GABA, a non-protein amino acid ubiquitous in food matrices

Abstract: GABA has attracted great attention over the last several decades due to its ubiquity in life. It is an important molecule naturally present in considerable amounts in many feed and food matrices of vegetable and animal origin. GABA occurs naturally in plants, animals and microorganisms, having diverse physiological functions and great potential health benefits. Extensive data demonstrates that GABA content is usually higher in plants than in animals and its concentration is in the range of mg g depending on plant matrix, development stage and postharvest processing conditions. In animals, GABA was found at significantly high levels in the brain and central nervous system and some specific peripheral tissues like livestock muscles in the range of μg g. Food items produced by different types of animals, such as eggs, milk or honey, also show remarkable GABA content without any processing steps. A healthy diet following the set of recommendations of WHO national food-based dietary guidelines (FBDG) or/and the Healthy Eating Plate (Harvard) will provide a considerable amount of GABA as a natural nutrient. Additionally, considering its potential health benefits, many efforts are being allocated to developing new technological processes for GABA enhancement in traditional foodstuffs or avoiding losses after processing treatments.

Dr. Roberto Ramos Ruiz is technical director at Servalesa, a company aiming to offer products for farming which provide a differential value through their innovation and contribution to ensure healthier crops for healthier consumers. Amongst other responsibilities, Roberto is in charge of research and development. Servalesa has developed for the past decade several research projects in collaboration with different universities and research organizations. The fundamental objective of these projects is to offer farmers technologies able to mitigate the effects caused by different kind of plant stress with impact on crops, either biotic or abiotic, with an acceptable efficacy, no residues, a minimal impact on the environment and a toxicological profile with no effect on users and consumers through food treated with these products. Within this research activity, Servalesa has a particular interest in studying the effects on crops and impacts on human health and the environment of naturally occurring substances.

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Tendency of experts and consumers towards a healthy diet includes an increasing interest on details about the nutrients (e.g. carbohydrates, fat, protein, vitamins and minerals) needed to achieved a healthy balance diet. GABA is a nonprotein amino-acid that occurs naturally in plants, animals, and microorganisms, having diverse physiological functions and great potential health benefits. Over the last several decades GABA has attracted great attention due to its different positive effects on mammalian physiology. The aim of this review is to compile the levels of GABA measured in nature, specifically in plant and animal products for the food industry.
from flavedo, albedo, pulp, seeds and the content of oil glands obtained from shelf samples of lemon (Citrus limon) and citron (Citrus medica). Only in albedo and pulp was it possible to detect GABA, with a relative average molar content of 0.26 and 0.28 in pulp of lemon and citron respectively. Using the same technique, (Corsaro et al., 2015), studied the cultivar "Interdonato" lemon, which is a hybrid between a cedar and a lemon. They evaluated the lemon juice for different samples of both Protected Geographical Indication (PGI) Interdonato lemon of Messina and Interdonato lemon from Turkey. In the Turkish lemon there was a higher amount of GABA (2.40 mM) [0.247 mg mL −1 ] than in the Italian lemon (1.05 mM) [0.108 mg mL −1 ]. (Sun et al., 2013) measured variations of GABA content in Hirado Buntan Pomelo (HBP; Citrus grandis). The harvested HBP fruits were stored at ambient temperature (16-20°C) and 85-90% relative humidity for 132 days. The average content of GABA in the pulp fruit was more than 140 μg g −1 [0.140 mg g −1 ] Fresh Weight (FW). This content slightly increased in the fruit over time from 145 μg g −1 FW [0.145 mg g −1 ] at day 12-190 μg g −1 FW [0.190 mg g −1 ] at day 78.

