THE EFFECT OF PROCESSING ON THE NUTRIENT AND ANTI-NUTRIENT CONSTITUENTS OF TWO SPECIES OF POTATOES: SOLANUM TUBEROSUM AND IPOMEA BATATAS

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ABSTRACT

Effect of two cooking methods (boiling and frying) on the nutrients and anti-nutrients of Irish potato and two varieties of Sweet potatoes (white flesh (Wf) and orange flesh (Of)) were evaluated in this work using standard techniques. The percentage proximate analysis of the raw samples revealed that the moisture (85.44 ± 0.78%), fats (1.04 ± 0.03%) and protein (2.10 ± 0.03%) contents of the Irish potato sample were significantly (p ≤ 0.05) higher compared to that of the sweet potatoes. While the sweet potatoes had a higher carbohydrate (25.99 ± 1.22%: Wf and 29.08 ± 1.46%: Of) and fibre (1.28 ± 0.03%: Wf and 1.39 ± 0.02%: Of) contents compared to the Irish potato. The white flesh sweet potato had the highest ash content (1.71 ± 0.03%) which was significantly (p ≤ 0.05) higher than others. The mineral analysis of these potatoes showed that the calcium (144.04 ± 1.69mg/100g) and magnesium (64.12 ± 1.75mg/100g) contents of the Irish potatoes were significantly (p ≤ 0.05) higher compared to the two varieties of sweet potato. The sweet potatoes had significantly (p ≤ 0.05) higher iron content (4.91 ± 0.03mg/100g: Wf and 3.97 ± 0.05mg/100g: Of) than the Irish potato (2.98 ± 0.04mg/100g). The orange flesh sweet potato sample had the highest potassium (127.74 ± 2.66mg/100g) and sodium (137.28 ± 0.53mg/100g) contents, while the white flesh sample had the highest selenium content (0.91 ± 0.03mg/100g) compared to the others. The Irish potato had the highest zinc content although it was not significantly (p ≥ 0.05) higher than that of the orange flesh sweet potato sample. The result of the vitamin analysis revealed that these potatoes have an appreciable amount of vitamin C. The orange flesh sweet potato had the highest contents of vitamin A (0.614 ± 0.017mg/100g), B1 (0.172 ± 0.006mg/100g), B9 (0.043 ± 0.005mg/100g), C (24.840 ± 1.721mg/100g) and K (0.152 ± 0.004mg/100g) compared to the other two potatoes, while the white flesh variety had the highest content of B2 (0.527 ± 0.005mg/100g) and E (0.884 ± 0.005mg/100g). Result of the anti-nutrient constituents analyses showed that the Irish potato had the highest oxalate content (2.11 ± 0.04g/100g), the white flesh sweet potato had the highest phytate content (1.81 ± 0.03mg/100g) and the orange flesh variety had the highest hydrogen cyanide content (0.76 ± 0.02mg/100g) compared to the other two potatoes. Boiling and frying had varying effects on the different nutrients and anti-nutrients of these potatoes. Boiling increased the moisture content of the potatoes while frying decreased it. Comparing the fried and boiled samples, the fried potato samples had a significantly (p ≤ 0.05) higher fat and carbohydrate contents while the boiled samples had significantly (p ≤ 0.05) lower contents. The fibre content of all the potatoes showed a significant (p ≤ 0.05) decrease after processing. The protein content of the processed Irish potatoes and the fried white flesh sweet potato sample were significantly (p ≤ 0.05) higher, while those of the orange flesh and boiled white flesh sweet potato were significantly (p ≤ 0.05) lower. Cooking had a decreasing effect on the potassium, calcium, iron and selenium contents of the potatoes and an increasing effect on their magnesium and sodium contents, while the zinc content was hardly affected. Frying had a more retaining effect on most of the minerals. The cooked potatoes had a significantly (p ≤ 0.05) higher vitamin A content compared to the raw samples with the fried potatoes having significantly (p ≤ 0.05) higher content compared to the boiled samples. The boiled and fried potatoes had a lower vitamin B1, B2, B9, C, and K contents compared to the raw samples. Vitamin E was significantly (p ≤ 0.05) higher and lower in the fried and boiled potatoes respectively. The anti-nutrient results of the boiled and fried tubers showed that their oxalate and phytate contents were significantly (p ≤ 0.05) lower compared to the unprocessed potatoes.