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Prior to the invention of refrigeration, meat curing was an ancient method of preserving foodstuffs that is still widely used today, effectively preventing the deterioration of meat after slaughter and prolonging the shelf life of food for a short period of time[1]. Nitrates or nitrites, sugar, ascorbic acid and other substances are usually added to cured meat products in addition to salt to enhance the color of cured meat. Nitrites, which give cured meat products a bright red color and prevent the production of botulinum toxin, have not yet emerged as a complete substitute for nitrites, despite their proven carcinogenicity[2]. The pigments in cured meats are dominated by the bright red nitrosomyoglobin produced by the reaction of nitric oxide from the decomposition of nitrates or nitrites with Mb[3]. Due to the carcinogenicity of nitrites, antioxidants such as ascorbic acid are added to cured meats to enhance nitrite coloration and inhibit excessive oxidation of Mb in order to increase the amount of nitrosoMb in the meat while limiting nitrite content[4].
The color of cured meat products is closely related to the quality of the raw meat. Animals with high Mb content, such as beef and lamb, are bright red, while animals with low Mb content, such as chicken and freshwater fish, are lighter in color. The content and activity of pigment-related enzymes such as high iron Mb reductase in meat products also affects the quality of meat. In recent years, there have been more studies on the effect of meat aging on meat color, and elevated temperatures during meat preparation soften the meat, and protein hydrolysis of must myogenic fibers and cytoskeletal proteins may affect the permeability of the muscle body to Mb as time increases[5]. In addition, during the curing and maturation of meat, the Melad non-enzymatic browning reaction produces brown polymers that affect the color of cured meat products, Starowicz & Zielinski showed that an increase in acrylamide concentration was associated with a darker color of the product[6]. The Maillard reaction also affects the production of a brown substance from fat and the formation of an eventual yellow fat due to factors such as lipid oxidation. Due to the huge market demand for cured meat products, packaging and storage of cured meat has become extremely important. This paper mainly reviews the above related issues affecting the color and luster of cured meat products to provide more theoretical basis for the color and luster changes and regulation in the processing and marketing of cured meat products.
Myoglobin
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Oxidation of Mb is a major internal factor affecting the color of cured meat[7]. As a result of the bloodletting of livestock animals after the completion of all slaughtering operations, this leads to a significant loss of hemoglobin, the higher the percentage content of Mb in the muscle tissue of animals that have been fully bled, the greater the redness value[8].
Mb is found primarily in the myoplasm and is responsible for the storage and distribution of oxygen to myocytes. The three-dimensional structure of Mb as seen microscopically is a single globular protein in the form of a flattened pike, consisting of a heme cofactor and bead protein. The amino acids of the hydrophilic group side chains of Mb are distributed almost on the outer surface, and the hydrophobic groups wrapped around the interior protect heme from oxidation by the external environment, allowing the central heme to bind oxygen, and the outer bead protein chains confer water solubility to the heme group, enabling the protein to maintain its function[9]. Heme consists of four pyrrole substituents and an iron atom in the center, and the resonance properties of the conjugated double bonds in the group determine the ability of Mb to absorb visible light, thus fulfilling its role as a pigment (Fig. 1)[7].
Mb itself is purple in color, and upon contact with O2 in the air it can quickly covalently combine with it to form bright red oxygenated Mb, which has a fresh bright red appearance. As the oxidation time continues to increase, Fe2+ in the heme cofactor is oxidized to Fe3+, and Mb or MbO2 becomes Metmyoglobin (Met-Mb). Mancini & Hunt[10] used MetMb as an indicator of the deterioration of fresh meat, if the content of MetMb is less than 20%, the meat color is bright red, up to 30% the meat color is dark, up to 50% the meat is reddish-brown, and up to 70% the meat turns completely brown. The key to maintaining bright red meat color is to prevent or reduce the production of MetMb substances, while the formation of high iron Mb in meat is also affected by a variety of factors in meat processing, and the reduction pathway and mechanism of high iron Mb are still the focus of the current meat color research work. During the storage of meat, the three forms of Mb are constantly transforming into each other, their relative content determines the color of the meat[11].
When curing meat, as the temperature increases, the color of the meat changes from dark red to pinkish gray and finally light brown. These color changes are related to Mb, and the most significant change in the surface of the meat occurs in the brightness, which is especially prominent between 42 and 56 °C[12]. Meat brightness also decreased with temperature, and the darker meat color also indicated thermal denaturation of heme proteins, so the brightness observed at lower temperatures may be related to the precipitation of other structural proteins that produce modifications within the muscle thereby affecting the meat color[12].