Tree nuts
This group comprises fruits that are composed of an inedible hard shell and a seed which is generally edible. It includes a wide variety of dried seeds, the most common being almonds, brazil nuts, cashew nuts, chestnuts, coconuts, hazelnuts/cobnuts, macadamias, pecans, pine nut kernels, pistchios and walnuts. Even though the concentration of GABA in chestnuts is significant (188 nmol g −1 Dry Weight (DW), [0.019 mg g −1 ] (Oh, Moon, & Oh, 2003)) not much information related to GABA content has been reported.
Accumulation of GABA was also observed in loquat fruit under cold conditions (Cao, Cai, Yang, & Zheng, 2012). The content of GABA increased steadily during storage time. After 35 days at 1ºC the amount of GABA in loquat pulp was 49 μg g −1 FW [0.049 mg g −1 ], almost three times more than at harvest (18 μg g −1 FW [0.018 mg g −1 ]).
More recently, (Lee & Hwang, 2017) investigated changes in the physicochemical properties of mulberry fruits at seven maturity stages during ripening. Content of GABA decreased during ripening. GABA contents of the immature mulberry fruits were 113.2 and 59.6 mg 100 g −1 DW [1.132 and 0.596 mg g −1 ] respectively). These concentrations were significantly higher than those of the mature fruits (MS-3-6, 17.1-33.6 mg 100 g −1 DW [0.171-0.336 mg g −1 ]). Final GABA content in the fully mature phase increased slightly to 42.1 mg 100 g −1 DW [0.421 mg g −1 ]. The concentrations of GABA in the leaf, stem, and root bark of mulberry have also been reported. (Kwon, Kim, Hwang, & Park, 2013), using a simple high-performance anion-exchange chromatography-integrated pulsed amperometric detection method, determined that GABA content was 2.22 ± 0.20 mg g −1 in leaf, 2.84 ± 0.20 mg g −1 in stem, and 1.87 ± 0.14 mg g −1 in root bark. These results are in line with those previously reported, showing that GABA content of mulberry root bark ranged from 1.70 to 2.62 mg g −1 (Bang, Lee, Choi, & Kim, 1998) and of mulberry leaf were 2.36 mg g −1 (Yoo, Kim, Kim, & Rhee, 2002).
Omija fruit is another example of a berry which contains GABA  at a substantial amount of 10 mg 100 g −1 [0.1 mg g −1 ] of fruits. It has also been reported (Kim, Lim, & Yang, 2016) that in the ethanol extract of the stems of Elaeagnus umbellata Thunb., GABA was the major free amino acid (300.17 mg 100 g −1 [3.002 mg g −1 ]).
2.1.5.2. Grapes. The amino acid profiles of grape berries harvested at a similar maturity from six different cultivars of Vitis vinifera L. were investigated (Stines et al., 2000). GABA content for Sangiovese, Riesling, Pinot Noir, Cabernet Sauvignon, Muscat Gordo and Grenache were 82. 51, 152.70, 174.30, 146.60, 79.56 and 90.53 mg g −1 FW respectively. The GABA distribution in seeds, skin and pulp was determined for berries of Riesling (RI) and Cabernet Sauvignon (CS). In both varieties, more than 65% of GABA was found in the pulp (RI, 85.36 mg g −1 FW, CS, 107.72 mg g −1 FW). Riesling berries had a higher GABA content in the seed than in the skin (30.88 and 14.03 mg g −1 FW respectively) and the contrary was observed for Cabernet Sauvignon berries (9.30 and 20.04 mg g −1 FW respectively). Data of GABA content in leaves of grapevines of Chardonnay (0.257 μmol g −1 DW [0.027 mg g −1 ]) and Meski (0.200 μmol g −1 DW [0.021 mg g −1 ]) have also been reported (Hatmi et al., 2015). Under drought stress GABA content increased significantly to more than 3,400 μmol g −1 DW [350.6 mg g −1 ].
A chemical study was carried out on deseeded berries of Carlos and Noble muscadine grapes (V. rotundifolia) during berry maturation (Marcy, Carroll, & Young, 1981). HIS plus GABA (also with Thr) were the predominant free amino acids in both cultivars at an immature berry stage (229.9 nm g −1 [0.024 mg g −1 ] fresh deseeded weight). At full berry maturity, HIS plus GABA content increased (397.0 nm g −1 [0.041 mg g −1 ]) and was only surpassed by Arg content. The mean concentration (nm g −1 fresh deseeded weight) of HIS plus GABA for three V. rotundifolia cultivars determined at normal harvest were very similar (Regale, 688.7 [0.071 mg g −1 ], Pride, 699.8 [0.072 mg g −1 ] and Magnolia, 714.3 [0.074 mg g −1 ]). The amount of HIS plus GABA for the Dixie cultivar was lower (444.0 nm g −1 [0.046 mg g −1 ]).
Data on GABA behaviour during maturation was reported by (Murch, Hall, Le, & Saxena, 2010) for wine grapes of Merlot varieties. GABA was found at approximately 115 μg g −1 [0.115 mg g −1 ] in 77% of early stage green grapes (pre-lag) and there was a significant linear decrease in both prevalence and concentration as the grapes matured through the process of véraison (green: 80 μg g −1 [0.080 mg g −1 ], transition, 70 μg g −1 [0.070 mg g −1 ] and purple 40 μg g −1 [0.040 mg g −1 ]). The analysis of metabolite variation throughout the physiological development was also analyzed for the Sardinian Vermentino grape berry (Mulas et al., 2011). The variability in metabolite concentration was investigated as a function of the clone, the position of berries in the bunch or growing area within the vineyard, environmental factors and grape maturity. GABA contents varied between 6 and 67 mg kg −1 [0.006-0.067 mg g −1 ] depending on these factors.
2.1.5.3. Musts, wines and vinegars. Free amino acid contents are of great physiological significance for the final taste and quality of wines and vinegars. They are considered as barcodes to wine authenticity. Many studies have been reported showing the detailed chemical composition and, more specifically, the GABA content of these products (see Table 1).
Considering vinegars, acetatos balsámico di Modena are the ones with the higher GABA content. Sherry vinegars (Spain) show much lower content. Red wine vinegars contain more GABA than vinegars from white wine.

Miscellaneous fruits
The amounts of GABA of different parts of jujube fruits were analyzed (Collado et al., 2014). Data indicated that edible parts (peel and flesh) with 1.4 g kg −1 (DW) [1.4 mg g −1 ] contain more GABA than the pits (shell plus seed), which contain 0.3 g kg −1 (DW) [0.3 mg g −1 ]. Contents decreased with low irrigation and limited soil water conditions. Among small fruits with inedible peel, kiwi has been well studied.  investigated the distribution of GABA in different tissues of kiwi fruit (Actinidia deliciosa) and changes in GABA concentration during maturation. Although amino acid concentrations in the fruit decreased during maturation, GABA, along with Arg, increased to become the predominant amino acids in fruit harvested at the end of May (GABA + Arg: 76.9 μg g −1 FW [0.077 mg g −1 ]) compared to 140.7 μg g −1 FW [0.141 mg g −1 ] in fruits harvested in February. Considering the outer  (Erbe & Brückner, 1998) cortex, Arg/GABA content increased significantly during development in the stem ends (from 26 to 44 μg g −1 FW [0.026 to 0.044 mg g −1 ]) and decreased in middle of the fruit and blossom (17 to less than 5 μg g −1 FW [0.017 to 0.005 mg g −1 ]). The same tendency was observed in the inner cortex, demonstrating increasing Arg/GABA concentrations in stem ends (from 120 to 195 μg g −1 FW [0.120-0.195 mg g −1 ]) and decreasing in middle of the outer cortex and blossom (from around 100 to less than 25 μg g −1 FW [0.100-0.025 mg g −1 ]). In the core, the Arg/GABA content showed the same pattern in all the parts of the fruit, being lower in May than in February (blossom: from 1750 to 1300 μg g −1 FW [1.75 to 1.3 mg g −1 ]; middle of the fruit: from 1200 to 200 μg g −1 FW [1.2-0.2 mg g −1 ], stem: from 600 to 75 μg g −1 FW [0.6-0.075 mg g −1 ]). It could be theorized that as the fruit ripens GABA is transported from the leaf to the fruit .
GABA is the most abundant amino acid in lychee flesh (1.7-3.5 mg g −1 FW), with a concentration approximately 100 times higher than in other fruits (Wu et al., 2016). The concentration varies among cultivars but remains relatively constant during development and maturation. When the amino acid composition of five kinds of lychee juices from cultivars of five regions in China were analyzed, GABA was found to be one of the major amino acids, with an average content of 104.69 mg 100 mL −1 [1.05 mg mL −1 ] (Cui et al., 2011). GABA was also found in other small fruits of inedible peel, such as in the flesh of rambutan, with a low concentration of 0.71 ± 0.23 mg g −1 (Meeploy & Deewatthanawong, 2016); or in "Chuliang" and "Shixia" cultivars of Longan fruit (Dimocarpus longan Lour.) with a GABA content of 13-14 mmol kg −1 FW [1.341-1.444 mg g −1 ] (Zhou, Ndeurumio, Zhao, & Zhuoyan, 2016).

