## 3 Conventional and Emerging Methods of Analysis of Quality Changes in Muscle Food
### Traditional Analysis Methods  
Traditional methods used to determine quality changes in muscle foods can be divided into methods to determine changes in proximate composition and methods to determine changes in specific components during processing. Proximate composition such as content of water/dry matter, ash, and total amount of proteins and lipids can give some information on changes taking place during processing, but the information is usually limited. However, the proximate composition is usually needed as a basis for other more specific methods. For muscle foods, the water content/dry matter is usually determined by drying at 105 \({}^{\circ}\)C for 24 h [106], but other methods such as drying using IR is also used. The ash content which reflects the minerals content is determined by combustion in an ashing/combustion oven at 550 \({}^{\circ}\)C overnight [107]. For determination of lipid content several methods exist. These are the Bligh and Dyer method [108] and the Folch method [109], which both rely on extracting the lipids in a mixture of chloroform/methanol and water. The Soxhlet method uses different solvents as extraction medium--usual solvents are hexane or mixtures of hexane and acetone or hexane and dichloromethane [110]. The microbial load is also an important quality parameter that is used both to determine shelf life and the effect of different processing and storage parameters such as the effect of heating, superchilling, and MAP packaging [111, 112]. The microbial load is measured as total viable count (TVC) but this cannot be used alone as an indicator of spoilage. To determine spoilage, the count of psychotrophic bacteria or sulfide reducing bacteria as well as number of specific spoilage bacteria are commonly used [111]. Changes during processing can be measured by determination of changes in different properties of proteins and lipids. To measure changes taking place in proteins and lipids, a wide range of different methods are used. One of the simplest methods to measure changes in proteins is to measure changes in solubility. Muscle proteins can be divided into three groups based on solubility--water soluble (sarcoplasmic), salt soluble (myofibrillar), and insoluble (connective tissue) proteins. The myofibrillar proteins are susceptible to denaturation during processing and changes in the solubility or extractability of these can reflect changes during processing. This has been used to study changes during storage (ice, frozen storage) as well as heat treatment [106, 113]. Iced and superchilled storage as well as freezing and thawing may also lead to leakage of enzymes from lysosomes. This has been used to detect changes during superchilled, iced storage, as well as to detect if product or sample has been frozen [114]. During processing and storage, muscle proteins may be degraded, and this is reflected in changes in molecular weight. This can be measured by an increase in acid soluble peptides and free amino acids. The proteins can also be denatured--and the denaturation can be determined by measuring changes in solubility properties as discussed above. It is also possible to analyze which proteins are changed by electrophoresis or chromatography [113, 115]. Changes in lipids are usually determined by determining degree of lipid oxidation. This is measured as a combination of primary and secondary oxidation products. The commonly used primary oxidation product is peroxide value (PV) usually measured by iodometric titration [116] but also spectrophotometric methods are used [117]. Content of conjugated dienes and trienes are also used as measured of primary oxidation products. For secondary oxidation products, determination of content of thiobarbituric acid reactive substances (TBARS) is commonly used in addition to determination anisidine value. The fatty acid composition and other related compounds can also be used to determine changes in lipids during storage and processing but is usually not sensitive enough to detect small changes [4, 118].