Raw meat
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The quality of meat is closely related to the functional properties of meat in cured meat processing (Table 1). Meat with a bright red uniform color and less drip loss is more likely to be preferred by consumers. There are three main factors that affect the color of raw meat (1) pH; (2) pigment content in the meat; (3) aging of the meat[13]. Numerous studies have shown a negative correlation between brightness (L*) values and pH. Meat with high pH has weaker light scattering, higher transmittance of muscle fibers, longer light paths through the tissues, and increased selective absorption of light by Mb and its derivatives, so the meat will look darker; while meat with a low pH has a high reflectance and light scattering, reduced water-holding capacity and tenderness, increased steaming and dripping losses, and a pale meat color[14]. L* closely related to water content and mobility of water molecules hemoglobin, and cytochrome c, which together influence the color of raw meat. The effect of pH on the brightness of the meat after maturation and curing depends on the initial color of the raw meat, and aging of the meat can also result in the loss of heme pigments leading to lightening of the meat color. For light-colored meat, pH is the main determinant of brightness; however, for dark-colored meat, pH changes may not be the only cause of the increase in L* values during ageing[15].
Table 1. Effect of raw meat quality on myoglobin.
Factor Influence Refs Species Mountain animals and marine mammals have high myoglobin content [16] Age With the increase of age, the content of myoglobin increased and the meat color became darker [17] Motion Highly athletic areas with high oxygen consumption, high myoglobin content and dark meat color [18,19] Feeding method Antioxidants such as polyphenols are added to the feed to help increase the redness value [20] Pre-slaughter stress Redness (a*) values were lower and yellowness (b*) values were elevated in pork with pre-slaughter stress than in the control group [21] Sexuality The pH value of female lambs at 4 h after death was significantly lower than that of male lambs, with higher tenderness and more stable color [21] At present, the direction of research on Mb before and after slaughter of livestock at home and abroad is mainly based on the feeding method, slaughter age, selection site, sex and breed. The final pH of meat and the rate of pH decline are influenced by ante- and post-mortem biochemical events that act on the structural components of muscle cells and their associated connective tissues.
Related enzymes and proteins in raw meat
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Met-Mb reductase is an enzyme that regulates protein oxidation in the muscle of living animals, and its activity gradually decreases after slaughter. A large number of studies have shown that the activity of Met-Mb reductase helps raw meat to maintain a bright color during curing and storage. Nicotinamide adenine dinucleotide (NADH) can provide electrons for Met-Mb reduction through enzymatic, non-enzymatic and mitochondrial-mediated pathways to form deoxy / oxidized Mb. In postmortem muscles, NADH-dependent Met-Mb reductase is still active. Processes affecting high iron myoglobinase activity or NADH regeneration may affect color stability[12]. Lactate dehydrogenase (LDH) and pyruvic acid dehydrogenase (PDH) belong to the class of enzymes related to mitochondrial function, and both are involved in the tricarboxylic acid cycle, which is a key enzyme in anaerobic glycolysis in living organisms (Fig. 2)[8]. Lactate dehydrogenase reacts more slowly in post-mortem anaerobic state in intramuscular tissues[22].
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The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.
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About this article
Cite this article
Han J, Wang Y, Wang Y, Hao S, Zhang K, et al. 2024. Effect of changes in the structure of myoglobin on the color of meat products. Food Materials Research 4: e011 doi: 10.48130/fmr-0024-0003
Effect of changes in the structure of myoglobin on the color of meat products
- Received: 10 November 2023
- Accepted: 23 January 2024
- Published online: 03 April 2024
Abstract: Color is an important factor in determining a consumer's desire to buy and an important indicator of meat quality. Processing and storage processes affect the color of meat products. Therefore, research on how to improve the color of meat products can not only improve the quality of meat products but also enhance a consumer's desire to buy. Nitrosomyoglobin is known to be the main substance that exerts color in meat products. Meanwhile, meat products undergo a series of chemical and physical changes during the curing process that also affect the color of cured meat products. This paper reviews the six main factors currently affecting the color of cured meat products: (1) Quality of raw meat and the content of myoglobin; (2) physical structure of the muscle and achromatic aberration; (3) lipid oxidation; (4) Maillard reaction; (5) additives; and (6) packaging methods. In addition, the article also explores the relationship between pH, temperature, water retention and color in cured meat products, in order to provide more ideas for the study of color in cured meat products.
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Key words:
- Cured meat /
- Color /
- Myoglobin /
- Nitrite /
- Light scattering