Ramos
Radish (Raphanus sativus L.) has been reported to contain 0.28 ± 0.01 mg of GABA per g of dry weight (DW) of root radish at harvest  and 1 μmol g −1 FW [0.103 mg g −1 ] in mature leaves (Streeter & Thompson, 1972). Powdered sprouts of radish had a GABA concentration of 18.7 mg 100 g −1 DW [0.187 mg g −1 ] (Nakamura et al., 2016). Post-harvest processing, such as dehydration by sun-drying or salt-pressing process, caused an increased of GABA content in roots to 7.30 ± 1.57 and 4.98 ± 0.06 mg g −1 DW, respectively. Leaves also showed an increase of GABA content when submitted to anaerobic stress.

Bulb vegetables
There have been few studies to determine GABA content of bulb vegetables, such as onions, garlics and shallots, among others.  showed that onions contain a low amount of GABA, 12 nmol g −1 DW [0.001 mg g −1 ]. However, (Moreno, Marta Corzo-Martínez, Del Castillo, & Villamiel, 2006) were not able to find GABA, measured as 2-furoylmethyl derivative (2-FM-GABA), neither in dehydrated onion nor in garlic. FM-GABA was only detected at low levels in stored onion. Thus, 2-FM-GABA showed a maximum concentration of 247 mg 100 g −1 [2.47 mg g −1 ] protein and remained constant from the fourth to the tenth day of storage.

Fruiting vegetables
Tomato (Solanum lycopersicum L.; Solanaceae) is a major crop worldwide. It has become an excellent model for the analysis of fruit development, ripening, metabolism and genomic research of solanaceous plants ( (Rastogi & Davies, 1990;Takayama & Ezura, 2015;Yin et al., 2010) and references therein). In comparison with other plants, this vegetable accumulates a large amount of GABA in the fruits (Choi et al., 2014;Morini, Stingone, Cornali, & Sandei, 2015), although the content differs greatly among the varieties (Table 3). GABA content in other parts of the plant has also been reported but is usually found in lower quantities.
Data on GABA content of tomato-processed products has also been reported (Morini et al., 2015). A recent example is the quantification of GABA for each step of the production of tomato vinegar (Koyama et al., 2017): raw tomato juice (422 mg 100 mL −1 ) [4.22 mg mL −1 ], tomato wine after alcohol fermentation (348 mg 100 mL −1 ) [3.48 mg mL −1 ] and tomato vinegar after acetic acid fermentation (398 mg 100 mL −1 ) [3.98 mg mL −1 ].   (Kader et al., 1978) Solanum lycopersicum    (Mori et al., 2013) reported the amount of GABA in nine selected varieties of aubergine (Solanum melongena L.). The results (23.3-38.1 mg 100 g -1 FW) [0.23-0.38 mg g −1 ] were very similar to those reported by (Horie, Ando, & Saito, 2013). The average content in the fruit was 24 mg 100 g -1 FW [0.24 mg g −1 ], and the difference among the varieties was not significant. Heat treatment at 60°C induced the accumulation of GABA in the fruit and doubled the contents of GABA after supplying glutamate to the fruit.
The variability of GABA levels was investigated in six cultivars of bitter melon (Momordica charantia L.) of different origins: Nikko and Peacock from Japan, Galaxy and Verde Buenas from Philippines and two native cultivars from China and Korea . The Philippines cultivar, Galaxy, contained the highest amount of GABA (19.3 μmol g −1 DW) [1.990 mg g −1 ] followed by the Chinese native (14.0 μmol g −1 DW [1.444 mg g −1 ]) which was around five times more than the other cultivars. The cultivars Peacock, the Korean native, Verde Buenas and Nikko contained as low as 3.5, 4.2, 4.8 and 5.2 μmol GABA g −1 DW [0.361, 0.433, 0.495 and 0.536 mg g −1 ], respectively. (Lee, 2016) also studied the GABA content of bitter melon and determined that it was rich in GABA, with a concentration of 283.8 mg 100 g −1 DW [2.838 mg g −1 ]. Courgette fruit (Cucurbita pepo) was found to have less GABA content in the exocarp, fluctuating from 26 to 40 μg g −1 [0.026-0.040 mg g −1 ] depending on variety. GABA content decreased during storage at 4ºC in two varieties, by approximately 70% in Natura fruit (more tolerant to chilling) and 35% in Sinatra fruit (more sensitive) at 7 and 14 days (Palma, Carvajal, Jamilena, & Garrido, 2014). The floral nectar of Cucurbita pepo L was analyzed and showed a GABA content in male and female flowers of 734 ± 86.3 and 678.6 ± 94.1 pmol μL −1 [75.69 ± 8.90 and 69.98 ± 9.70 mg mL −1 ] respectively (Nepi et al., 2012).
Data related to the most common legumes used as food dietary or forage crop are reported in Table 4.

