EFFECT OF PROCESSING ON THE NUTRIENTS, ANTI-NUTRIENTS AND FUNCTIONAL PROPERTIES OF WHOLE AND EXRACTED STARCHES FROM DIFFERENT TUBERS

Abstract

Three different tuber samples namely water yam (Dioscorea alata), cassava (Manihot esculenta) and Irish potato (Solanum tuberosum) were subjected to two processing methods. The processing methods are boiling and roasting and the unprocessed (raw) served as the control. Each sample was divided into two portions, one portion was analysed with respect to their proximate compositions, minerals, functional properties, phytochemical screening, anti-nutrients and functional groups using Fourier transform infra-red rays (FTIR). The second portion was subjected to starch extraction and the extracted starch granules were granules were analysed characterised to different sugars before and after digestion with α-amylase enzyme. Functional groups were also determined as well as the structural morphology of the starch granules using scanning electron microscopy (SEM). Results on proximate revealed that the moisture content (MC) ranged from 7.58g/100g in boiled water yam to 9.18g/100g in fresh of the same sample. Crude protein (CP) was significantly different (≤ 0.05) with the highest value of 9.01g/100g found in roasted Irish potato and the lowest value (4.83g/100g) in fresh cassava. Fibre content (FC) was significantly high (≤ 0.05) in the roasted Irish potato (12.39g/100g) and significantly (≤ 0.05) low (1.63g/100g) in fresh cassava. Crude fat (CF) was highest (4.45g/100g) in fresh Irish potato and lowest value of 1.61g/100g was revealed in fresh cassava. Ash was generally highest in all roasted samples while similar values were observed in nitrogen free extract (NFE) except in roasted Irish potato. Among the functional properties on the average water absorption capacity WAC ranged from 25.52g/100g in roasted Irish potato to 250.00g/100g in boiled cassava. Oil absorption capacity (OAC) was highest (195.00g/100g) in roasted water yam and lowest value (53.64g/100g) in roasted Irish potato, emulsion capacity (EC) ranged 50.67g/100g in fresh water yam to 67.73g/100g in Irish potato. Highest values ranged11.79g/100g -39.35g/ and 1.04-1.20g/100g were found in foaming capacity (FC) and bulk density (BD) respectively in Irish potato while the least gelation (LG) had highest value 14.00g/100g in boiled water yam and least value of 2.00g/100g in fresh cassava. Results on the minerals compositions showed a general decline in the mineral contents except in K and P after processing especially in boiled samples. Anti-nutrients contentsshowed that saponins, terpenoid, oxalate and phytate were present in all the samples in the range mg/g 4.55- 56.38; 1.48- 3.90; 0.63- 1.35; and 7.42- 23.08 respectively. Tannin was not detected only in roasted cassava. Cardiac glycoside was present only in the water yam andboiled Irish potato. Alkaloid was only detected in cassava and only boiled Irish potato while flavonoid ranged 1.18- 4.88mg/g was absent in roasted water yam, boiled cassava and roasted cassava. Results on functional groups before and after starch extraction revealed the presence of OH, C2H2n, CnH2n-2, RX and ROR. Results on thescanning electron microscopy showed an increase in the surface areaof the boiled samples. Complex carbohydrate digestion on the whole tubers and their extracted starches showed that digestions highest in all the roasted samples. Thus, processing had significant effect on nutritive characteristics of all the tubers analysed.