Graduate Thesis Or Dissertation
 

Ultrafiltration of grape juice : effects on composition, quality and preservation

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  • Ultrafiltration (UF), using Romicon pilot-scale hollow-fiber cartridges (Model HFXS-MK11) in the batch mode, was investigated as a possible procedure for clarifying, stabilizing and preserving White Riesling (WR) and Thompson Seedless (ThS) grape juice. The effect of UF processing of SO₂-treated or untreated juice, of process temperature and membrane molecular weight cut-off (MMWCO) on UF juice composition and quality, was determined. "Sparkling clear" pale light-colored UF permeates were obtained and subjected to storage stability trials at different temperatures. Juice parameters evaluated included proteins, phenolics, polyphenol oxidase (PPO), microorganisms, browning, haze and sensory properties. UF at elevated temperatures of up to 50°C, compared to lower temperatures, had the advantage of increased rate of filtration with decreases in browning or haze of clarified permeates and of markedly reduced microbial loads. UF of SO₂-treated juices (13- 20 ppm free) had several advantages including further reduction in browning, haze, and microbial load, and complete inhibition of PPO activity. More proteins (34% - 92%) than phenolics (2% - 27%) were retained by UF membranes. However, more proteins and less phenolics were retained in SO₂-treated than untreated juices. Increasing MMWCO from 10,000 to 100,000 had no effect on the retention of browning, haze, or microorganisms but decreased the retention of proteins from 58 percent to 34 percent in SO₂-treated WR juice. A MMWCO of 50,000 or less retained essentially all PPO. ThS juice browned more than WR during juice preparation and UF processing, yet its clarified permeates were lighter. It contained less phenolics, more proteins and more PPO activity, and higher percentages of its proteins and phenolics were retained by UF membranes. UF juices processed with or without low concentrations of SO₂ (13-20 ppm free) could be stored for at least one year at 1°C without fermentation spoilage. At 21°C only the treated juices could be stored for this long, untreated juices fermenting within one week. At 1°C, there was no increase in browning, but a gradual increase in haze upon storage, the latter being greater in untreated juices. At 21°C, browning increased gradually, resulting in pale golden juices but haze formation was markedly less than in juices stored at 1°C. Similar but accelerated changes occurred with storage at 38°C. The above changes correlated with the disappearance of free SO₂. Although juices were preserved for up to one year, "absolute juice sterility" was never obtained. Leakages around the cartridge gaskets, rather than failure of membranes, was believed to be responsible. Sensory analysis detected no significant differences (p = 0.05) in color, aroma and taste between juices processed with SO at 30°C and 50°C. Juices processed without SO₂ were not significantly different in color, but demonstrated small differences (p = 0.05) in aroma and taste. The SO₂-treated and untreated juices at both processing temperatures, as well as treated juices stored at 21°C and 1°C for twelve months, were significantly different (p = 0.01) in color, aroma and taste. Although these juices had different flavor "characters," only small or no significant differences (p = 0.05) in desirability were detected. It is concluded that good quality clarified grape juice but of different flavor "character" can be produced by UF, either in the presence or absence of SO₂. Both juice "styles" (± SO₂) may find preferred use, but juices processed with SO₂ have the advantage of ambient storage, retaining good quality after one year at 21°C. Recommended optimum processing conditions are 50°C, in the presence of minimal amounts of SO₂ (13-20 ppm free), using membrane molecular weight cut-off of 30-50,000. However, application of the process to "cold-sterilization" of grape juice requires further evaluation.
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