FROST PRECIPITATION OF PROTEINS OF PLANT JUICE

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Abstract
An exposure of 5 hr. at − 7 °C. caused maximum precipitation of the proteins of press-juice from leaves of unhardened winter wheat grown in the greenhouse. The value of sucrose added as a protection against precipitation increased with concentration up to about 8%. This concentration of total sugar is often attained by hardy varieties of winter wheat in the field. Sucrose and dextrose were about equally effective. By microscopic examination of smears caused by drops of juice, to which various quantities of dilute acid or base had been added, flowing across glass slides, it was ascertained that maximum precipitation occurred at about pH 5.1 and 7.3, respectively. Added sugar reduced the percentage precipitation by acid, base or "salting out." The addition of acid or common salt, insufficient to cause any immediate effect microscopically visible, increased subsequent frost precipitation; the addition of base or alkaline buffers decreased it. The removal of natural electrolytes by dialysis increased subsequent frost precipitation. Added sugar stabilized the juice proteins under all conditions. Precipitation by frost, acid or base are all irreversible. The coagula produced by different agencies acting on the juice have distinctive characteristics, which are illustrated. It is concluded that dehydration is the basic cause of frost precipitation, and that ice formation, acidity, salt concentration, and possibly pressure, are all contributing factors.

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