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METHODS OF PROCESSING YAM

 

CHAPTER ONE

INTRODUCTION

 

          Yams are major crops in West Africa, the Caribbean – island of the South Pacific, South East Asia, India and Parta of Brazil.  Nigeria alone accounts for considerable more than half the world’s total production of yam. (Ihekoronye and Ngoddy, 1985) noted that in many African Communities, yam is recognized as a prestige food crop.  In Nigeria especially in Igbo land, it features prominently in many traditional festivals.  Yam also occupies a place in many traditional marriage ceremonies and in special diets for mothers in conferment after child birth.

          Yam have about ten species that c an be considered domesticated.  The tuber is the main economically utilizable part of the yam.  As with other crops, there are considerable variations in composition, not only between species but also within a single species or even a single cultivar, according to the cultural, climatic and edaphic factors of the environment under which it was grown, its maturity at harvest and the length of time for which it has been stored.  Yam like other root crops composed primarily of carbohydrate on material and the have high water content.  The water content generally his between 70 and 80%; yam may be taken as being two – thirds water. The carbohydrates consist mainly of starch with only small amounts of sugar, usually much less than one percent.

          The main constituent of yam starch is amylopectin.  Amylose occurs as 10 – 20% of the starch and influences the properties of the starch.

 

1.1     VARIETIES OF YAM

 

          There are about five varieties of yam to be reviewed.  These are D., alata, D. cayenesis, D., ratundata, D., esculenta and D. elumentorum.

A.      DIOSCOREA ALATA (WATER YAM); Leaves are broadly ovate and are borne opposite each other on the stem and the vines are angular.  There are no prickles or stipules.  The tuber flesh is watery and creamy white and the tube is unevenly shaped.  The skin is thick and there is a creamy or bright purple epidermal layer immediately below the skin.  Maturity is in about 8 – 10 months.  The vines are weak and twin in a counter clock – inflorescence and produces fruits which are three called capsule containing two seeds in each cell.

B.      DIOSCOREA CAYENESIS (YELLOW YAM)

          This has leaves that are as broad as they are and has cylindrical vines.  Prickles where not joined are long and slender, and cylindrical in cross sect stipules are generally narrow and constricted towards the base.  The tuber skin is thick and brittle.  Vines twin anti-clockwise.  This specie has a male inflorescence which occurs singly or in pairs, but not in groups.

C.      DIOSCOREA ROTUNDATA (WHITE YAM)

          D. Rotundata has cylindrical vines with leaves which are long and broad, it is mostly cultivated in West Africa and the West Indies.  The tuber is white and requires about seven months to mature.  Tuber is generally thin skinned and the vines twin anti-clockwise.

D.      DIOSCOREA ESCULENTA (CHINESE YAM)

          Here the vines twin in a clockwise direction and bear alternate, pale green relatively small cordate leaves.  The plant produces a bunch of soft sugary tubers at the base of the stem.  Tubers are small rounded structures.  It maturity is about eleven months.  The tubers bruise easily and do not store longer before it start sprouting within a short time.

E.      DIOSCOREA DUMENTORUM (THREE – LEAF YAM)

          D. Dumentorum have cyclindrical stems with digitate compound leaves of 3, 5, or 7 leaflets.  Twining is clockwise.  Tubers are bunched and the flesh colour varies from white to creamy white or yellow.  Tubers may be bitter and sometimes they are soaked for about three days before being prepared. Some varieties are poisonous.  Tubers reach maturity in about 10 months.

 

1.2     COMPOSITION OF YAM

 

          The tuber is the main economically utilizable part of the yam.  As with other crops, there is considerable variation in composition, not only between species, but also within a single species, or even a single cultivar, according to the cultural, climatic and eclaphic factors of the environment under which it was grown, its maturity at harvest and the length of time for which it has been stored.  The approximate composition of the various species of yam is river in table 1: the analytical data of Nigeria.