Karel Grohmann

FRACTIONATION AND PRETREATMENT OF ORANGE PEEL BY DILUTE ACID HYDROLYSIS

Abstract Solubilization and depolymerization of carbohydrates by treatment of orange peel with dilute (0·06 and 0·5%) sulfuric acid at 100, 120 and 140°C has been investigated. The acid treatments solubilized a large portion of total carbohydrates in orange peel. However, only soluble sugars and sugars derived from hydrolysis of hemicelluloses were efficiently released by the treatment with hot dilute sulfuric acid. Cellulose and segments of pectin containing galacturonic acid units were very resistant to acid-catalyzed hydrolysis. The treatment with dilute sulfuric acid had a positive effect on the rate of subsequent enzymatic hydrolysis of orange peel by a mixture of cellulolytic and pectinolytic enzymes.

Hydrolysis of orange peel with pectinase and cellulase enzymes

SummaryHydrolysis of polysaccharides in comminuted orange peel by commercial cellulase and pectinase enzymes has been investigated. High levels of conversion to monomeric sugars were observed after treatment with pectinase enzyme, but cellulase enzyme achieved only limited solubilization. The combination of cellulase and pectinase enzymes appears to be a most efficient system for enzymatic hydrolysis of polysaccharides in orange peel.

Fermentation of galacturonic acid and pectin-rich materials to ethanol by genetically modified strains of Erwinia

Evaluation of the four ethanologenic constructs of bacteria in the genus Erwinia indicates that two strains E. chrysanthemi EC16 and E. carotovora SR38 show promise for development of direct hydrolysis and fermentation of pectin-rich substrates to mixtures of ethanol and acetate. Both strains fermented glucose to ethanol in nearly theoretical yields, but produced mainly acetate and ethanol by fermentation of D-galacturonic acid. Both strains depolymerized citrus pectin, polygalacturonic acid and polysaccharides in citrus peel and converted resulting sugars to carbon dioxide, acetate, ethanol and lesser amounts of formate and succinate.

Changes in citrus leaf flavonoid concentrations resulting from blight-induced zinc-deficiency

Increased flavonoid concentrations were found to correlate with the elevated levels of leaf phenolic compounds occurring in blight-induced zinc-deficient citrus. In orange (Citrus sinensis L.) leaves, the increases occurred primarily in hesperidin and diosmin, whereas in grapefruit (C. paradisi Macf.) the largest increases occurred in naringin and rhoifolin. Zinc-deficiency occurring in the blighted citrus leaves appeared to be the important contributing factor to the increased flavonoid content. Although the leaves from trees with blight were typically smaller than leaves from unaffected trees, the increased flavonoid content was not significantly due to a concentration effect. Large differences occurred in the percent increases in concentrations of certain citrus leaf flavonoids. While large increases occurred for a number of flavanone and flavone glycosides, much smaller percent increases occurred for other minor flavone glycosides, and the polymethoxyflavone aglycones. The parallel increases occurring in the concentrations of certain flavone glycosides and their flavanone analogs provide a further indication that flavanone glycosides are precursors in the biosynthesis of flavone glycosides in citrus.

Flavonoids of the Orange Subfamily Aurantioideae

The flavonoids and related compounds of the orange subfamily Aurantioideae have attracted the attention of generations of chemical researchers, beginning with the first description of hesperidin by Lebreton (1828) to the many current pharmacological studies of these compounds in living systems. For many reasons (medicinal, herbal, agricultural), citrus fruit have been collected and used by societies throughout the centuries (Webber, 1967). However, our modern focus on the impact of citrus flavonoids on human health was perhaps started by the work of Szent-Gyorgyi, who, in calling citrus flavonoids Vitamin P, first indicated the importance of flavonoids in capillary function (Armentano et al., 1936; Rusznyak and Szent-Gyorgyi, 1936; Bentsath et al., 1937). While the term Vitamin P fell into disuse, the importance of flavonoids and ascorbic acid in proper capillary function was firmly established. Without question, the importance of the capillaries in many different aspects of human health cannot be overstated, and aspects of this are discussed in the chapters by Middleton and Kandaswami (1998), Gerritsen (1998), and Attaway and Buslig (1998). Extending from this, many pharmacological studies now show the important antioxidant and anticancer activities that citrus flavonoids contribute to human health through the diet. Much of this research relies directly on the isolation and structural characterizations of these diverse citrus phenolics, much of which was done by chemists at the U.S. Department of Agriculture. Although many of the major citrus flavonoids have now been well characterized, much still remains unclear about the biological activities of these compounds in mammalian systems, and about the biosynthesis, transport, and physiological roles of these compounds in the plants in which these compounds occur. It has been noted that in developing citrus plant tissue tremendous amounts of metabolic energy are expended in the biosynthesis of these compounds. In fact, flavonoids can constitute well above 50 percent of the dry weight of immature citrus fruit and leaf tissue undergoing rapid cell division. Yet, very little is known why this occurs, or how the biosyntheses of the different groups of flavonoids in citrus are connected. As part of this chapter, the remarkable diversity and distribution of the flavonoids in the orange subfamily Aurantioideae are reviewed, and evidence pertinent to the biosynthetic pathways of citrus flavonoids is reported.