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Total Kjeldahl N
Protein content in rice improves its nutritional value but affects its texture and color. Protein is determined using classical method of using the value of Total Kjeldahl N and multiplying this value with a conversion factor (Protein content in rice = Total Kjeldahl N data, % x 5.95).
Total Kjeldahl N is analyzed by converting the various organic nitrogen forms to NH4+ and then measuring the NH4+ concentration. To accomplish this, a measured weight of oven-dried ground sample material is digested in a mixture of concentrated H2SO4, K2SO4, and selenium at about 380 °C for a certain period of time. The ammonium formed is determined colorimetrically by reacting it with sodium salicylate in the presence of sodium hypochlorite and nitroprusside. This will form an emerald-green complex (the Berthelot reaction) whose intensity is measured at 660 nm. This method only partially converts nitrate; thus, samples with high nitrate levels must be pretreated with salicylic acid-thiosulfate to ensure complete conversion (Bremner and Mulvaney, 1982; Bremner, 1996).
Note: To determine protein by Kjeldahl nitrogen, the sample weight required is at least 3 g of milled rice or 2 g of rice flour placed in a paper bag. In order to keep the integrity of the customers' samples, senders are requested to place them in secure plastic boxes.
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