CARBOHYDRATE FRACTIONS OF LEMON PEEL

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Abstract
The whole peel and albedo of lemon fruits were extracted with 80% ethyl alcohol and separated into 2 distinct fractions, the alcohol-soluble and alcohol-insoluble solids. The carbohydrate constituents of these fractions were investigated, and certain of their physical and chemical properties determined. The soluble sugars from whole peel or albedo averaged 54.0-96.8% of the total alcholic extractives, the proportion depending upon the maturity of the fruit. In addition to soluble sugars, the alcohol-soluble fraction contained substances such as essential oils, waxes, organic acids, and various concns. of undetermined constituents. The alcohol-soluble fraction also contained a substance which liberated a small amt. of CO2 on boiling with 12% HCL. As the alcohol-insoluble solids contained the pectin, most of the CO2 occurred in this fraction. The sum of the water-soluble and ammonium citrate-soluble pectin (as Ca pectate) of the whole peel amounted to 35.29% (avg. value) of the alcohol-insoluble solids. Similarly, the sum of water-soluble and acid-soluble pectin (as Ca pectate) of the albedo amounted to 37.33% (avg. value) of the alcohol-insoluble solids. These values are much lower than those for total pectin calculated from CO2 and methoxyl values, respectively. To establish criteria of purity, the Ca pectate values were accompanied by corresponding values jfor CO2, Ca, and furfural. The percentages of Ca in the Ca pectates were somewhat higher than the 7.50% usually reported for pure C pectate. The furfural equivalent to the uronic acid anhydride was equal to approx. 50% of the total furfural of the alcohol-insoluble fraction. The furfural derived from substances other than uronic acids is reported as pentosans. The residue that remained after extraction of the pectin from the alcohol-insoluble solids was composed of cellulose and hemicellulose and a comparatively small amt. of firmly bound pectin that is difficult to extract and determine quantitatively. The insoluble residue that remained after hydrolyzing the cellulose and hemicellulose fraction with 2% HCL for 3 hours amounted to approx. 20% of the alcohol-insoluble solids. This insoluble fraction is soluble at room temp. in the cellulose solvent, Zn-Cl2 soln.