D.A. Ledward

Catalysts of lipid oxidation in meat products.

In emulsions consisting of refined lard, egg white and corn starch haemoglobin, initially a mixture of the oxy and met forms, at levels similar to the haemoprotein contents found in fresh meat, was a far more powerful catalyst of lipid oxidation, as measured by TBA number, than inorganic iron compounds at levels appropriate to those found in meat. This was true over the pH range 4·5 to 7·5. When added and evenly distributed to exhaustively washed muscle fibres (WF), at levels appropriate to those found in meat, haemoglobin was again a powerful catalyst, but all forms of inorganic iron appeared to have little prooxidant activity. The rate of oxidation of the lipid was very dependent on the haemoprotein concentration, being maximal in the range 10(-4) to 10(-5) M. This equates to an approximate unsaturated lipid to haem molar ratio of several hundred to one, similar to the values reported for model linoleate haem systems. In heated systems the haemoprotein again appeared to be a more effective catalyst than inorganic iron at levels appropriate to those found in meat. It is concluded that the conflicting results as to the roles of haem pigments and inorganic iron in lipid oxidation found in the literature are due, at least in part, to the difficulty of evenly dispersing the catalysts in the washed fibres, especially if heat or freezing leads to subsequent phase separation, and that H(2)O(2), formed by autoxidation of the oxypigments, may be necessary for ferric haem pigments to be effective catalysts.

Protein hydrolysates from meat industry by-products

Hydrolysates were prepared from bovine lung, bovine rumen and from partially defatted tissue by treatment at 50°C with either pepsin at pH 3·0, papain at pH 5·5, neutrase at pH 7·0 or alcalase at pH 8·5. For all substrates papain was the most effective hydrolysing agent of those studied whilst neutrase was the least effective. Yields of soluble hydrolysate were high with 45-85 % of the protein being solubilised. In addition, the tissue is, to some extent, defatted during hydrolysis. All enzymes, with the exception of alcalase, readily solubilised the collagen of heated by-products although undenatured (unheated) collagen was, to some extent, resistant to enzymic digestion. Amino acid analysis of the soluble hydrolysates indicated that there was no major loss of any amino acid following prolonged enzymic hydrolysis. In addition, no increase in tyramine concentration occurred during hydrolysis.

Lipid and haemoprotein oxidation in meat emulsions

Emulsions of refined additive-free lard, egg albumin and water were designed to study the effect of haemoproteins, iron salts and sodium chloride on lipid oxidation. It was found that ferric haematin pigments (metmyoglobin, methaemoglobin and heat denatured myoglobin and haemoglobin) were all catalysts of lipid oxidation whereas the oxy and carboxy derivatives were not effective. Emulsions prepared using meat of high and low metmyoglobin contents supported these findings. At the levels present in meat products, both ferrous and ferric salts were only very weak catalysts of the lipid oxidation compared with the ferric haematin complexes. Sodium chloride (1·5%) also possessed little or no pro-oxidant activity in these systems. Emulsions prepared from fresh meat and fats from various sources and of different histories indicated that although, in model systems, peroxidising lipids can catalyse the oxidation of myoglobin, in meat and meat-based emulsions the effect is of minimal importance.

Effect of oxygen and storage temperature on intermediate moisture meat products.

During storage of glycerol desorbed intermediate moisture meats at 38°C it was found that, for both protein crosslinking and loss of haemoprotein character to occur, oxygen must be present. However, collagen degradation, as monitored by the formation of water-soluble hydroxyproline containing fragments, still occurred in the absence of oxygen although the rate of degradation was slower than that observed in aerobically stored samples. The rates of the various deteriorative reactions also varied with storage temperature, there being the expected decrease in rate with temperature for both the crosslinking and haemoprotein breakdown reactions. However, the temperature dependence of the collagen breakdown reaction was apparently more complex as there was no measurable breakdown, even during several months of storage, at temperatures of 17°C and below.

