Protein changes in muscle foods due to freezing and frozen storage

There is a progressive deterioration in the organoleptic and other properties of meat, poultry and fish, frozen and stored at temperatures down to ?25°C, due to undesirable processes taking place in lipids and proteins. Deterioration due to changes in proteins is shown mainly by a loss of extractability of the microfibrillary fraction, changes in enzyme activity and loss of functional properties of the meat such as water-holding capacity, ease of emulsification of the fats, and the ability to form a gel. Long term frozen storage normally results in a marked hardening of the flesh, considered to be due to crossing-over of the fibrillary proteins. However, in species characterised by a low level of protein denaturation, it has not been possible to detect any marked increase in hardness during storage.Since 1951 the author has considered that the properties of proteins in the tissues could be changed or protected by lipids, fatty acids, nucleotide compounds or even by glucides, and that little may be done to avoid deterioration during storage without a better knowledge of the chemistry and physics of muscular proteins. During 25 years many experimental results have been noted on the influence of various factors, depending on the type of frozen muscular tissue as well as the parameters relating to treatment and storage, on the state of proteins in muscles and the quality of the produce. Most of the research dealt with the influence of high concentrations of salts in the meat due to freezing out of water as ice, the break-up of cells of muscle following freezing, mechanical mincing, neutral lipids, fatty acids, oxidation products of lipids, several types of metabloite in tissues compounds, eg formaldehyde from the enzymic breakdown of trimethylamineoxide in fish, and the addition of several antioxidants and hydrophilic cryoprotectors.The type of protein change following freezing and storage in the frozen state is made up from a partial decomposition of molecular and the formation of new primary and secondary linkages. The number and energy of these depend on participation of individual factors of deterioration, the properties of the tissues and also on the parameters of treatment and storage. It is not yet known precisely which microfibrillary proteins take part especially in the reactions which lead to the undesirable changes in the quality of frozen foods, nor how to avoid these interactions. However, to some extent, the knowledge gained during the several past decades help to prolong the shelf life of many products.