Stem vegetables
Harvested fresh asparagus has a GABA content of 0.15 mg g −1 FW (Zhao & Jiang, 2007). This concentration is positively affected by post-harvest processes, such as soaking treatments in different conditions. Best results were obtained after soaking in citric acid-disodium hydrogen phosphate buffer of pH7.0 or 100 μmol L − 1 CaCl 2 solutions for 2 h, resulting in an increase of GABA content by 73 or 62.23% over the control, respectively.
Standard processing of green asparagus (Asparagus oficinalis, L.) for commercial purposes negatively affected the GABA content (Lopez et al., 1996). No substantial differences were found on the GABA contents of green asparagus, classified by commercial sizes according to the Spanish Quality Classification for processing vegetables (Fine (≤8 mm), Middle (9-11 mm), Thick (12-14 mm); Very thick (15-19 mm; Extra Thick ≥20 mm). Treatments like washing, blanching (5 min in 90°C water by gradual immersion) and canning (time elapsed from harvesting to obtain the processed product was between 18 and 24 h) decreased the amount of GABA to 1.39-1.67 mg g −1 DW, 0.62-1.39 mg g − 1 DW and 0.37 −0.84 mg g − 1 DW respectively. Extra thick asparagus had the highest GABA content once canned.

Fungi, mosses and lichens
The kingdom Fungi is one of the most diverse groups of organisms and is generally recognized as comprising 1.5 million species, classified in five different phyla. Mushrooms generally are considered to be the spore-bearing fruiting body of higher fungi and most belong to the Basidiomycota. As they have been used as foods and food flavouring materials for centuries, their profile of volatile and non-volatile compounds has been widely studied Oka, Tsuji, Ogawa, & Sasaoka, 1981;Rotzoll, Dunkel, & Hofmann, 2006) and references in Table 5). Data for GABA content in different species of culinary-medicinal mushrooms are summarized in Table 5. Concentration of GABA varies between species and strains. (Chen, Kung-Jui, Hsieh, Wang, & Mau, 2012) classified mushrooms into five levels depending on the amount of GABA (GABA content of >200 mg kg −1 DW, 100-200 mg kg −1 , 10-100 mg kg −1 , <10 mg kg −1 , no detection; [>0.200, 0.100-0.200, 0.010-0.100, <0.010, mg g −1 , no detection]). However, due to the variability of the reported data, this classification system is not commonly used.
Huitlacoche or cuitlacoche (Ustilago maydis) is an edible corn smut fungus consumed in Mexico, and it is becoming internationally known as a delicacy for its flavour. Its GABA concentration (0.75 mg g -1 DW) is much lower than most other reported fungi (Lizárraga-Guerra & López, 1996).
Mushroom mycelia of Antrodia camphorata, Agaricus blazei, Hericium erinaceus and Phellinus linteus have been used to substitute 5% of wheat flour to make bread. After baking, myceliumsupplemented bread still contained substantial amounts of GABA (0.23-0.86 mg g −1 DM) (Ulziijargal, Yang, Lin, Chen, & Mau, 2013). (Bent & Morton, 1964) reported a study of the free and combined amino acids of the fungus Penicillium griseofulvum throughout its life cycle. GABA content of Penicillium griseofulvum, during growth in shaken culture, varied depending on the type and duration of the culture (7.3-25.3% of total free amino-N). After 45 h, the shaken culture showed levels of GABA of 4.9% (conidia) and 10.1% (sporogenous mycelium).

Algae and prokaryotes organisms
Marine algae comprises thousands of species that represent a considerable part of the littoral biomass. They are classified as green (Chlorophyta), brown (Phaeophyta) and red (Rhodophyta) algae on the basis of their nutrient and chemical compositions. Brown seaweeds are a very large group and, since ancient times, have been part of the diet in Asian countries. (Cao, Duan, Guo, Guo, & Zhao, 2014) reported the chemical analysis of 24 brown algae samples collected from different locations in China, all of them of the Sargassaceae family (Saccharina japonica, Sargassum pallidum, S. fusiforme, S. thunbergii and S. muticum). GABA was detected in some samples (33%), but generally in trace amounts (1.6-8.6 μg g −1 [0.0016-0.0086 mg g −1 ]).
Some examples of GABA content in green edible algae have been determined. Ulva lactuca (Seher et al., 2013) and green laver  have been analyzed showing an amount of GABA of 71.5 μmol g −1 FW [7.373 mg g −1 ] and 37 nmol g −1 DW [0.004 mg g −1 ], respectively. Chlorella vulgaris is an unicellular green algae that, under cultivation, produces GABA with the highest production rate of 1.90 μg L −1 per day [1.9 10 −6 mg mL −1 ] under favourable conditions (Kim, Lim, Hong et al., 2016).
Due to its high GABA content, it is worth noting the data reported for the aquatic plants Nymphaea alba and Iris kaempferi, 787.76 μmol g −1 FW [81.23 mg g −1 ] and 738.14 μmol g −1 FW [76.12 mg g −1 ], respectively (Seher et al., 2013). (Lahdesmaki, 1968) reported that the amount of GABA in the leaves of Salvinia natans increases with age.
2.2.9.1. Prokaryotes organisms. Phytoplankton represents an important source of carbon and nitrogen in marine systems. (Kittredge, Simonsen, Roberts, & Jelinek, 1962) described for the first time that the dinoflagellate Gonyaulax polyedra had high concentrations of GABA, among other amino acids.
Sinking particles obtained in Breid Bay, Antarctica, at different depths, were analyzed for organic materials, stable carbon and nitrogen isotopes (Handa, Nakatsuka, Fukuchi, Hattori, & Hoshiai, 1992). The acid hydrolysates of these particles consisting of diatoms (mainly Thalassiosira Antarctica) had 15 types of protein amino acids with traces of GABA, β-Alanine and ornithine. The traces of these amino acids indicated that the sinking particles were fresh and that little microbial degradation had occurred.
The analysis made by (Nguyen & Rodger Harvey, 1997) on the contribution of the diatom Thalassiosira weissflogii, the cyanobacterium Synechococcus sp. and the dinoflagellate Prorocentrum minimum, to the amino acid and the particulate carbon and nitrogen pools during their microbially mediated degradation, gave similar results. In the oxic and anoxic dinoflagellate decay experiments, GABA plus β-Alanine reached a maximum of 3.6 and 5.6% respectively of the total hydrolysable amino acids and, thereafter, decayed to concentrations not significantly different from the initial concentrations (1.5%).
Particles in sea water can originate from a variety of sources including phytoplankton biomass, fragments and moulds of crustaceans, faecal pellets and exudates, resuspension of sediments, terrestrial inputs from rivers and even Aeolian transport. Biochemical and transformation processes occur in these sinking particles throughout the water column, ending as sediments, where they continue their metabolic degradation. Examples of processes will be discussed in the section regarding GABA in soils.

