Purification of citrus peel juice and molasses.

Citrus peel juice and molasses are extremely bitter and unpalatable byproducts of orange and grapefruit juice production. Major components of interest are soluble sugars, glucose, fructose, and sucrose, which account for 60-70% of the dry solids. Analyses indicate that the remaining components are suspended tissue fragments, proteins, organic acids, mineral ions, phenolic compounds, and polyols. A purification sequence that removed a majority of bitter limonoids and phenolic compounds by adsorption on nonionic, macroporous resins was tested. Residual phenolic compounds were removed by adsorption on activated carbon or anion-exchange resin, which also removed anions of organic and inorganic acids. Taste panel results suggested that debittered products could be acceptable for food uses.

Effect of juice extractor settings on juice cloud stability.

Juice was extracted from Valencia oranges using three different extractor settings. Differential juice cloud stability was observed. Soft-extracted juice was the most stable, and hard-extracted juice was the least stable. The medium-extracted juice had intermediate cloud stability. Yearly (1997 versus 1998) differences were observed, but the relationship among the juices did not change. Addition of protein extracts, obtained from each juice, to pasteurized juice also resulted in differential cloud stability. Using pectinmethylesterase (PME) activity estimated at pH 4.5, the effects of the protein extract mirrored results from raw juice. Estimating PME activity at pH 7.5 produced contradictory results, indicating that predicting consequences of PME activity estimated at pH 7.5 is unreliable.

Fermentation of orange peel hydrolysates by ethanologenic Escherichia coli. Effects of nutritional supplements.

Orange peel, an abundant byproduct of the citrus processing industry, is converted to a mixture of glucose, galacturonic acid, fructose, arabinose, galactose, and xylose by hydrolysis with mixed pectinase and cellulase enzymes. All these sugars can be fermented to ethanol or ethanol and acetic acid by the recombinant bacterium Escherichia coli KO11. The fermentation efficiency is improved by the addition of yeast extract, tryptone, mixed amino acids, corn steep liquor, or by proteolytic digestion of endogenous proteins. Batch fermentations of supplemented peel hydrolysate containing 111 g/L of initial total sugars produced 35-38 g/L of ethanol in 48-72 h and a 75-85% yield.

Fermentation of sugars in orange peel hydrolysates to ethanol by recombinant Escherichia coli KO11.

The conversion of monosaccharides in orange peel hydrolysates to ethanol by recombinant Escherichia coli KO11 has been investigated in pH-controlled batch fermentations at 32 and 37 degrees C. pH values and concentration of peel hydrolysate were varied to determine approximate optimal conditions and limitations of these fermentations. Very high yields of ethanol were achieved by this microorganism at reasonable ethanol concentrations (28-48 g/L). The pH range between 5.8 and 6.2 appears to be optimal. The microorganism can convert all major monosaccharides in orange peel hydrolysates to ethanol and to smaller amounts of acetic and lactic acids. Acetic acid is coproduced in equimolar amounts with ethanol by catabolism of salts of galacturonic acid.

Production of ethanol from enzymatically hydrolyzed orange peel by the yeast Saccharomyces cerevisiae.

We extended our previous investigations of enzymatic hydrolysis of polysaccharides in orange peel by commercial cellulase and pectinase enzymes to higher, more practical concentrations of orange peel solids. High yields of saccharification could be maintained even at substrate concentrations as high as 22-23%, but the rates of solubilization and saccharification decreased 2-3-fold. We also tested the fermentability of these hydrolysates by the yeast Saccharomyces cerevisiae, which revealed the presence of inhibitory compounds. These compounds could be removed by the filtration of hydrolyzed peel. Successful fermentations of filtered hydrolysates were achieved after pH adjustment with calcium carbonate.