Preparation and storage stability of dried salted mutton

Minced (8 or 18 mm plate) mutton with salt (25%) and sorbate (0·4%) was pressed into cakes about 11 cm in diameter and 3 cm high. The cakes were partially dried in an air oven at 40°C for 48 h to a water activity of about 0·75. The cakes were packed, either in vacuo or in air, and stored at 30 or 2°C for up to 60 days. Objective assessment of quality showed that these dried salted meats can be kept for up to 60 days at 30°C with little loss of textural or nutritional quality although some fading, due to haemoprotein breakdown, occurs. Packaging in vacuum, however, minimises this loss of colour and would be recommended for centralised manufacture prior to distribution in developing, tropical countries.

Reactivity of sorbate and glycerol in intermediate moisture meat products

Abstract Intermediate moisture meats were prepared by cook-soak equilibration with glycerol/salt, sucrose/salt or salt only solutions to an aw of 0.85. Sorbate or propionate was added to some products and all were stored at 38°C in the presence of air. It was found that sorbate, but not propionate, caused increased rates of protein aggregation and accelerated loss of haemoprotein character. Glycerol may also take part, albeit more slowly, in these deteriorative reactions. However, sorbate did not appear to lead to any marked increase in the rate or extent of the reactions leading to solubilisation of the collagen present in these samples; glycerol, however, significantly increased the rate and extent of such reactions. The results are discussed in the light of the results obtained with the model systems described in the previous paper.

Effect of frozen storage of minced meats on the quality of sausages prepared from them

Mutton, pork and beef sausages were prepared from minced meats which had been frozen and stored up to 52 weeks at -18°C. There was a significant decrease in texture (W-B shear force) of all the sausages with increased age of the meat. However, concomitantly there were no significant differences in the colour, juiciness or shrinkage (cooking loss) of the sausages. All sausages were organoleptically acceptable even when made from meats stored for 52 weeks at -18°C.

Enthalpy changes associated with the denaturation of collagens of different imino acid content

The enthalpy changes associated with the denaturation of acid-soluble and insoluble collagens prepared from sheep, cod, halibut and pike skin were determined by differential scanning calorimetry. The enthalpy change associated with the soluble collagens decreased with decreasing imino acid content (from 1420 cal/mol for sheep to 736 cal/mol for cod) while the value for insoluble collagens was approximately constant at 1360 cal/mol. A possible explanation for these values in terms of the nautre of the bonds present in collagen is discussed.

Lipid oxidation and metmyoglobin formation in sausages containing chickpea flour

Rapid lipid oxidation and metmyoglobin formation in sausages containing up to 30% chickpea flour is due to the presence of lipoxidase in chickpea flour. This enzyme oxidises the unsaturated fats present to peroxides (or related compounds) which then catalyse myoglobin oxidation. Heat treatment of chickpea flour at 80° C for 1 h in water prior to its addition to the batter will prevent both accelerated lipid and myoglobin oxidation in these sausages. An antioxidant containing α-tocopherol and ascorbyl palmitate inhibited the lipid oxidation in these products but had no effect on myoglobin oxidation in sausage batter containing unheated chickpea flour. The relevance of these results to the interdependence of lipid and myoglobin oxidation in meat and meat products is discussed.

Effects of calcium and pH on spun fibres produced from plasma-alginate mixtures

When a solution of blood plasma and sodium alginate (protein: polysaccharide ratio 3:1) was extruded into coagulating baths of calcium chloride there was a rapid increase in shear strength of the fibre bundles with a rise in salt concentration up to 3% calcium chloride. Above this concentration no further increase in shear strength was observed. The ratio of plasma protein to alginate was between 0·84 and 1·1 in all tows produced from unbuffered calcium chloride baths. Extruding the dope into 5% calcium chloride baths of pH values between 2 and 8 indicated that fibre strength was independent of pH in the range 4 to 8. Below pH 4 the tows rapidly decreased in shear strength to a minimum at pH 3·5. Decreasing the pH further led to an increase in fibre strength as acid denaturated protein coprecipitated with the polysaccharide. The ratio of plasma to alginate in the tows was around 1·0 for pH values above 3·5, rapidly increasing to 2·8 at pH 2·0.