Nitric acid extraction of pectin from citrus peel [oranges, grapfruit, lemons]

Pectins were extracted from grapefruit, orange and lemon peels with nitric acid during the 1975-76 Florida citrus season. Temperature and time of extractions were varied while extraction acidity remained constant at pH 1.6 ± 0.05. Parameters for evaluation of extracted pectin from peels were yield, jelly grade and jelly units. Maximum yields of pectin calculated to 150 grade obtained from lemon, or ange and grapefruit were 11.0, 8.15 and 6.35% respec tively, while highest jelly grades were 254, 225 and 263, respectively. Highest jelly units were found for lemon peel (16.5), followed by orange (12.2) and grapefruit (9.5). There are several ways to hydrolyze protopectin in citrus peel to water-soluble pectin and many of these methods have been reviewed by Kertesz (6). The commercial extraction of pectin is usually accomplished with mineral acids such as sulfurous (5, 11), sulfuric (10), hydrochloric (7, 8, 9), a combination of hydrochloric with ion-exchange resins (1, 2, 3) or nitric acid. Nitric acid is used in Europe. However, conditions for obtaining maximum jelly units from citrus peel extracted with nitric acid have not been published. Generally, extraction temperatures range from 80° to 100°C, time of extraction from 20 to 60 minutes and pH from 1.4 to 2.6. One purpose of this paper is to present the effects of extraction time and temperature variations at optimum pH on jelly units, yields and the amounts of 150 grade pectin obtainable from wet citrus peel extracted with nitric acid. Another purpose is to present tabular data on total solids found in fresh and leached citrus peel and the percent re covery of dry solids for peel with advancing fruit maturity. Materials and Methods During the 1975-76 season the following citrus peels were leached and extracted: 'Duncan' grapefruit from October through early June, 'Pineapple' oranges in January and February, 'Valencia' oranges from March through early June and 'Sicilian' lemons in July. Leaching Procedure for Peel. One part of freshly ground peel was stirred with 2.5 parts of heated tap water for 10 min at 74°C. After draining, another 2.5 parts of tap water at room temperature were added to the peel and stirred for 5 min. to bring the temperature of this leach to 42°C. The mixture was again drained and pressed to remove excess water. Samples of peel were taken before and after the leach and oven dried at 80°C for total solids determination. Extraction Procedure for Pectin from Peel with Nitric Acid. Series of extractions were made on each prepared leached peel. Temperature and times were as follows: 95°C for 30 and 45 min; 90°C for 30, 45 and 60 min; 85°C for 60 min. Preliminary experiments had shown that pH 1.6 -± 0.1 was optimum for protopectin hydrolysis. Each extract contained leached citrus peel equivalent to 17g dry solids, 1100 ml deionized water and 1.0 N nitric acid to pH 1.6. The mixture was vigorously stirred for a given time and the temperature and the vol of liquid in the beaker were maintained throughout the extraction. The mixture was then cooled in ice water to 55°C, centrifuged and the liquor decanted into a beaker. Semi-solids from the centrifugation were reextracted with 400 ml of 40°C deion ized water, stirred 5 min, recentrifuged and the liquor de canted into the beaker containing the original liquor. The combined liquors were heated to 55°C with 0.5% standard super eel and filtered through a prepared mat in an 8-inch Buchner funnel with gentle suction. The prepared mat in the funnel consisted of Cenco #13255 filter paper coated with a mixture of lOg standard super eel and lOg hyflo. The filtrate was precipitated in 2 vol of 99% isopropyl alcohol, washed 3 times in 68% alcohol and finally in 99% alcohol. This precipitate was dried under vacuum at 60°C for 14 hr. Definitions of Terms. Yield of pectin is the amount of dry alcohol precipitate extracted from leached peel calcu lated on a fresh peel basis. Jelly grade denotes the quality of pectin and is estimated by measuring jelly sag of a standard 65% soluble solids jelly on a Ridgelimeter (4). Yield of 150 grade pectin is a calculated value that indi cates to the processor the amount of this grade of pectin that can be produced from a given peel. Commercial pectin for the jelly, jam and marmalade trade is based upon the use of 150 grade. Jelly units are calculated by multiplying yield of pectin by jelly grade. Results and Discussion Recovery of dry solids declined with maturity (Table 1) in leached 'Duncan' grapefruit peel (57.6% in December to 46.8% in June) and in 'Valencia' orange peel (65.2% in March to 57.0% in June). Recovery of peel solids was high est in lemon, 74.8% followed by orange and grapefruit which averaged 61.6 and 52.2%, respectively. Table 1. Total solids and recovery of solids for fresh and leached citrus peels of fruit sampled monthly (1975-76).