| dc.description.abstract | The world population is increasing and valuable nutrients from fish is in high demand. Fish, especially fatty fish, is a nutritionally valuable raw material rich in lipids and proteins. The rest raw material such as heads, backbone and viscera are usually not used for direct human consumption but may be transferred, using innovative methodology, from feed to food as valuable ingredients for human consumption. Lipids and proteins are extracted from the rest raw material by enzymatic hydrolysis, producing fish oil and fish protein hydrolysates intended for human consumption. A limitation in utilization of rest raw material is the risk of lipid oxidation due to the high content of unsaturated fatty acids. Therefore, to produce high value products acceptable as food ingredients, reducing lipid oxidation is essential. Antioxidants are well known to limit lipid oxidation during storage and processing of raw material, but the effect on properties of proteins has not been studied to the same degree.
The overall aim of the study was to investigate whether antioxidants postpone and reduce lipid and protein oxidation, and thereby improve the solubility and other properties of the proteins in both raw material and the products derived from the raw material. In the first study, the effect of antioxidant during frozen storage of fillets of Atlantic mackerel (Scomber scombrus) and the cold storage of minced rainbow trout (Oncorhynchus mykiss) heads were studied. Both lipid oxidation and protein oxidation increased during cold and frozen storage. Rosemary extract was effective in delaying oxidation during frozen storage of mackerel, BHT was effective in delaying the oxidation of trout heads during cold storage. During storage, lipid oxidation was detected prior to protein oxidation.
A relatively few numbers of processing plants compared to slaughtering houses makes transportation and storage of the rest raw material necessary prior to processing. Hence, the rest raw material may be oxidized to some degree prior to processing. The state of raw material was hypothesized to affect the properties of the hydrolysate. The effect of using oxidized raw material for preparing protein hydrolysate was investigated in the second study. Protein solubility in hydrolysates was related to the level of protein oxidation in the raw material, resulting in lower protein solubility in hydrolysates made from stored raw material. However, neither the amino acid composition nor the degree of hydrolysis was affected. Addition of antioxidant prior to storage of raw material reduced the formation of protein oxidation in hydrolysate compared to hydrolysate made from stored heads without addition of antioxidant, but the addition of antioxidant did not increase the solubility of proteins.
How an addition of antioxidant added to enzymatic hydrolysis influences the oil, protein hydrolysate, and storage stability of dried protein hydrolysates was investigated in the third study. Addition of antioxidants did not affect the solubility, amino acid distribution or the size distribution of peptides in the protein hydrolysates. However, the antioxidants influenced the oxidation parameters of both the oil and protein hydrolysates. Addition of BHT and propyl gallate was most effective in limiting lipid oxidation in the oil, but lipid oxidation increased with addition of citric acid. All protein hydrolysates with added antioxidants had a higher amount of thiol groups compared to hydrolysates without antioxidants, showing that use of antioxidants decreased the protein oxidation. Citric acid resulted in the highest content of thiol groups in hydrolysates from both salmon and trout heads. The color of the hydrolysates was also affected by addition of antioxidant, resulting in less yellow hydrolysates.
During storage, propyl gallate was able to retain the thiol groups in protein hydrolysates made from salmon heads. This effect of propyl gallate was not observed in protein hydrolysates made from salmon, showing that antioxidants may have different effectivity in different species. Protein hydrolysate prepared with propyl gallate resulted in significant less yellow color during the whole storage period compared to the other protein hydrolysates. Addition of ascorbic acid resulted in formation of red pigments in the protein hydrolysates.
The results in this PhD thesis demonstrate how adding antioxidants not only will decrease the rate of lipid oxidization, but also protein oxidization. Addition of antioxidants increase the stability of the raw material, rest raw material and the ingredients made from the rest raw material through storage and processing. As such, it contributes with valuable knowledge for the fish industry in their effort to contribute to a more sustainable and effective food supply making use of the entire fish. | en_US |
