Development of a Method to Isolate Glutamic Acid from Foodstuffs for a Precise Determination of Their Stable Carbon Isotope Ratio

Abstract

Recent growing health awareness is leading to increasingly conscious decisions by consumers regarding the production and traceability of food. Stable isotopic compositions provide useful information for tracing the origin of foodstuffs and processes of food production. Plants exhibit different ratios of stable carbon isotopes (δC) because they utilized different photosynthetic (carbon fixation) pathways and grow in various environments. The origins of glutamic acid in foodstuffs can be differentiated on the basis of these photosynthetic characteristics. Here, we have developed a method to isolate glutamic acid in foodstuffs for determining the δC value by elemental analyzer-isotope-ratio mass spectrometry (EA/IRMS) without unintended isotopic fractionation. Briefly, following acid-hydrolysis, samples were defatted and passed through activated carbon and a cation-exchange column. Then, glutamic acid was isolated using preparative HPLC. This method is applicable to measuring, with a low standard deviation, the δC values of glutamic acid from foodstuffs derived from C3 and C4 plants and marine algae.

Distinguishing glutamic acid in foodstuffs and monosodium glutamate used as seasoning by stable carbon and nitrogen isotope ratios

“…When isotopic fractionation is observed, isotope ratios are altered during pretreatment procedures, such as purification and derivatization, prior to measuring stable isotope ratios ( Macko et al., 1987 ; Silfer et al., 1991 ). Nevertheless, we previously confirmed that subjecting a reagent-grade MSG sample to hydrolysis by hydrochloric acid did not significantly change δ 13 C values ( Kobayashi et al., 2018 ) in a series of pretreatment steps. Isotope carbon ratio values before and after hydrolysis step of the reagent MSG were −14.0 ± 0.1‰ and −14.1 ± 0.1‰, respectively.…”

“…The isotope ratio values for the carbon and nitrogen from the glutamic acid in foodstuffs were separately measured. Carbon isotope ratio values were measured using EA/IRMS after the glutamic acid was isolated according to the preliminary optimization of our pretreatment method ( Kobayashi et al., 2018 ). Nitrogen isotope ratio values were measured using GC/C/IRMS according to a pretreatment method previously reported ( Chikaraishi et al., 2007 ).…”

“…This process was performed according to our preliminary optimization study ( Kobayashi et al., 2018 ) to isolate glutamic acid for carbon isotope ratio analysis. A total of 20 mL hydrochloric acid was added to the vegetables, beans, and mushrooms samples (6 g) and to the kelp, marine products, and livestock products samples (3 g) followed by heating at 110 °C for 20 h under a nitrogen atmosphere to hydrolyze proteins.…”

“…Given the potential differences that occur because of geographical factors, δD and δ 18 O were excluded as measurement targets. Recently, we have developed a method to isolate glutamic acid from various foodstuffs for precise determination of their δ 13 C ( Kobayashi et al., 2018 ). We employed different pretreatment methods to measure the δ 13 C and δ 15 N values of glutamic acid.…”

Establishment of a method for determining the origin of glutamic acid in processed food based on carbon and nitrogen stable isotope ratios

“…Thus, to establish a method for distinguishing whether the glutamic acid present in food products is added seasoning MSG or natural glutamic acid, we utilized isotope-ratio mass spectrometry (IRMS), as reported in our previous publications [5, 6]. IRMS has often been used in ecology and the geosciences to precisely measure the stable isotope ratios of hydrogen, carbon, nitrogen, oxygen and sulfur [7, 8].…”