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Chapter 1 Lactic Acid Bacteria and ��-Aminobutyric Acid and Diacetyl ����Shunhe Wang�� Pei Chen�� and Hui Dang ����1.1 Lactic Acid Bacteria and ��-Aminobutyric Acid ����1.1.1 Introduction of ��-Aminobutyric Acid ����1.1.1.1 ��-Aminobutyric Acid ������-Aminobutyric acid (GABA)�� or ��-ammonia butyric acid�� is a kind of nonprotein amino acid which widely exists in vegetables and animals. It exists in the seeds�� roots�� and tissue fluid of many plants�� such as Glycine L.�� Panax�� herbal�� and in animals almost exclusively present in nervous tissues. The content of GABA in brain tissue is 0.1�C0.6 mg/g�� and immunology research show that substantia nigra contain the highest concentration of GABA (Krajnc et al. 1996). Meanwhile GABA is also present in microorganisms�� such as yeast�� Lactobacillus�� and Escherichia coli. ����GABA was discovered in the metabolite of plants and microorganisms in 1883. Awapara et al. (1950)�� Roberts and Frankel (1950)�� and Udenfriend (1950) found that the content of GABA was relatively high in the brain of mammals�� but the biological significance was unknown. Roberts reported that GABA was a product of glutamate through ��-decarboxylation. Krnjevi. and Schwartz (1966) researched that GABA played a part in the brain of mammals as an endogenous neurotransmitter and GABA was confirmed as an inhibitory neurotransmitter in the Second International Symposium on GABA in 1975. ����Many studies in recent years have reported the physiological function of GABA�� including delaying senescence of nerve cells�� lowering blood pressure�� repairing the skin�� treating mental illnesses�� and regulating cardiac arrhythmia and hormone secretion (He et al. 2007). Scientists from all around the world studied the synthesis technology and its related product. GABA could be produced by chemical synthesis�� plant enrichment�� and microbiological fermentation. Sawai et al. (2001) researched that the content of GABA in tea could be enhanced through a variable anaerobic-aerobic process. Meanwhile�� it also could be produced by controlling the germination condition of brown rice and rice germ. Xia Jiang et al. reported that Lactobacillus brevis CGMCCNO.1306 was isolated from unpasteurized raw milk and the capacity of GABA was 76.36 g/L after the mutation breeding (Xia 2006; Xia et al. 2006). ����1.1.1.2 General Physicochemical Property ����The chemical formula is C4H9NO2 and molar mass is 103.120 g/mol. It appears as a white microcrystalline powder�� easily soluble in water�� slightly soluble in ethanol�� and insoluble in cold ethanol�� benzene�� and ether. The dissociation constant of pKCOOH and pKNH3 is 4.03 and 10.56�� respectively. GABA has no optical rotation�� the melting point was 203�C204��. Its products of decomposition were pyrrolidone and water (Chen et al. 2010). In most part�� GABA exists in the form of amidogen with positive charge or carboxyl with negative charge. The state of GABA determined its molecular conformation. In the gaseous state�� the molecular conformation is highly folded due to the electrostatic interaction of two charged groups; in solid state�� the molecular conformation is extended due to the intermolecular interaction between the two groups. When in liquid state�� molecular conformations exist in both states. The various conformations of GABA are combined with different receptor proteins�� which play significant physiological functions. ����1.1.2 GABA Production of Lactic Acid Bacteria ����GABA In microorganisms GABA is produced from L-glutamate or its sodium salt by ��-decarboxylation reaction at the catalysis of glutamate decarboxylase. Glutamate decarboxylase is the single enzyme to generate GABA from L-glutamate or its sodium salt�� which was discovered in bacteria�� archaea�� and eucaryotic microbes. Compared with chemical synthesis and plant enrichment�� GABA from microorganism fermentation�� which has low cost and high yield�� can be used in food safely. ����Lactic acid bacteria (LAB)�� as a safe microorganism (GRAS)�� are widely used in foods�� such as yogurt�� cheese�� and pickles. Many studies show that LAB have glutamate decarboxylase which generates GABA from L-glutamate by decarboxylation reaction. Nomura et al. (1998) reported that the content of GABA was 383 mg/ kg in cheese�� in which a strain of Lactococcus lactis 01�C7 with high GABA was isolated. Komatsuzaki et al. (2005) researched that a strain of Lactobacillus paracasei NFRI7415 with high GABA was isolated from traditional fermented food and the capacity of GABA was 302 mmol/L. Xu et al. (2002) isolated a strain of Lactococcus lactis with high GABA for the first time interiorly�� and the amount of GABA in the fermentation broth was 250 mg/100 ml�� which was fermented in 25 L tank for 72 h. It could be used in the development of yogurt or as an ingredient in other foods. Cui et al. (2005) also reported the screening and fermentation conditions of high GABA-producing Lactobacillus�� and the capacity of GABA was 5.4 g/L in the fermentation