Nitrates in vegetables and changes of their contents due to storage and processing.

摘要

The aim of the thesis was to set realistic guidelines for assessing the intake of nitrates in fresh, stored and processed vegetables, and to provide guidelines for its reduction. For this purpose, we validated two methods for determining nitrate content with liquid chromatography: EN 12014-4 and the method by Cheng and Tsang, and compared them with a standard spectrophotometric method EN 12014-7. Using the validated methods we determined nitrate levels in samples of lettuce (Lactuca sativa L.), dandelion ( Taraxacum officinale agg. F.H. Wigg), spinach (Spinacia oleracea L.), white cabbage (Brassica oleracea var. capitata L. convar. capitata (L.) Alef. var. capitata L. fo. alba DC.), beetroot (Beta vulgaris L. subsp. vulgaris var. conditiva Alef., syn. B. vulgaris L. var. cruenta Wittm.) and potato (Solanum tuberosum L.). The research included samples of vegetables grown in different seasons, by different means of production, and of various origins and varieties. The vegetables were processed using 11 different techniques in a large kitchen under controlled conditions and in large cabbage factories. The conclusion of the thesis is based on the analysis of 1043 estimates of nitrate, nitrite (determined by Griess) and vitamin C (determined by liquid chromatography with PDA detection) levels. In the whole sample, nitrate levels ranged from 14 to 7513 mg/kg fresh weight. The highest nitrate levels were determined in beetroot (2816 mg/kg), spinach (1682 mg/kg), lettuce (1529 mg/kg), white cabbage (374 mg/kg), dandelion (195 mg/kg), and potato (143 mg/kg). Storage did not have a significant effect on nitrate and nitrite levels in lettuce. In samples of potato, nitrate levels did not significantly change during storage, however, the ratio vitamin C/nitrate decreased by 48 %. Cooking is the most effective method for reducing nitrate levels in spinach (by 53 %), in fermented white cabbage (by 22 %), and in roots of beetroot (by 10 %). In samples of spinach, cooking and blanching significantly decreased nitrate content (by 36 %), whereas sauteing significantly increased it (by 25 %). Pureed cooked spinach had significantly lower nitrate levels than pureed sauteed spinach. Fermenting cabbage did not significantly affect nitrate content. Varieties of ingested vegetables and type of preparation both have a major impact on the intake of nitrates.