Humans cannot produce ascorbic acid in their own bodies and it has to be obtained from food sources. It is required in the diet in greater amounts than all other vitamins combined (FAO/WHO 1970). Major functions of ascorbic acid include hydrolysis of certain amino acids, formation of collagen base or intracellular cement substance necessary for body growth, tissue repair and wound healing. It also plays an important role in iron, copper and folic acid metabolism. It also helps in synthesis of the hormone adrenaline (epinephrine) and the hydroxylation of anti inflammatory steroids in the adrenal gland (Marks, 1975).

Fresh vegetables are considered as a significant source of ascorbic acid in regular diets. Broccoli is high in ascorbic acid and a single serving of broccoli provides more than 30mg of ascorbic acid. Cabbage is mostly eaten raw as part of a salad but cooking and SF of cabbage is also done, which can lead to the loss of certain nutrients especially vitamin C. Capsicum is a rich source of ascorbic acid and green leafy vegetables (GLV) are micronutrient dense, which is nature's gift to mankind that provide more vitamins than any other food. They are rich sources of calcium, iron, β-carotene, vitamin C, dietary fiber and many trace minerals and are consumed in cooked form in almost all the countries of the world (Gupta and Prakash, 2008) and (Adefegha and Oboh, 2011).

Processing, particularly cooking is a crucial factor that has an impact on the ascorbic acid content of any food material.

It is particularly important for vegetables, since most of them are commonly eaten in the cooked form (Ejoh et al., 2007). Vegetables vary greatly in their ascorbic acid content. It is a heat labile vitamin, that is, loss of ascorbic acid occurs when vegetables are subjected to heat treatments. Vegetables are mainly consumed cooked; domestic handling and processing, including cooking processes, have been shown to considerably affect this vital nutrient (Berenguer et al., 2007). It is conventional wisdom that processed fruits and vegetables have a lower nutritional value than their respective fresh commodities, mainly due to the loss of ascorbic acid content in the processing (Dewanto et al., 2002 and Monreal et al., 2009) however, there is a need to determine the extent of losses during cooking of vegetables, so that the consumers can be guided to choose the cooking method, which causes the least loss of this vital nutrient.

Procurement of vegetables: Five commonly consumed vegetables namely spinach, fenugreek leaves, broccoli, cabbage and capsicum with reported high vitamin C content were procured from the local market during the peak season. The vegetables were randomly sampled from the shelf. Vegetables were washed with tap water after removing inedible parts manually with the knife and then were cut into almost equal pieces and mixed well. Four hundred grams of each vegetable was taken and divided into four portions. One portion was retained raw; others were cooked by three most common methods to cook vegetables namely Cover Lid Cooking (CLC), Microwaving (MW) and Stir Frying (SF).

The results of the sensory evaluation revealed that maximum overall acceptability of vegetables, that is, spinach, fenugreek, broccoli, cabbage and capsicum was at 15, 10, 5, 10 and 10 min during CLC and MW, while it was at 10, 15, 5, 10 and 10 min during SF (Table 1).

The ascorbic acid (AA) content of selected vegetables ranged between 28.0–109.9 mg/100g fresh weight (Figure 1). The maximum and minimum AA was found in capsicum and spinach, respectively. AA in cabbage was 105.3mg, which was comparable to capsicum while intermediate values were found for broccoli (45.7mg) and fenugreek leaves (42.0mg). Narasinga Rao et al. (1989) reported the AA values for capsicum as 137 mg/100g. The results of the present study were lesser when compared to the reported study. Singh et al. (2007) studied variability of ascorbic acid in Brassica vegetables and maximum mean ascorbic acid (52.9 mg/100g) was recorded in broccoli, which was higher than the value for the vegetable observed in the present study. The variation in the vitamin C content could be due to several factors such as varietal differences, agronomic conditions and the stage of harvesting etc.

The ascorbic acid content of vegetables calculated on dry weight basis when subjected to three cooking methods for optimum cooking time adjudged by sensory evaluation has been shown in Table 2. A decrease in ascorbic acid (AA) was observed when all the selected vegetables were cooked by three methods. The maximum loss of AA in leafy vegetables, that is, spinach and fenugreek leaves were during SF, that is, 35.7% and 43.8%, respectively. MW resulted in 29.7% and 34.7% losses in two vegetables while CLC caused 30.9% and 31.4% loss respectively. Yadav and Sehgal (1995) and Adefegha and Oboh (2011) reported a significant decrease in ascorbic acid content of green leafy vegetables on cooking. In comparison to other analysed vegetables, broccoli showed the least loss of AA when subjected to different cooking methods. The losses were 19.0% and 18.4% for CLC and SF, respectively. The minimum loss was during MW (14.8%). The lesser loss in broccoli during three methods of cooking was attributed to shorter cooking time as compared to other vegetables. Similar to leafy vegetables, cabbage and capsicum showed maximum loss of AA, that is, 39.1% and 36.9% during SF, while the least loss was during MW, that is, 27.2% and 26.6%, respectively. The results revealed that SF caused maximum loss in all the vegetables except broccoli, where it was comparable to CLC. MW resulted in minimum loss of AA in case of spinach, broccoli, cabbage and capsicum.

Howard et al. (1999) also showed that MW had minimal effects on ascorbic acid. Similarly, Yamaguchi et al. (2001) reported that ascorbic acid content of the vegetables cooked in a microwave were generally higher than in those cooked by boiling, because microwave heating does not stimulate the release of either AA or other antioxidants from cooked tissue into cooking water.

The comparison of AA content in vegetables when subjected to three cooking methods is shown in Figure 2. During CLC, the losses ranged between 19.0%–33.8%, the least loss was in broccoli while maximum was in cabbage. SF caused the maximum loss, however the least was found in broccoli (18.4%), while the maximum was found in stir fried fenugreek leaves. Though loss of AA are inevitable when vegetables are cooked, choosing the appropriate cooking method, which results in minimum loss is the right option for obtaining maximum content of this vital nutrient. The present study concluded that MW is a better method as it resulted in the least loss of AA as compared to CLC and SF.

References