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Storage changes in natural and model lipid-protein systems El-Lakany, Safaa

Abstract

The primary objective of the study was to investigate the nutritional significance of the interactions which occur between oxidized fat and protein in foods during storage at room temperature (21°C) and at -20°C. Herring meal, because of its high content of protein and of highly unsaturated fat, was selected as the material for investigation. Studies were also conducted on a considerably less complex system, consisting of egg albumin and herring oil (a model system). The effects of antioxidant-treatment on the chemical and nutritive properties of herring meal were determined as well. Since freeze-drying has become an important process for the preservation of foods, freeze-dried herring meal was also included as a test material. Oxidation of lipids in the herring meals and in the model systems was monitored through measurements of iodine value, peroxide value, thiobarbituric acid value and ultraviolet spectrophotometry. The changes in the amount of ether-extractable material were also followed during the storage period. The following conclusions were drawn from the results of the study: 1. The antioxidant, 6-ethoxy-1, 2 dihydro-2, 2, 4-trimethyl-quinoline (ethoxyquin), added to the herring meal at a concentration of 0.025% considerably limited the decrease in the ether-extractability of lipids and the decrease in iodine value of the ether-extract which occurred in meal not treated with antioxidant. The effect of the antioxidant was evident at both storage temperatures. 2. Peroxide value, thiobarbituric acid absorbance at 535 nm, and the ultraviolet absorption at 233 nm were much higher in the untreated herring meals than in the antioxidant-treated ones at both storage temperatures. 3. The available lysine in the herring meals declined during the storage period. The rate of decline was most pronounced in the untreated meal stored at room temperature. 4. Pepsin digestibility tests showed that antioxidant-treated meals and untreated herring meal stored at -20°C contained the highest amounts of digestible protein while the untreated herring meal stored at room temperature contained the lowest amounts of digestible protein throughout the storage period. 5. The solvent-extracted meals, whether stored at room temperature or at -20°C exhibited only a slight decrease in the available lysine content and in pepsin-digestibility. 6. The peptide maps showed no marked changes in the pattern of peptides released by the action of pepsin on the different herring meals during the storage period. 7. With one exception the herring meals were similar in nutritive value as sources of supplementary protein or of energy, even after storage for 10 months. The unstabilized herring meal stored at room temperature, however, showed significant depressions of metabolizable energy value and supplementary protein quality. 8. In vivo digestibility tests showed that the antioxidant-treated herring meals were digested much faster in the small intestine of chickens compared to the untreated meals regardless of the storage temperature. It was also noted that antioxidant-treated and untreated meals which had been stored at -20°C for 11 months were digested at faster rates than those similarly treated but stored at room temperature for the same period of time. 9. Solvent-extracted meal stored at -20°C gave higher metabolizable energy values and showed better protein quality than the solvent-extracted meal stored at room temperature. Similarly, in vivo digestibility tests indicated that solvent-extracted meal stored at -20°C was digested at a faster rate as compared to the meal stored at room temperature. 10. Ether-extractable fat from freeze-dried meals (whether stored under air or under nitrogen) and from presscake showed- considerable oxidative deterioration as measured by iodine value, thiobarbituric acid value, peroxide value and ultraviolet spectrophotometry. A decrease in the available lysine during the storage period was also evident. Protein digestibility, as measured by pepsin solubilization, declined slightly throughout the storage period. No changes in the peptide maps of these meals were observed. 11. In the model system, the presence of protein promoted oxidation in the oil fraction, irrespective of the storage temperature. In addition, some alterations occurred in the protein fraction as a result of storage in mixture with oil. There were no differences in the peptide maps of the pepsin digests of the model system and of albumin stored at the two temperatures. There was, however, in both cases, a general decrease in the number and intensity of the peptide spots in the maps after storage of the samples.

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