GABA in pulses
According to the Food and Agriculture Organization (FAO), pulses are defined as "Leguminosae crops harvested exclusively for their grain, including dry beans, peas and lentils". Pulses are categorized into 11 groups as follows: dry beans, dry broad beans, dry peas, chickpeas, blackeyed peas, pigeon peas, lentils, bambara groundnut, vetch, lupins and other "minor" pulses. It is difficult to differentiate between fresh and dried seeds within literature on pulses, including metabolite profile during development . Therefore, within this review the information related to seed leguminosae, either fresh or dry, has been included in the section on legume vegetables.

Oil seeds
Soybean or soya bean is a legume classified as an oilseed due to its high oil content. Soybean accounts for more than a half of the overall world oilseed production. Soybean is a very versatile crop with many different applications from human food to industrial uses. It has been recognized for its healthy properties, being the focus of multiple research projects. GABA content in soybean has been well studied, including the influence of culture conditions during development, postharvest treatments and processing.
The GABA content of dried soybeans was reported to be 211 μg g −1 [0.211 mg g −1 ] of soybeans (Zazzeroni et al., 2009) and it was found to be higher for powdered sprouts, 1.16 mg g −1 (Nakamura et al., 2016). Similar to the observations for other seeds, soybean germination caused a significant increase of GABA. GABA content increased around four-fold from 0.25 to 0.9 mg g −1 DW in Glycine max var. Jutro after 4 days of germination compared to var. Merit, which took 6 days of germination to reach 1.09 mg of GABA g −1 DW (initial GABA content, 0.26 mg g −1 DW) (Martinez-Villaluenga et al., 2006). (Tiansawang et al., 2016) obtained the best results after 6 h of incubation, from 0.1222 g kg −1 DW [0.1222 mg g −1 ] at time zero to 0.4977 g kg −1 DW [0.4977 mg g −1 ]. The influence of germination in isolated germs was also evaluated. GABA content increased considerably as germination progressed, from 26.5 mg 100 g −1 [0.265 mg g −1 ] in ungerminated soy seeds to 718.0 mg 100 g − 1 [7.180 mg g − 1 ] after 24 h of germination. The total GABA content of whole soybeans (var. Daepung) was 5.79 mg 100 g −1 [0.058 mg g −1 ] . (Abe & Takeya, 2005) reported the quantitative differences in free amino acids and GABA in the cotyledon of immature seeds harvested at 35 days after flowering (DAF) of six vegetative-type soybean (Edamame) and two grain-type soybean. The concentration of GABA varied greatly among cultivars. Immature seeds of two vegetative-type soybean cultivars, Shirayama-dadacha and Wase-shirayama, had the highest content of GABA at 15 DAF with over 50 mg 100 g −1 FW [0.50 mg g −1 ], and it remained high until 35 DAF, after which it decreased until 50 DAF.
Processing treatments also modify GABA concentration. Drying of immature seeds of vegetable soybean (Glycine max L. Merrill) at a maximum temperature of 40ºC increased the GABA content more than 5 times. Untreated seeds contained 79.6 mg of GABA 100 g −1 DW [0.796 mg g − 1 ] and heat-dried seeds accumulated 447.5 mg 100 g −1 DW [4.475 mg g −1 ] (Takahashi, Sasanuma, & Abe, 2013). Cooking processes had a negative influence on the GABA content of germinated soy beans. The nutritional evaluation of sesame seeds and sprouts has also been reported. GABA content of untreated Sesamum indicum seeds was very low, 24.12 μg g − 1 DW [0.024 mg g −1 ]. As germination progressed an increase of GABA was found, almost three times higher than in seeds 5 days after seeding, 95.28 μg g −1 DW [0.953 mg g − 1 ] (Liu, Guo, Zhu, & Liu, 2011). The same behaviour was reported by (Tiansawang et al., 2016), who found that the initial amount of GABA, 90.8 μg g −1 [0.091 mg g −1 ] increased to 165 μg g − 1 [0.165 mg g −1 ] after 6 h of germination. (Bor et al., 2009) observed 150 μg g −1 FW [0.150 mg g −1 ] of GABA in sesame seedlings.
The influence of post-harvest processing treatments in sesame seeds on GABA content is very similar to that described for soybean seeds. Temperature treatments induced GABA enrichment with a maximum GABA content of 0.84 μmol g − 1 [0.087 mg g − 1 ] when the seeds were heated at 100ºC (GABA content of raw seeds was 0.06 μmol g − 1 [0.006 mg g −1 ]). Moreover, when water was added to sesame seeds, heating treatment increased GABA production to a maximum of 4.2 μmol g − 1 [0.433 mg g − 1 ] when the seeds were heated at 60ºC (Katsuno et al., 2015). On the contrary, cooking decrease GABA content of germinated sesame seeds. The amount of GABA in germinated sesame was 0.165 g kg − 1 DW [0.165 mg g − 1 ], which decreased after boiling, steaming, microwave cooking and open pan roasting to 0.072, 0.073, 0.158 and 0.093 g kg − 1 DW [0.072, 0.073, 0.158 and 0.093 mg g − 1 ] respectively (Tiansawang et al., 2016).

Oil trees
The main focus of the studies performed with olives was to determine the chemical composition related to fatty acids, vitamins and volatile components. There is very little information related to the amino acid profile. (Rosati et al., 2014) reported that olive (Olea europaea L.) of Leccino cultivars had larger amounts of GABA than Frantoio cultivars.

