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A study of the influence of extraction conditions (pH: 1.5 to 2; temperature: 80 to 90 degrees C; extraction time: 1 to 3 h), on the yield and purity of apple pomace pectin without elimination of impurities by alcohol washing was carried out. The alcohol precipitate yields varied from 2.9% to 8.9% depending on the pH. At pH 1.5, these yields were higher than those obtained at pH 2 contrary to the galacturonic acid purity (%w/w). Compounds other than pectins were solubilized from the cell walls of apple pomace at pH 1.5, and they were precipitated with alcohol. The apple pectins obtained from the different extraction procedures were highly methylated (54.5% to 79.5%), especially when the conditions (temperature, pH) were drastic. Similar conclusions can be drawn for the neutral sugar content that decreased at pH 1.5 (arabinose, xylose, and galactose) or at the highest temperatures and extraction times (arabinose and galactose). The phenomenon of demethylation and pectin degradation of neutral sugars chains can be observed at acid pH, and for long extraction times. The presence of high quantities of mannose or fructose, glucose, and xylose in the alcohol precipitate showed that pectin precipitation with ethanol was not specific.

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A new method for the determination of the main neutral sugars in pectin has been developed. The sample preparation involves a mild chemical attack followed by an enzymatic hydrolysis. The completeness and nondestructive character of the method are demonstrated by comparison of the results obtained with different acids such as H2SO4, HCl, and trifluoroacetic acid (TFA) at different concentrations (2, 1, or 0.2 M) at two temperatures (80 or 100 degrees C). The chemical hydrolysis of pectin neutral sugar chains with strong acid (1 or 2 M) and high temperature (100 degrees C) shows that the liberation of the pectin sugars is not realized at the same rate for each sugar. Different optimum conditions are thus obtained. However, the chemical pectin hydrolysis with 0.2 M TFA at 80 degrees C is characterized by the liberation of pectin neutral sugar side chains without any degradation within 72 h of hydrolysis. Under these conditions, the liberation of some pectin sugars, essentially galactose, glucose, and rhamnose, was not complete. An enzymatic hydrolysis is necessary to obtain a complete release of all the sugars. The combination of the two treatments, a chemical hydrolysis realized with diluted acid (0.2 M) for 72 h at low temperature (80 degrees C) on one hand and an enzymatic hydrolysis on the other hand, allow a total liberation of pectin sugars. The quantitative analysis of the carbohydrates is realized with accuracy, high selectivity, and sensitivity with high-performance anion-exchange chromatography with pulsed-amperometric detection. The sugars can be analyzed without any derivatization with a limit of quantification of 0.1 mM.

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