starch affect the textural characteristics of the food preparations made from the grain. The behaviour of starch in water is temperature and concentration dependent (Malleshi and Desikachar, 1985). Starches in general show very little uptake of water at room temperature and their swelling power is so small. At higher temperature, water uptake increases and starch granules collap--- which leads to solubilisaiton of amylose and amylopectin to form a colloidal solution.  This is the gelatinisation stage. Genetic and environmental factors affect the gelatinisation temperature of starch (Freeman and Bocan, 1973).

Heat treatment of starch in a limited amount of water lead to swelling the granules  with very little loss of soluble material and partial gelatinization of the starch (Watson, 1970). On cooking, the gelatinized starch tends to return from the soluble,  dispersed and amorphous state to an insoluble crystalline state. This phenomenon is known as retrogradation or set back; it is enhanced with low temperature and high concentration of starch (Rooney, 1991). Amylose, the linear component of the starch, is more susceptible to retrogradatioln than the amylopectin (Freeman and Bocan, 1973).

1.1       STATEMENT OF THE PROBLEM

Steeping of sorghum for a longer period consumes time and imparts undesirable flavour to the starch extracted from the sorghum due to the action of some spoilage microorganisms. Hence the need to reduced the steeping time and detect its effect on the yield and their sensory properties.

1.2 The overall objective of the project work is to develop a process for extracting starch from two different cultivar of sorghum by subjecting to different steeping time.

The null hypothesis is that cultivar and steeping period will not have effect on the yield and functionality of the extracted starch. Specific aims are:

To determine the effect of steeping period on

1. The yield of the starch

2. The functional properties of the starch

3. Organoleptic properties of the starch

4. The glycosidic cyanide level of the starch

5. The metabolysable energy value of the starch.

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