GABA in cereals
Cereals are the edible seeds or grains of the grass family, Gramineae, including corn, barley, oats, triticale, millet, sorghum and rice. On a worldwide basis, wheat and rice are the most important crops, accounting for over 50% of the world's cereal production. Cereals are basic foods, and are an important source of energy, carbohydrates, proteins and fibres, as well as containing a range of micronutrients such as vitamin E, some of the B vitamins, magnesium and zinc. They are also an important source of GABA compared with other vegetables . Many studies are available reporting GABA content in cereals (Tables 6 and 7) and nearly half of them focused on rice ( Patil & Khan, 2011), references in Table 7).
Data of GABA content in cereals other than rice and data related to rice are shown in Tables 6  and 7 respectively. 2.6. GABA in teas, coffee, herbal infusions, cocoa and carobs 2.6.1. Teas Tea is one of the most widely consumed beverages in the world made from the leaves and buds of Camellia sinensis (L.). Tea contains many chemical components such as amino acids, polyphenols (catechins and flavonoids), polysaccharides, volatile oils, vitamins, minerals and alkaloids (Syu, Lin, Huang, & Lin, 2008). Amino acids account for approximately 1-4% of the dry weight of fresh tea leaves, which mainly comprise theanine, glutamic acid (Glu), Asp, Arg and GABA (Zhao et al., 2013). As in other vegetables, the amino acid profile differs among species and cultivars, incubation under stress conditions (Sawai, Yamaguchi, Miyama, & Yoshitomi, 2001;Tsushida & Murai, 1987) or fermentation (Jeng, Chen, Fang, Chien-Wei Hou, & Yuh-Shuen, 2007). Table 8 details some of the data reported in the literature.

Herbal infusions
Information about the free amino acid pool and the role of these substances in non-Camellia teas is not as well studied. (Bi et al., 2016) studied 33 non-Camellia teas collected in China. GABA was detected in all teas except tea from Sarcandra glabra. The GABA content in teas from Ampelopsis grossedentata (2.26 mg g − 1 ), Isodon serra (1.82 mg g − 1 ) and Hibiscus sabdar-iffa (1.03 mg g − 1 ) were the highest, much higher than that in green tea (0.28 mg g − 1 ).
The GABA content of some extracts was also studied (Sahin, Eulenburg, Kreis, Villmann, & Pischetsrieder, 2016) to identify specific allosteric GABAAR modulators. Reported results of the amount of GABA in 1 mg mL −1 plant extracts were as follows: Sage leaves: 0.33 μg mL A study about the composition of free amino acids in 19 species of botanical plants was reported (Carratu, Boniglia, Giammarioli, Mosca, & Sanzini, 2008). GABA was detected in almost all the extracts of dried plants (from 5 to 629 mg 100 g −1 FW [0.05-6.29 mg g −1 ]). GABA was one of the major amino acids observed in the following 13 extracts: Camellia sinensis, Coleus forskohlii, Echinacea angustifolia, Echinacea pallida, Echinacea purpurea, Ginkgo biloba, Glycine max, Griffonia simplicifolia, Hypericum perforatum, Panax ginseng, Passiflora incarnate, Serenoa repens, Sutherlandia frutescens and The GABA content of Asian ginseng (Panax ginseng C.A. Meyer) was studied in more detail (Kuo, Ikegami, & Lambein, 2003). The seeds and some parts of the plants (one to three years old) were analyzed. GABA concentrations increased dramatically after germination and reached its maximum in 70% ethanol extracts of the 3-year-old plants: Seed 0.051 mg g −1 ; 1 year whole plant: 1.175 mg g −1 ; 2 years root and stem plus leaves, 0.972 and 2.284 mg g −1 , respectively and 3 year root, stem and leaves plus buds 1.778, 2.335 and 2.774 mg g −1 of GABA respectively.
Processing also changed the amino acid profile of Panax ginseng. GABA content of White Ginseng (0.876 mg g −1 DW) decreased to 0.659 mg g −1 DW after steaming at 100ºC (Red Ginseng) and to 0.161 mg g −1 DW after steaming at 120ºC (Cho et al., 2008).    (Bytof, Knopp, Schieberle, Teutsch, & Selmar, 2005). In order to produce tradable standard green coffee, beans are usually dried, leading to a GABA accumulation during the process. Unwashed Arabica beans produced by drying processes had a higher GABA content (1009-2619 nmol seed −1 [0.104-0.270 mg]) than washed Arabica beans (89-264 nmol seed −1 [0.009-0.027 mg]) resulting from a less stressful wet processing method. (Kramer, Breitenstein, Kleinwächter, & Selmar, 2010) reported that GABA accumulation in coffee beans could be associated to the drought stress induced by the drying process.
GABA was detected in C. arabica L. (arabica) green coffee from Burundi, Colombia and Guatemala in different ratios. Interestingly, lower levels of GABA were observed in speciality beans compared to commercial-grade green coffee beans (Kwon et al., 2015).
Little information is available in literature regarding GABA content in cocoa. (Marseglia, Palla, & Caligiani, 2014) provide an overview on the GABA content in 39 fermented and dried cocoa beans from different geographical origins (13 from Africa, 20 from Central/South America, 4 from Asia and 2 from Oceania). Results showed that cocoa beans are an excellent source of GABA and its content is extremely variable as a function of the geographical origin. Cocoa beans from Africa showed a GABA content ranging from 35 to 93.9 mg 100 g −1 [0.35-0.939 mg g − 1 ]; cocoa beans from America had a GABA concentration from 31.7 (minimum found in Grenada beans) to 101.2 mg 100 g −1 [0.317 to 1.012 mg g −1 ]. The maximum was measured in Ecuador beans. Cocoa beans from Asia and Oceania had a GABA content in the ranges of 47-95 and 45-68 mg 100 g −1 [0.47-0.95 and 0.45-0.68 mg g −1 ], respectively.
Concentrations of free amino acids in cacao tissues depend on the ontogenic stage of the somatic embryos and the culture conditions (Niemenak, Saare-Surminski, Rohsius, Ndoumou, & Lieberei, 2008). The contribution of GABA to the total free amino acids in embryogenic callus is substantial compared to non-embryogenic callus and further developmental stages of cacao somatic embryos.

