A rapid gas chromatographic method for direct determination of short-chain (C₂–C₁₂) volatile organic acids in foods

Abstract

A simple, rapid and accurate GC analytical method for direct quantification of short-chain volatile organic acids in liquid foods was established. Hydrophilic 1,3-butanediol was selected as the internal standard. Thirteen volatile organic acids including acetic, propionic, isobutyric, butyric, isovaleric, valeric, caproic, heptanoic, caprylic, capric, lauric, lactic and levulinic acids were simultaneously determined with detection limits 0.025–1 ng . The recovery rates of tested acids from fruit juice and vinegar were 92–109% with coefficients of variation below 9.4%. The contents of volatile organic acids in 37 commercial liquid food samples were determined.

Introduction

Short-chain volatile organic acids (VOAs) with carbon numbers ranging from two to 12 significantly affect the flavor and quality of food (Alur, Doke, Warrier, & Nair, 1995). These volatile acids, mainly acetic acid and less often propionic acid and butyric acid, may originate from raw materials or be generated by fermentation during processing and storage. The levels of volatile acids in a variety of foods have special indications. For instance, in 100 ml of wine the volatile acid content as represented by acetic acid should be <0.05 g (Anonymous, 1992). However, in cases of unqualified materials, contaminated utensils, inadequate sterilization and microbial contamination during storage, sugar fermentation results in the formation of volatile acids and impairs the quality of products. Therefore, in some foods volatile acid content is an index to quality assurance.

The determination of volatile acids can be traditionally conducted by either indirect or direct ways. The former is to measure residual non-volatile acids after evaporation of volatile acids and then subtract the amount of non-volatile acids from that of total acids. On the other hand, the direct method is to titrate distilled volatile acids with standard alkaline solutions (AOAC, 1984, Blanco Gomis & Mangas Alonso, 1996). Nevertheless, both methods determine the total amount rather than the composition of volatile acids.

There are many ways to determine organic acids in foods (Blanco Gomis & Mangas Alonso, 1996). Among which gas chromatography (GC) with split injection is unsatisfactory for the analysis of free C₁–C₁₂ short-chain organic acids, since their high polarity retards the separation of these compounds from food and the response of flame ionization detector (FID) to them is quite low (Larsson & Roos, 1983). Therefore, these short-chain organic acids usually need to be derivatized before analysis (Van Huyssteen, 1970). But the derivatization process is time-consuming. In addition, the volatility of the derivatized compounds is so high as to reduce the recovery and affect the accuracy and repeatibility of GC quantification (Mccalley et al., 1984, Burke & Halpern, 1983, Moffat et al., 1991). To solve these problems, headspace analysis was considered (Mulligan, 1995). Alternatively, the short-chain organic acids were prepared into less volatile derivatives for GC analysis (Shaw & Bickling, 1986, Chauhan & Darbre, 1982, Bahre & Maier, 1996, Kim et al., 1987, Molnar,-Perl & Szakacs-Pinter, 1986). However, the procedure is still tedious and not applicable to routine analysis.

Gas chromatography, which provides high resolution as well as excellent sensitivity, is one of the most important modern analytical techniques. In our experience, direct injection gave higher detection response than the usually used split injection mode, and the insertion of glass wool into the glass liner of the injection port helped to prevent the analytical column from being contaminated by non-volatiles as to reduce the interference from contaminants (Choong et al., 1997, Lee et al., 1998, Wang et al., 1997, Lin & Choong, 1999). We also found the commercial megapore GC columns were quite resistant to water (Lin & Choong, 1999). Even when the water solution was directly injected into the column, the separation effect and the retention time remained the same as of the new column. That means aqueous samples without any pretreatment were eligible for direct injection into GC columns and being quantitatively analyzed.

In the present study, C₂–C₁₂ volatile organic acids in aqueous samples such as vinegar, fruit juice, lactic acid drink, fermented milk and soy sauce were quantitatively analyzed by the direct injection method. All samples were mixed with an appropriate amount of aqueous internal standard solution and then directly injected into a gas chromatograph without derivatization. The purpose of this study is to establish a simple, rapid and accurate GC analytical method for direct quantification of short-chain volatile organic acids in liquid foods.