Effect of freezing on raw and cooked beef quality during five month of storage

Red meat is one of the most vital ingredients of human diet because of being an important source of protein, Further studies have proven that methods of cooking and processing of meat are effective not only on the fat, but also in increasing its oxidation index, as well. Freezing is a common practice in the meat industry because it preserves meat quality for an extended time and offers several advantages such as insignificant alterations in product dimension, and minimum deteriorations in product color, flavor, and texture. The limit of storage or shelf life of meat has been prevented from being reached by microbiological and physicochemical spoilage under conditions such as refrigeration and freezing. Today, due to the lack of time and employment of women, one of the concerns of home cooking is how to store meat. Usually the meat is kept frozen at home, and whether the meat is cooked in the frozen state or raw is one of the questions facing most women. Therefore, the purpose of this study was to compare the effect of freezing on the quality characteristics of raw and cooked meat to determine the quality characteristics of frozen and then cooked meat. In this research, effects of freezing treatment on beef was investigated in 6 replicates with two different treatments, precooked and then frozen (T1), pre-frozen first and then cooked (T2). The samples physicochemical features have been studied during 5 months of storage. A control sample was prepared without any treatment.

pH measurement, moisture content, fat, protein content and peroxide value of meat samples were all analyzed according to national Iranian standards.TBA index was measured according to the method described by AOAC (1990).Iron content was measured with a flame atomic absorption spectrometer according to national Iranian standard. Color measurement as ΔE was analyzed by image processing with Photoshop 8. Texture analyzing test was performed using a texture analyzer (Brookfield LFRA 4500, USA). In this method, the specimen was placed 2 cm in diameter on a porcupine and under a hemisphere. The test was accelerated to 1 mm / s, and the infiltration of the specimen was performed; the infiltration of the specimen and its perforation were reported at high power (Honika, 1997). Analysis of variance (ANOVA) was performed in a completely randomized design using linear plot (G.L.M) with Minitab 16 software. Comparisons of means were performed using Tukey's test at 95% confidence level. Treatments were performed in six replications.

The treatment effect was significant on all the analyzed parameters: pH, moisture, fat, protein, peroxide, TBA and cooking loss (p < 0.05). Results of the mean comparison showed that fat, pH and iron had significantly decreased (p < 0.05), while protein content, peroxide and TBA had remarkably increased (p < 0.05) during storage. The decrease in moisture after slaughter is inevitable due to the decrease in pH (close to the isoelectric pH of proteins), the completion of ATP, and the effects of myofibrillar contractions. These factors all lead to the release of water, which subsequently releases water into the extracellular and sarcoplasmic reticulum of proteins. Water retention was less for frozen samples. Although frozen foods are microbial-resistant, they are subject to chemical changes and their enzymatic activities continue at low intensity. The reason for the decrease in fat was due to the lipid oxidation Fresh meat fat was higher than frozen meat and cooked meat. Certainly, fat removal during baking causes this decrease. Fat content of treatment 2 was higher than treatment 1 and oxidation in treatment 1 was higher. In fact, initial cooking resulted in more fat removal in treatment 1 than in post-freezing cooking (treatment 2), which also differed during the storage period, resulting in lower fat content in the fifth month. Freezing damage the structure of muscle cells by releasing lysosomal and mitochondrial enzymes, iron and other peroxidants, which increase the intensity and speed of protein oxidation. In treatment 1 due to the thermal process initially and inactivation of some enzymes, the proteolysis process is slower. However, in treatment 2, the process of proteolysis has improved. The non-frozen water portion is important in determining oxidation, since some chemical reactions can also occur during freezing storage, primary oxidation during baking, for example, can lead to secondary lipid oxidation during melting (Own & Laurier 1975). As treatment 1 was first cooked and then frozen, oxidation began at the initial heating stage and continued at a lower rate during freezing. For this reason, it had a higher peroxide index than treatment 2 overall. In the overall study, the lowest thiobarbituric acid was in control and the highest in treatment 2 (frozen and cooked) after 5 months of storage. Both treatments (1 and 2) had oxidative corruption within 5 months of storage, although the severity of treatment 2 was higher than that of treatment 1. In fact, the initial curing by enzymatically deactivating was able to reduce the intensity of oxidation during frozen storage. The results of this study showed that the amount of iron decreased significantly over time. Freezing time in particular affects the retention of vitamins and minerals. Iron is in the structure of myoglobin, denaturation leads to increased sensitivity of myoglobin to auto-oxidation and co-oxidizes with iron myoglobin oxidation (Otter et al. 2012). In texture analysis, the results showed the meat become more stiff during storage. The raw state showed more stiffness due to the effect of annealing on protein deterioration. Finally, raw meat (treatment 2) showed higher firmness after 5 months and the nearest firmness to control raw meat. Increased stiffness of meat due to lower moisture content is one of the most common problems that occur due to freezing and storing of frozen fresh meat in the meat texture (Akhtar et al., 2013). Regarding the color, treatment 2 showed less difference with the raw meat which can be concluded that the raw meat in the freezer retains its color better.

This research showed that both samples had an oxidative spoilage within 5 months, which offers not to store frozen meat more than 3 months. Considering nutritional features expected from meat, it could be concluded that sample T1 which was the meat that was first cooked and then frozen was preferred to sample T2 because of preserving more iron, protein and having lower TBA index. In sum, cooking meat before freezing is recommended.