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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.

References:
Anonymous Manual. 1977. Technicon AutoAnalyzer II Industrial Method No. 334-74W/B. Release January 1976/Revised March 1977. Individual/simultaneous determination of nitrogen and/or phosphorus in BD Acid Digest.
Bremner JM. 1996. Chapter 37- Nitrogen-Total: In Methods of Soil Analysis. Part 3. Chemical Methods. SSSA Book Series no. 5.
Bremner JM, Mulvaney CS. 1982. Nitrogen-Total. P. 595-624, In A.L. Page et al (eds). Methods of soil analysis. Part 2. 2nd ed. Agron. Monograph 9.
Kalra YP, Maynard DG. 1991. Methods manual for forest soil and plant analysis. Information Report NOR-X-319. Edmonton, Alberta (Canada): Forestry of Canada,Northwest Region, North Forestry Centre.
Mulvaney RL. 1996. Chapter 38- Nitrogen-Inorganic Forms: In Methods of Soil Analysis. Part 3. Chemical Methods. SSSA Book Series no. 5
Varley JA. 1966. The determination of nitrogen, phosphorus and potassium ion plant materials. Analyst. 91: 119-126.
Juliano, B. Rice Chemistry and Quality. Philippine Rice Research Institute, 2007.
Sreenivasulu, N. (2018). Rice Grain Quality: Methods and Protocols. New York, NY: Humana Press.