Authored by Augustine I Airaodion
Abstract
The use of seasonings to enhance the flavor of food has been on the increase in recent time. Different types of seasonings are produced daily. The constituent of these flavor-enhancers is unknown to ignorant consumers. They only want to eat food with good taste without consideration of the effect of these additives on their health. These seasonings contain monosodium glutamate (MSG) which really spiced the food. This review focuses on the toxicological effect of monosodium glutamate in seasonings on human health. MSG was found to induce oxidative stress, renal and hepatotoxicity. It increased total protein and cholesterol. It also induces fibroid. MSG increases the number of platelets, bleeding time and clotting time. It adversely perturbed some sex hormones: testosterone, Estrogen and progesterone. It also leads to increase in body weight and thus obesity. However, these effects can be minimized by the consumption of garlic, ginger, and turmeric. Consumption of food rich in vitamins C and E and other antioxidants can also minimize the effect of MSG on human health. Locust bean has been reported to be a good alternative to seasonings containing MSG.Keywords: Monosodium glutamate; Seasoning; Free radicals; Toxicological effect; Human health
Introduction
Monosodium glutamate (MSG) is a sodium salt of glutamic acid. It is usually a white powder. Water ionizes it into free sodium ions and glutamic acid, which is an organic compound consisting of five carbon atoms. It has a carboxylic (-COOH) group and an amino (-NH2) group attached to an “alpha” carbon atom (a carbon atom joined directly to the -COOH group) (David, 2008). It is an alpha amino acid. The molecular formula of MSG is C3H8NNaO4 and its molecular mass is 169.11 gmol-1. MSG has the same basic structure of amino acids, with an amine group (-NH2) and carboxylate ion instead of the carboxylic group (-COO-). MSG has almost same structure with glutamate. The difference is that one hydrogen atom at the carboxylic chain has been replaced with a sodium atom, hence, the name monosodium glutamate [1] (Figure 1&2).Monosodium glutamate has a distinctive taste that falls outside the region of the four classic tastes: sweet, sour, salty, and bitter. This taste is called “Umami,” also referred to as “Xien Wei” in Chinese or “savory, “broth-like” or “meaty taste” in English. Due to this special taste, many food producers use MSG to enhance the flavor of their product [2]. Recently, Chaudhari et al. [3] identified a specific glutamate taste receptor on the tongue. Three umami substances (glutamate, 5-inosinate, and 5-guanylate) were found by Japanese scientists, but umami has not been recognized in Europe and America for a long time. In the late 1900s, umami was internationally recognized as the fifth basic taste based on psychophysical, electrophysiological, and biochemical studies. Three umami receptors (T1R1+T1R3, mGluR4, and mGluR1) were identified. There is a synergism between glutamate and the 5-nucleotides. Among the above receptors, only T1R1+T1R3 receptor exhibits the synergism [4]. Since glutamate and 5-inosinate are contained in various foods, umami tasted is induced by the synergism in daily eating [4].
The safety and toxicity of MSG had become controversial in the last few years because of reports of adverse reactions in people who have eaten foods that contain MSG. Many studies had confirmed the adverse reactions of MSG [1,5,6]. MSG has been reported to cause headache, vomiting, diarrhea, irritable bowel syndrome, asthma attacks in asthmatic patients and panic attacks [1]. Obuchi et al. [6] studied the effect of garlic extracts on MSG induced fibroid in wistar rats and reported that MSG alone increased total protein, cholesterol and estradiol (estrogen), which in turn, induced fibroid in the rats. However, treatment with garlic extracts near-completely abrogated/mitigated any effects that have been induced by MSG alone.