GABA in hops
Studies related to the activity of hops (Humulus lupulus L.) have been published. Although some of them relate to GABAergic functions (Moir, 2000;Zanoli & Zavatti, 2008), most do not provide any data on the GABA content in hops. (Sahin et al., 2016) reported that extracts of 1 mg mL −1 of hop cones contains 0.44 μg mL −1 [0.00044 mg mL −1 ] of GABA.

GABA in spices
Spices have a long history of both culinary uses and of providing health benefits (Tapsell et al., 2006). Nevertheless, quantitative GABA contents in the different species have not been extensively studied. GABA content of Curcuma sp. varies depending on the species. In Curcuma aromatica Salisb. from India the GABA concentration was 0.04 μg mg −1 [0.04 mg g −1 ] but Curcuma longa L, both from Korea and Myanmar contain more than 1 μg mg − 1 [1 mg g − 1 ] of GABA, (1.11 and 1.31 μg mg −1 [1.11-1.31 mg g −1 ], respectively (Jung et al., 2012)). It was also reported that the concentration of GABA in ethanol extracts from Zingiber officinale Rosc. from Korea reached 1.12 μg mg −1 [1.12 mg g −1 ]. GABA content in Zingiber sp. was also identified by (Anju, Moothedath, & Shree, 2014).

GABA in sugar plants
Metabolomic studies on sugar beets are not common (Kazimierczak et al., 2014) and the amino acid content has not been determined. (Sekiyama, Okazaki, Kikuchi, & Ikeda, 2017) demonstrated that GABA could be found in early growth stages in leaves of sugar beets (Beta vulgaris L). As a pool of amino acids, GABA with Orn, Pro, Thr Iso, Val, Leu, Ala, His, Lys and Phe, was shown to increase in roots of chicory during plant development from 27.16 nmol mg −1 DW [2.801 mg g −1 ] at day 21 to 49.81 nmol mg −1 [2.166 mg g −1 ] at day 56 (Druart, Goupil, Dewaele, Boutin, & Rambour, 2000).

GABA in miscellaneous plants
This section collects some examples of the presence of GABA in plants that are not included in the previous groups. Examples of GABA content for pollen and floral nectar have also been included here. As discussed throughout the review, GABA content differs among species, tissues and development stages (Table 9). Values are in similar ranges than those reported in other sections.
Reported data for pollen samples also showed high variability among species.

GABA in animals
GABA is widely distributed in nature independently of the type of organisms, species and even phylum. In animals, it seems that GABA plays multiple roles and functions considering the location of GABA receptors in nearly all tissues.
Due to the significant amount of information retrieved, this section describes the results measured in rodents, mammals different than livestock and human tissues. Studies on livestock are described in a separate section.

GABA in humans
Medical and pharmacological effects of GABA have been widely studied due to its numerous physiological functions and positive effects on metabolic disorders. However, abnormal GABA levels do not always result in an illness. Considering different tissues, one of the most common analysis is the measurement of the amount of GABA in plasma. It seems that there are no marked effects on plasma GABA concentrations due to gender, exercise, diet, season, time of day or menstrual cycle (Petty, 1994). GABA levels in other tissues are summarized in Table 11, including results from in vivo analysis (Goddard, Mason, & Almai et al., 2001;Terpstra, Ugurbil, & Gruetter, 2002).

GABA in other mammals
Several studies have been performed to obtain information on amino acids and related compounds from tissues of different mammals. Reported results demonstrated a similar pattern to that shown previously, brain and ocular tissues contain higher concentrations of GABA, although it is distributed in several other organs of mammals. Table 12 shows GABA content in different tissues from cats, dogs and monkeys.

GABA in livestock
Consumer demand and regulatory requirements for food items of high quality and nutritional properties are constantly increasing. A wide number of methods and breeding programs have been developed to maintain healthy livestock and improve production, yield and quality, from adapted feeding diets to controlled facilities and regular health inspections. Within these procedures, many controls evaluate livestock performance and evolution of plasma metabolites during development. Amino acid profiles, besides the nutritional properties, are of crucial importance due to their particular contribution to the taste. Free amino acids are classified into four categories (saccharinity, amino acids with sulphide, fragrant amino acids and essential amino acids.) However nonprotein amino acids are not considered and GABA is not usually quantified (Bermúdez, Franco, Carballo, Sentandreu, & Lorenzo, 2014;Iida et al., 2016;Lim, Jo, Seo, & Nam, 2014;Lisa, Spraggins Jeffrey, Reyzer Michelle, Norris Jeremy, & Caprioli Richard, 2014;Mullen et al., 2000;Soriano-Santos, 2000;Subbaraj, Brad Kim, Fraser, & Farouk, 2016).
In recent years, along with the increasing knowledge of its functional properties, GABA has become part of the diet of livestock (Li et al., 2015;Tang & Chen, 2016;Wang, Wang, Liu, Liu, & Ferguson, 2013;Zhang, Zou, Li, Dong, & Zhao, 2011;Zhigang, Sheikhahmadi, & Li, 2013). The final objective is to obtain feeding material with good nutritional and sensory properties improving the life quality of the animals. Table 13 summarizes some data related to GABA content in livestock. Although most of the results are not from tissues included in human diets, they provide evidence of the natural occurrence of GABA in these different animals. Data for muscles show a GABA content of more than 120 μg g − 1 in Longissimus lumborum muscle of swine. GABA distribution is similar to that described previously for humans and other mammals.
Food items produced by different type of animals, such as eggs, milk or honey, also show remarkable GABA content without any processing steps. It is interesting that even human milk for baby nutrition contains 0.01 µg mL −1 GABA. Data are shown in Table 14.
These receptors have become the target of numerous commercial insecticides and plants use this pathway as defence against invertebrate pests (Bown et al., 2006). Table 15 shows some examples of GABA content in insects.

