ACTA ALIMENTARIA VOL. 15 (An International Journal of Food Science, 1986)
1986 / 1. sz. - CSAPÓ, J.-TÓTH-PÓSFAI, L.-CSAPÓ-KISS, ZS.: Optimization of hydroliysis at determination of amino acid content in food and feed products
Acta Alimentaria, Vol. 15 (1), pp. 3 — 21 (1986) OPTIMIZATION OF HYDROLYSIS AT DETERMINATION OF AMINO ACID CONTENT IN FOOD AND FEED PRODUCTS J. CSAPÓ, I. TÓTH-PÓSFAI and Zs. CSAPÓ-KI ss Agricultural College, H-7401 Kaposvár, Dénesmajor 2. Hungary (Received: 7 June 1984; revision received: 9 November 1984; accepted: 16 November 1984) The authors determined the amino acid composition of maize, soybeans, milk powder and meat meal using the 6 M HCl, the 3 M para-toluenesulphonic acid and the 3 M mercaptoethanesulphonic acid methods of protein hydrolysis, and studied the decomposition of the individual amino acids due to the effect of hydrolysis and the processing of the hydrolysate. It was found that hydrolysis with 6 M HCl at 105 °C for 24 hours, followed by neutralization with 4 M sodium hydroxide gave the optimum result for all amino acids with the exception of tryptophan. Under acidic conditions tryptophan is almost entirely decomposed, while 20-50% of the cysteine is decomposed, depending on the quality of the material under examination, compared with the quantity determined in the form of cysteic acid after oxidation with performic acid. Due to the proven loss of cysteine the authors elaborated a rapid, accurate method for determining the cysteine content of feed and food products. The method consists of oxidation with performic acid, followed by hydrolysis with 6 M HCl, after which 5-8 samples are fed onto the ion exchange column of the amino acid analyser at 2—3 minute intervals. This method allows the precision of the cysteine determination to reach the level obtained for the other amino acids. For tryptophan, results suitable for evaluation were only obtained using the sulphonic acid methods. A 30% higher value was found for the tryptophan content after hydrolysis with mercaptoethanesulphonic acid than after hydrolysis with para-toluenesulphonic acid. In the course of their examination the authors used cysteic acid as an internal standard, which had the effect of eliminating the reduction in colour intensity caused by the decomposition of ninhydrin. A description is given of how the instrument can be calibrated for cysteic acid, and the results achieved after reference to an internal cysteic acid standard are compared to those obtained using the traditional standard calibration. All the analyses were carried out using an LKB-4011 automatic amino acid analyser. Keywords: amino acids, protein hydrolysis, analysis of amino acids, protein in food and feed When determining the amino acid composition of feed and food products the following errors are the most frequent: - sampling errors; — errors in hydrolysing the sample; - instrumental errors committed during the analysis of the sample; errors in interpreting the chromatograms. The report by MOORE and STEIN (1951) on the development of a method of analysing amino acids using ion exchange column chromatography gave 1* Akadémiai Kiadó, Budapest D. Reidel, Dordrecht