Egbuonu et al. [7] reported a study aimed at investigating the potentials of low concentration administration of monosodium glutamate in inducing hepatotoxicity in male albino rats. In that study, it was observed that treating rats with monosodium glutamate at a low concentration (5mg/kg of body weight) could be hepatotoxic without significant cholestasis or pathologies of the bone. Onyema et al. [8] reported that MSG at a dose of 0.6mg/g body weight induced the oxidative stress and hepatotoxicity in rats and vitamin E ameliorated MSG-induced oxidative stress and hepatotoxicity. Meraiyebu et al. [9] reported that MSG increased the number of platelets, bleeding time and clotting time in MSGtreated rats. Onyema et al. [10] tested the hypothesis that alteration in glucose metabolism following MSG administration might be a contributor to the changes in the markers of oxidative stress observed in the animals. The pattern of induction of oxidative stress and alteration of glucose metabolic enzymes in the animals was an indication that oxidative stress induced by MSG in the renal tissues of rats might be contributed by increased tissue glucose concentration resulting from enhanced renal gluconeogenesis [10]. Nwajei et al. [11] reported that four selected food seasonings (labeled IS, KC, SMC and BS) commonly consumed in Nigeria adversely perturbed some sex hormones: testosterone, Estrogen and progesterone of wistar albino rats due to the presence of MSG in these seasonings. Kolawole [12] investigated the effect of orally administered MSG on food consumption, body weight and some biochemical and hematological parameters in adult wistar rats and reported that MSG at the doses or 5-15mg/kg body weight was not hazardous to health.
Production of MSG
The Chinese have used certain seaweeds to enhance the flavor of food for some 2000 years. In 1908, the flavor enhancing agent was identified as glutamic acid [1]. Shortly thereafter, methods for extracting glutamic acid from seaweeds were developed [1]. MSG is manufactured through a process of protein hydrolysis, where glutamic acid is freed from protein through enzymatic fermentation or use of chemicals. MSG is also manufactured by a fermentation process in which bacteria are grown aerobically in a liquid nutrient medium. The bacteria release glutamic acid as a by-product of metabolism into the liquid nutrient medium in which they are grown. The glutamic acid is then separated from the fermentation broth by filtration, concentration, acidification and crystallization and conversion to its sodium salt.
The name “monosodium glutamate” refers to a 99% pure combination of glutamic acid and sodium [13]. About 1.9 million tons of monosodium g1utamate are produced worldwide per year by fermentation using Corynebacterium glutamicum or related species [13]. These bacteria are biotin auxotrophs and biotin (vitamin B7) is used as a cofactor. Producers prefer the use of sugars to produce MSG. Some sugar sources used includes, sugarcane, starch hydrolysates gotten from corn or cassava tubers, among others. Ammonia and ammonium salts are added as Nitrogen source. Vitamins and other nutrients are added to finish off the process. Glutamate accumulation in the medium occurs only under biotin-limiting condition. The requirements for biotin limitation prevented the use of standard raw materials such as sugar molasses because they contained biotin. Addition of penicillin, or the use of microorganisms auxotrophic for glycerol or oleate, that allows the bacteria to produce large amounts of glutamate without biotin limitation (Figure 3).
Impurities Found in MSG
Monosodium glutamate contains D-glutamic acid, pyroglutamic acid, and various other contaminants in addition to L-glutamic acid [2].D-Glutamate
Every amino acid (except glycine) can occur in two isomeric forms, because of the possibility of forming two different enantiomers around the central carbon atom. By convention, these are called L- and D- forms, analogous to lefthanded and righthanded configurations. Only L-amino acids are manufactured in the cells and incorporated into proteins. Some D-amino acids are found in the cell walls of bacteria, but not in bacteria proteins. Glutamate has both the D- and L- enantiomers and only the L-glutamate enantiomer has flavor enhancing properties [14]. Manufactured monosodium glutamate contains over 99.6% of the naturally predominant L-glutamate form, which is a higher proportion of free glutamate ions of fermented naturally occurring foods. Fermented products such as soy sauce steak sauce and Worcestershire sauce have levels of glutamate like foods with added monosodium glutamate. However, 5% or more of the glutamate may be D-enantiomer. Nonfermented naturally occurring foods have lower relative levels of D-glutamate than fermented products [14] (Figure 4).
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