GABA in products for human diet
Consumer preferences towards healthier lifestyles and safe and nutritional food products are the main drivers for producers to offer best-quality products. Studies with the aim of understanding and evaluating the real composition and functional claims of conventional products in our diets have become common Hermanussen et al., 2010). The food industry increasingly develops the functionality of traditional products, trying to improve their properties and to add health claims.
Considering the interest of GABA as a functional food ingredient, many efforts have been made to improve the GABA content during food manufacturing, not only by just adding GABA but also by using ingredients with high GABA content or "in-situ producers of GABA" (Dhakal et al., 2012;Kook & Cho, 2013;Poojary et al., 2017;Quílez & Diana, 2017).
Nervous system

113-153
μg g −1 Ventral nerve cord (Lin & Cohen, 1973) (Continued) Radial nerve-cord (ectoneural tissue) (Osborne, 1971) Asterias rubens  Table 16 shows the GABA levels in different examples of usual foodstuff like cheese, yoghurt, flour or bread; fermented food like Kimchi or Tempeh, very common in Asian countries and with high content of GABA; new developments mostly based on fermentation processes with GABAproducer starters and commercial processed products.

GABA in the environment
In the past, studies about the role of nitrogen in soil to sustain crop production in agricultural systems have been focused on inorganic nitrogen dynamics. The ability of numerous crop species to take up organic nitrogen from the free amino acids pool has increased the interest in the organic chemical composition of soils (Amelung, Zhang, & Flach, 2006;Bol, Ostle, Petzke, Chenu, & Balesdent, 2008;Friedel & Scheller, 2002;Miltner, Kindler, Knicker, Richnow, & Matthias, 2009;Scheller & Raupp, 2005).
Amino acids are widely present in soils (approximately 20-30% of total nitrogen) either in a free or a polymeric state (e.g. protein-humic complexes and peptides). The majority of amino acids are in a polymeric state and only 0.04-0.5% of the total weight is free amino acids. Most amino acids in the soil are derived from plant residues and root exudation but also from dry and wet deposition, microbial activity and animal inputs. The final level of amino acids in soil is influenced by many parameters from the soil matrix to the local environment, including plant life, external human intervention or microbial communities (Jones, Owen, & Farrar, 2002;Vieublé Gonod, Jones, & Chenu, 2005). Amino acid concentrations ranged from 3.9 to 16.5 g kg −1 [3.9-16.5 mg g −1 ] soil and correlated with the organic C and N contents at the sites (Amelung et al., 2006). The most abundant amino acids in soils were Glu, Gly, Asp, Arg and Ala.
In sediments, the amino acid composition may be altered with an increased turnover of proteins (Dauwe, Middelburg Jack, Peter, & Heip Carlo, 1999). In fact, the chemical composition of the sediments is considered to be a maturity indicator to estimate the relative degradation state of the organic matter. In order to understand compositional changes and evolution of oceanic and deepsea sediments, many studies of analysis of the water column at different depths have been reported (Goutx et al., 2007;Ittekkot, Deuser, & Degens, 1984). Specifically, the examinations of non-protein amino acids like GABA and β-Alanine in sea particles are used as long-term signs of organic degradation and chemical evolution of sediments. As an example, GABA and β-Alanine, degradation products of Asp and Glu, tend to accumulate in older sediments and their relative molar concentration is lowest in the surface sediments and highest in the bottom sediments (Gupta & Kawahata, 2003a, 2003b. Surface waters or surface sediments have no GABA or very low levels (Müller, Suess, & AndréUngerer, 1986;Zhao, Shan, Tang, & Zhang, 2015). Examples of concentrations of GABA in sediments and soils are shown in Table 17.
Concentrations of free and combined amino acids in atmospheric particles have also been investigated. (Zhang & Anastasio, 2003) reported that the average concentrations of combined amino compounds (proteins and peptides) were generally four to five times higher than those of free amino compounds (amino acids and alkyl amines). GABA accounted for only 1% of the free amino acids in the atmospheric fine particles but 7% in the water fog. A different result was obtained by (Filippo et al., 2014) who did not detect GABA as a combined amino acid. Data are included at the end of Table 17.

Conclusions
Reviewed literature shows the great attention GABA has attracted over the last several decades due to its ubiquity in life. GABA occurs naturally in plants, animals and microorganisms, having diverse physiological functions and great potential health related benefits. Extensive data demonstrates that GABA content is usually higher in plants than in animals and its concentration is in the range of mg g −1 depending on plant matrix, development stage and postharvest and processing conditions. GABA is present in almost all types of fruits and vegetables investigated, including wheat and rice as the worldwide most important crops, and in several food crops like tomato,        potato, asparagus or spinachs, GABA contents are above 1 mg g −1 . In animals, GABA was found at significantly high levels in the brain and central nervous system and some specific peripheral tissues like the pancreas, female reproductive tissues and retina. In the other peripheral tissues, GABA was also present in less abundant levels in the range of μg g −1 .
GABA is an important molecule naturally present in considerable amounts in many feed and food matrices from vegetable and animal origin. A healthy diet based on plant products (cereals, vegetables and fruits) following the WHO food-based dietary guidelines (FBDG, adapted to different countries and graphically represented in several food guide pyramids) or/and the Healthy Eating Plate, 4 will provide a considerable amount of GABA as a natural nutrient. Additionally, considering its potential health benefits, many efforts are being allocated to developed new technological processes for GABA enhancement in traditional foodstuff or avoiding losses after processing treatments. Of particular relevance is the use of microorganisms such as yeast fungi or lactic acid bacteria with the ability of producing GABA within the food matrix. GABA research has been intensified in recent years in parallel with the interest of the food industry in its roles as a health-related compound. The increased tendency of consumers to support functional food will contribute to maintain this research into GABA and its physiological roles.

Funding
The authors received no direct funding for this research.