Analytical, Nutritional and Clinical Methods SectionA rapid gas chromatographic method for direct determination of short-chain (C2–C12) volatile organic acids in foods
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.
Section snippets
Materials
Thirty-seven test samples, including six kinds of fruit juice (orange, plum, mango, cherry, grape and apple juice), five samples each of vinegar, vinegar beverage and soy sauce, four samples of fresh milk, two samples of spoiled milk (as control), five samples of fermented milk, three samples of yogurt drink and two samples of lactic acid beverage, were purchased from the local supermakets in Tainan and Pintung areas. The authentic compounds such as acetic acid (2C), propionic acid (3C),
Conditions of GC analysis
For selection of the analytical column, polar Chrompack CP-Wax column (30 m×0.53 mm) as well as CP-Sil 24 CB and CP Sil 8 CB columns (30 m×0.53 mm) of medium polarity was considered. Results showed the polar CP-Wax column was the best choice for the analysis of polar volatile organic acids (data not shown).
The column temperature gradient for this direct injection method was described in Section 2. Fig. 1 shows the chromatogram of authentic compounds. The retention times of acetic, propionic,
Conclusions
A simple, rapid and accurate GC method for direct determination of 13 short-chain volatile organic acids in liquid foods was established. With the current method, tedious pretreatment of samples such as distillation and extraction was omitted. Therefore, it took only 40 min to simultaneously determine the contents of 13 VOAs in comparison with 4 h to determine acetic and lactic acids with AOAC methods. Furthermore, the recoveries of VOAs, 92–109% from vinegar and 93–108% from orange juice, and
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