Effect of packaging, storage temperature and storage time on the physico-chemical qualities of cooked meat

Meat is an excellent source of many essential nutrients and makes an important contribution to a balanced diet. Specialty frying has been demonstrated to great effect on fruits and vegetables, but has been used only rarely with animal products and, until now, not at all with beef. In this study, strips of marinated beef were fried and stored in two different packaging materials in order to determine the effects of packaging and storage. The packaging types were polyethylene terephthalate (PET) and aluminium foil laminate. The water vapour transmission rate (WVTR) was slightly higher for PET and it also had a higher oxygen transmission rate (OTR) (14.7 compared to 3.0 cc/m².day). The cooked beef samples were stored at 11°C, 15°C, 25°C, 35°C and 39°C for three, five, 10, 15 and 17 weeks and the samples and oil extracted from the samples were analysed for quality changes. Response surface methodology (RSM) was used to analyse the responses, and the equations generated were also used to predict responses during storage of up to 32 weeks at 15, 20 and 25°C. The integrated force (kg.sec) was the only significantly affected texture parameter but only for the samples stored in PET packaging; there were no significant changes in breaking force (kg), linear distance (kg.sec) and number of peaks for either packaging type. Moisture content and water activity were also only affected after storage in PET packaging. The moisture content in the stored samples increased from the initial moisture content of 4.17% (dry weight basis) but decreased with increasing storage temperature and was dependent on the relative humidity inside the incubators. Water activity increased from 0.19 to 0.33 after storage for 15 weeks at 15°C, and was predicted to increase to 0.55 after 32 weeks of storage. There were significant changes in colour for both packaging types, expressed as total colour change and browning index. Lipid decomposition was evidenced by significant increases in free fatty acids (FFAs) for both packaging types and the Totox value for PET packaging, although these changes were considered to be minor, with FFAs reaching only 4.0 (% oleic acid) during 17 weeks of storage and predicted to increase to over 8% after 32 weeks of storage and the Totox value was not predicted to exceed 32 even after 32 weeks of storage. FFA levels were higher in the oil from the samples stored in aluminium foil laminate packaging. There were no significant changes in the unsaturated fatty acid content but the levels of PUFA, C18:2 and C18:3 fatty acids decreased during 17 weeks of storage, and mean levels were higher in the aluminium foil packaging, indicating less oxidation had occurred. The tocopherol contents decreased significantly from initial concentrations and mean levels were higher in the oil from cooked beef strips stored in aluminium foil laminate packaging, but the response could not be satisfactorily modelled by RSM. The neo-formed contaminants hydroxymethylfurfural (HMF) and sterol oxidation products (SOPs) were also monitored; HMF increased in both packaging types but it was not possible to model the SOPs data as it was too variable. The results of this experiment suggest that fried beef strips can be stored for up to 32 weeks at temperatures from 15-25°C without large changes in quality. Aluminium foil laminate packaging reduced most of the changes that occurred due to its lower OTR and WVTR, but both materials were acceptable. Future research into the shelf-life of fried beef strips should concentrate on the use of antioxidants to reduce oxidative changes, SOPs formation and a sensory analysis to determine the acceptability of the products after storage.