Meat quality is described by a range of physical and chemical attributes including color, flavor, tenderness, juiciness, and texture. These characteristics are influenced by intrinsic and extrinsic factors such as animal species and age, muscle structure and its chemical composition, antemortem stress, product handling, processing, and storage conditions. Color is an important attribute of meat, as the consumer uses it to judge the product's freshness and wholesomeness. The concentration and chemical state of the pigment myoglobin determines the color of meat, and is dependent upon a number of antemortem factors, including the animal species and age, the physiological function of the muscle, the nutritional status of the animal, and the dietary regime.[1] It is thus due to these reasons that beef muscle has a bright, cherry-red color compared to the lighter color of pork or poultry meat; that meat from an older animal appears darker than that from a younger animal; and that breast meat from poultry is lighter than its leg counterpart.[2] The color of meat is also affected by storage conditions, such as the gaseous environment, storage time, and temperature. For example, the lack of oxygen in vacuum-packaged beef results in conversion of oxymyoglobin to deoxymyglobin of purple-red or brown color, which is perceived as undesirable by consumers.[2]

The flavor of meat is composed of: 1) meat flavors derived from water-soluble compounds; 2) species flavors derived from fat-soluble compounds that are stored in the lipid depots of the animal; and 3) off-flavor development due to lipid oxidation or other chemical reactions.1-2-1 Lipid peroxidation or oxidative rancidity occurs when molecular oxygen reacts with unsaturated lipids to form lipid peroxides, which are colorless, tasteless, and odorless. Decomposition of these primary products, however, gives rise to a complex mixture of low-molecular weight compounds that have distinctive flavor and odor characteristics (e.g., rancid, fatty, pungent). Cooking of meat results in an increase in lipid oxidation in muscle and to the development of warmed-over flavor (WOF) in refrigerated cooked meat. This flavor defect can also be encountered in uncooked emulsion-type, ground, flaked, or restructured meat products in which the muscle structure is disrupted, exposing muscle lipids to a pro-oxidative environment. Lipid peroxidation is positively linked to pigment oxidation, and also affects the odor of meat, its nutritive value, and potentially the safety of the meat.[2,3] Factors affecting the flavor of meat include species, sex, age, and diet of the animal.[1,2]

Meat tenderness is a quality attribute determined by the connective tissue and type and state of muscle fibers, whereas texture is determined by the definition and fineness of muscle fiber, as well as by the amount and distribution of fat in the muscle. Juiciness is a sensory term indicating the moisture content of meat, which is critically affected by its water-holding capacity.1-1-1 More details on these aspects of quality can be found in other sections of this encyclopedia.

The quality of meat is also affected by the metabolic activity of microorganisms.[4] Substrates utilized for growth by spoilage bacteria include glucose, glucose-6-phosphate, amino acids, proteins, and lactic acid, whereas major end products of metabolism of these bacteria include sulfides, amines, acids (lactic, acetic, isovaleric, isobutyric), esters, and nitriles.[4,5] Spoilage characteristics of meat and poultry are similar, despite the differences in properties among muscle tissues of different animal and bird species.[6] Spoilage of meat and poultry is characterized by an offensive appearance (e.g., green, brown, or gray discolorations) and/or an off-odor (e.g., sour, fruity, cheesy, putrid) or off-flavor. The growth of bacteria on the surface of meat reportedly accelerates the oxidation of meat pigments, resulting in discoloration of the product.[3] Off-odors are sensorially detectable first, when bacterial numbers exceed 107 per cm2, whereas bacterial slime becomes evident when numbers reach 108

The spoilage flora of refrigerated meat and poultry is composed of psychrotrophic bacteria, including Pseudomonas spp., Enterobacteriaceae (Serratia liquefaciens, Hafnia alvei, Enterobacter agglomerans), Brochothrix thermosphacta, lactic acid bacteria (Lactobacillus, Car-nobacterium, Pediococcus, Streptococcus, Lactococcus, and Leuconostoc spp.), Shewanella putrefaciens, and species of Moraxella, Psychrobacter, Acinetobacter, and Aeromonas.[8] Under aerobic storage conditions, Pseudomonas spp., specifically Pseudomonas fragi, Pseudomonas fluorescens, and Pseudomonas lundensis, usually predominate (normally >50%), while under modified atmosphere or vacuum conditions, lactic acid bacteria (e.g., species of Carnobacterium, Lactobacillus, and Leuconostoc) and Brochothrix thermosphacta are domi-nant.[7,8] Thus, under aerobic and chill conditions, Gramnegative bacteria predominate, whereas under modified atmospheres, Gram-positive bacteria form the major portion of the spoilage flora. Processed meats (e.g., cooked ham, corned beef, emulsion-type sausages, luncheon meats) stored at cold temperatures and packed under vacuum or modified atmosphere are also predominantly spoiled by lactic acid bacteria. Spoilage in these products is characterized by a sour odor and flavor, greening, gas production, and slime formation. Yeast and mold spoilage of meat and poultry can also occur, but only under conditions where bacterial competition is reduced (e.g., reduced water activity, presence of preservatives or antimicrobial treatments, and long-term storage).[4,9]

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