Trials to Reduce Nitrate and Oxalate Content in Some Leafy Vegetables. 2. Interactive Effects of the Manipulating of the Soil Nutrient Supply, Different Blanching Media and Preservation Methods Followed by Cooking Process

Abstract

Experiments were carried out to find ways to prevent undesirably high nitrate contents in lettuce and high nitrate and oxalate contents in spinach. Treatments included manipulating the soil nutrient supply and the effects of processing and cooking on the reduction of nitrate and oxalate contents, and the maintenance of vitamins and minerals in the spinach. Best lettuce and spinach qualities, ie highest vitamin and mineral contents with lowest nitrate and oxalate levels, were achieved by fertilising them either with farm yard manure (FYM) or Ca(NO3)2+(NH4)2SO4(1: 1)+N-serve (nitrification inhibitor), at the rate of 178·5 kg N ha-1. High spinach yields were also obtained by fertilisation with urea at 178·5 kg N ha-1, but resulted in high nitrate and oxalate levels. Losses of vitamins A, B1, B2, niacin and C as well as minerals Ca, Fe, Mg, Na, K and P from spinach were smaller following freezing compared with canning, especially when 0·1% calcium citrate and 0·01% sodium ascorbate solutions were used for blanching at ∽77°C for 2 min. © 1997 SCI.