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RESEARCH TOPIC- On the Nature and Reactivity of the Haematin Complexes Present in Intermediate Moisture Beef

On the Nature and Reactivity of the Haematin Complexes Present in Intermediate Moisture Beef

 

ABSTRACT

The colour of intermediate moisture (i.m.) beef prepared by cook-soak equilibration in glycerol was studied. The freshly processed cook-soakequilibrated beef samples were spectrally similar to ordinary cooked beef. With storage at 38OC and 28°C the spectra changed from that typical of cooked meat haemoprotein to one more typical of free haematin. This reaction produced a subjectively detectable change (lightening) in meat colour.

The rate of change varied with water activity. However, the amount of haematin that could be extracted by 40% pyridine decreased during storage suggesting that if freed the haematin is either involved in further complexing reactions or broken down to pyrrole fragments and non-haematin iron. The nature and nutritional availability of the iron in the i.m. beef may depend on these reactions.

Introduction

It has recently been established that the haematin complexes present in cooked meat are formed by reaction between the haematin of myoglobin and haemoglobin and any of several of the denatured proteins present in cooked meat (Ledward, 1971, 1974). The pigments are thought to be diimadazole complexes of ferric haematin, these complexes being further stabilized by salt linkages, hydrogen bonds and hydrophobic interactions. In the previous papers on the nature of protein in i.m. beef (Obanu, Ledward & Lawrie, 1975a, b) it has been established that, in intermediate moisture beef (a, = 0.82- 0-86), the proteins present are, in several respects, different from the proteins present in fresh and cooked beef and that with storage further changes occur. These differences are apparently due to both increased crosslinking and degradation of the proteins present in the glycerol-infused intermediate moisture samples.

From the nutritional point of view meat is an excellent source both of protein and iron. Marked changes in the nature of the proteins occur in intermediate moisture beef, and as the haematin is presumably bound to these proteins, a study of the haematin Authors’ address: Food Science Laboratories, Department of Applied Biochemistry & Nutrition, University of Nottingham, Sutton Bonington, Loughborough, Leics. LEI2 5RD. 675 676 /Z. A. Obanu and D. A. Ledward complexes present in these samples was considered desirable.

As well as being important nutritionally the haematin complexes present will also be of importance in determining consumer acceptability of the product (i.e. the colour) and possibly the keeping quality of the samples as some haematin complexes may catalyse lipid oxidation (Kendrick & Watts, 1969). Materials and methods Preparation and storage of the samples The intermediate moisture beef samples were prepared and processed as described previously (Obanu et al., 1975a). Storage was in cryovac impermeable PVDC bags (W.R. Grace Ltd, London) at 38OC and 28°C.

The control cooked beef was prepared as described (Obanu et al., 1975a) and stored at 38°C in both polythene and airimpermeable cryovac bags. Rejectance sbectrophotometl;y Reflectance spectra of the intact 1 cm3 meat pieces were recorded against a MgO standard, using a Perkin-Elmer Model 124 double beam spectrophotometer. The range scanned was 700 nm to 340 nm at a rate of 60 nm per minute.

 

On the Nature and Reactivity of the Haematin Complexes Present in Intermediate Moisture Beef

Solubility studies

Absorption spectra of the solutions obtained on homogenizing, for 60 sec, and filtering 1 g samples of the pulverized beef with 10 ml of 40% pyridine at neutral pH were recorded in a Unicam SP800 double beam spectrophotometer. The range scanned was 700-370 nm using a pathlength of 1 cm. Subjective assessment of meat colour After forty days’ storage fifteen non-specialist judges ranked the meats of different water activities on a scale of 1 (brightest) to 7 (darkest).

This assessment was done on a different L. dorsi muscle over a, 0.80-0.86. To see the effect of storage at 28 and 38°C on colour portions were placed at – 10°C, at three-week intervals, and subjectively assessed by three judges at the end of the experiment (twelve weeks). Due to the antifreeze property of glycerol and depression of freezing point by the infused solutes,

 

On the Nature and Reactivity of the Haematin Complexes Present in Intermediate Moisture Beef

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