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United States Department of Agriculture

Agricultural Research Service

Research Project: Enhanced Utilization of Carbohydrates and Polysaccharides from Citrus Processing Waste Streams

Location: Citrus and Other Subtropical Products Research

2013 Annual Report


1a.Objectives (from AD-416):
1. Develop green and scalable integrated processes which improve production economics to obtain modified pectins, ethanol or other biofuels, and other co-products such as limonene and flavonoids from citrus process waste streams. 2. Develop new commercially viable industrial bioproducts made from pectin. a) Enzymatic modification of citrus pectin nanostructure to tailor functionality. b) Determine ion exchange properties of enzyme modified pectin and peel particles. c) Determine rheological and water holding properties of chemically modified pectin and peel particles.


1b.Approach (from AD-416):
Commercial success for development and recovery of byproducts from citrus waste streams depends on the ability to economically recover sufficient quantities to meet market demands, favorable recovery costs and market value. Integration of processes to separate and recover limonene, fermentable sugars, pectin and other polysaccharides, flavonoids and other components to produce multiple high value co-products will be investigated. Recovery of pectin or modified pectin along with other polysaccharides after water extraction of fermentable sugars will be investigated for utilization in industrial applications and integrated with a steam stripping treatment for recovery of volatile terpenes. Hydrolysis of citrus peel waste utilizing commercial enzyme products and subsequent fermentation of released sugars will be evaluated for efficacy in liquefaction, conversion of cellulose to glucose, ethanol production, and cost. This will be compared to extraction, concentration and utilization of isolated sugar and pectin/polysaccharide fractions. Separation, concentration and recovery schemes to separate fermentable sugars from non-fermentable components will include residue hydrolysis, use of ion-exchange and absorbent resins, ultrafiltration, nanofiltration, reverse osmosis, water/solvent extractions and selective precipitation. Mass balances and extraction efficiencies will be determined for major byproduct components

Pectin is a major component of citrus peel with extensive functionality and the degree of methylesterification has a very strong influence on functionality. Techniques to reliably produce novel, non-random patterns of methylesterification in pectin molecules and accurately characterize their distribution will be investigated. Fractions containing pectin or other polysaccharides from citrus processing waste will be characterized for macromolecular and nanostructural properties. They will then be treated with pectin modifying enzymes at varying pH, temperature, and salt concentrations and the resulting changes in functionality and nanostructure determined. Chemical modifications will be performed using nucleophilic reagents to modify functionality alone or in combination with enzymatic treatments. Materials generated will be tested for biosorption properties as amorphous powdered materials and after conversion via chemical crosslinking. In addition, water holding capacity, viscosity, and other rheological functional properties such as yield point will be determined along with changes in fragmentation size, molecular weight distribution, degree of polymerization, degree of substitution of added groups, as well as thermal and pH tolerance. Materials with appropriate properties will then be tested in applications such as drilling fluids, dry strength additives for paper, cement additives, and absorbents for spill applications.

The economics of producing newly developed by-products will be evaluated and compared with those products currently utilized for targeted applications. Economic information will include raw materials, consumable, and energy costs, fixed capitol investment cost, and a breakdown of operating and capital cost estimates.


3.Progress Report:
Citrus processing waste contains 8-10% sugars fermentable by conventional yeasts and after enzyme liquefaction and fermentation can yield 3.5 – 4.5% ethanol. This is on the very low end of ethanol concentrations considered to be economically distillable and the enzymatic liquefaction process degrades the pectin in citrus waste limiting use. Water extraction of fermentable sugars to allow subsequent pectin extraction from the waste residue as an additional co-product yields sugar extracts which are dilute. Concentration by thermal means of either the dilute sugar extract prior to fermentation or the dilute ethanol stream produced after fermentation is not economical. In order to be able to obtain modified pectins and ethanol as outlined in objective 1, a reverse osmosis system for concentration of dilute sugar extracts from citrus prior to fermentation was tested. With correct pretreatment of citrus processing waste and the sugar extracts obtained, no fouling of the reverse osmosis membrane was encountered and concentration of the sugar extracts by reverse osmosis was found to be much more economical than using thermal means. The citrus residues left after extraction of sugars were also suitable for pectin extraction or modification. In the development of new commercially viable industrial bioproducts made from pectin, progress was made pertaining to value-added research into the polysaccharide structure and modification of citrus processing residues. Work was initiated on the functionalization of citrus peel via chemical and enzymatic modifications. Preliminary studies indicate that gelling and viscosity properties can be manipulated. Testing and data on enzymatic deesterification of citrus pectin suggest that only a subfraction of pectin molecules are modified and potential properties of pectin may not be fully realized without further improvements to enzymatic modification procedures or use of chemical deesterification. Fully deesterified pectins were produced chemically from peel waste under chelation conditions and shown to form stable gels useful for suspension applications.


Last Modified: 10/22/2014
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