Acetic acid

The following standards were used for liquid chromatography analyses: xanthines (caffeine, theobromine and theophylline), monomeric flavan-3-ols ((+)-catechin, catechin gallate, (−)-Epicatechin, Epicatechin-3-gallate, epigallocatechin, epigallocatechin-3-gallate (EGCg), green tea catechin mix), theaflavins (theaflavin, theaflavins mix (tea extract from Camellia sinensis)) and gallic acid. They were purchased from Merck (Barcelona, Spain) and Phytolab (Vestenbergsgreuth, Germany) for the identification and/or quantification of characteristic bioactive components from tea (Camellia sinensis). Trifluoroacetic acid, acetic acid, acetonitrile, dimethyl sulfoxide and water of chromatographic quality were purchased from VWR (Barcelona, Spain).
Regarding gas chromatography, the standards used were as follows: 1-penten-3-ol, n-hexanal, n-hexanol, cisZ-3-hexenal, transE-2-hexenal, cisZ-3-hexenol, transE-2-hexenol and pentanol, and for Group II, linalool, linalool oxides, methyl salicylate, phenyl acetaldehyde, geraniol, benzyl alcohol, 2-phenylethanol, benzaldehyde, α-ionone and β-ionone were purchased from Sigma Aldrich, Steinheim, Germany. Methanol, acetic acid and furfural were obtained from VWR, Darmstadt, Germany.

The catalytic activity in the esterification reaction (acetic acid and n-butanol) was studied in a 50 mL plastic centrifuge tube at 80 °C for 1 h. Prior to reaction, the mixed oxides were activated by heating under vacuum at 150 °C for 5 h. A stock solution was prepared by adding acetic acid (6.00 g, 0.1 mol, VWR chemicals 99–100%) and 1-butanol (14.80 g, 0.2 mol, Acros Organics 99.5%). Toluene (Fisher chemical 99.8%) was added to obtain a 100 mL total volume. The esterification reaction was carried out by adding 5 mL of the stock solution to the catalyst. For each catalyst sample, two tests were realized: a fixed mass of catalyst (100 mg) and a fixed mole percentage of Al. The mass weighting of the catalyst was calculated to obtain 20 mol% of Al related to the number of mole of acetic acid. A reference reaction was also carried out in the absence of sample (labeled “blank”). After 1 h at 80 °C, the tubes were cooled down to room temperature. The tubes were centrifuged at 20,000 rpm for 20 min. The supernatants were analyzed using GC. The ester yield (butyl acetate) was defined as the number of moles of ester produced per moles of acetic acid introduced. Byproducts were not detected.

Ethanol (99.99%; Fisher Scientific, UK), glucose (laboratory reagent; Merck Extra Pure, England), fructose (laboratory reagent; Pharmacos Ltd, England), sucrose (analytical reagent; Guangdong Guanghua Chemical Factory Co. Ltd, China), and maltose (laboratory reagent; The British Drug Houses Ltd, Poole, England) were used to prepare synthetic calibration and validation sets. p-Amino benzoic acid ethyl ester (ABEE) (Riedel-de Haen AG, Seelze, Hanover, Germany), acetic acid (99.5%; BDH Chemicals Ltd, Poole, England), and sodium borohydride (BDH Chemicals Ltd, Poole, England) were used for derivatizing sugars. Sulfuric acid (Sigma-Aldrich, Germany) was used for the determination of total carbohydrate. Chloroform (Carlo Erba Reagents, France) was used for purifying the derivatized compound. Distilled deionized water was used for the preparation of standards and dilution of samples.