Effects of gamma radiation on the pectic constitutents of Citrus unshiu

Abstract

Changes which occurred in the pectic substances of Satsuma orange (Citrus unshiu) induced by gamma radiation were studied. Satsuma oranges were purchased fresh through the market. Irradiation was performed with gamma rays from the Cobalt -60 source, does being at 0, 0.9×104, 9.6×104, 53.2×104, and 117.8×104 rep.Preparation and extraction of pectic substances: Juice, peel and pulp were separated from each other. Peel and pulp were extracted with boiling 70% ethyl alcohol to remove sugar. Extractions from sugar-free dried samples were performed successively with three agents: cold water; cold 0.4% sodium hexametaphosphate; 0.05N hydrochloric acid at 85°C (HCl-soluble A). Water soluble pectinate and pectate were extracted from cold water, water insoluble pectinate and pectate from hexametaphosphate solution, and protopectin from hydrochloric acid solution. Total pectin is the sum of the 3 fractions.Also, extractions from sugar-free dried peel and pulp were immediately performed with 0.05N hydrochloric acid at 85°C (HCl-soluble B).Determination of pectin content: Each of the extracts was determined by the calcium pectate method of CARRE and HAYNES.Inherent viscosity measurement: Approximately 0.1% pectin solution containing 0.6% sodium chloride and 0.4% sodium hexametaphosphate at pH 6.0 ±0.2 was used for determining relative viscosity (ηγ). Viscosity was measured at 30°C by OSTWALD-CANN0N-FENSKE viscosimeter. Inherent viscosity of pectin was calculated from relative viscosity ({η}=ln ηγ/C, C is concentration of pectin in the solution [wt./vol.%]).1. In juice, soluble pectin increased and inherent viscosity decreased by gamma radiation.2. In peel and pulp, water-soluble and hexametaphosphate-soluble pectin contents increased by gamma radiation and that of HCl-soluble A decreased, and then inherent viscosities of all fractions decreased, especially that of hexametaphosphate-soluble fractions in peel and pulp, and that of water-soluble one in pulp. This indicated that depoly merization occurred during irradiation. Both pectin content and viscosity decreased more in pulp than in peel.3. The pectin contents and viscosities of three fractions, especially of water-soluble and hexametaphosphate-soluble fractions, in the peel, differed from those in the pulp, and consequently it was apparent that properties of pectin between peel and pulp were different.4. Owing to the reduction of jelly units obtained by multiplying together the yield and jellly grade calculated from inherent viscosity of the pectin in HCl-soluble B, irradiated peel and pulp were unsuitable as the material of manufacture of pectin.