Acidity changes associated with the keeping quality of apples under various storage conditions
Iowa Agriculture and Home Economics Experiment Station Research Bulletin: Volume 10, Issue 131
This investigation reports a study of. certain acidity changes occuring in Grimes and Jonathan under various storage conditions. Measurements were made of the total titratable acid, the H-ion concentration and, in some instances, the specific gravity of the juice.
Altho total acidity values declined rapidly in the Grimes variety during two weeks of delay at the orchard, its H-ion concentration remained quite uniform. Prolonging the delayed storage period to three weeks, however, resulted in a rather abrupt lowering of the active acidity of the fruit in cold storage. The appearance of soggy breakdown in the stored fruit occurred almost coincidently with a rather marked diminution in the H-ion concentration. This relationship suggests the possibility of an intimate balance between the active acidity and the carbohydrate reserves of the Grimes apple at low temperatures. The H-ion concentration during delayed storage at the orchard, was decidedly more constant in Jonathan. This fact suggests a difference in buffer stability between the two varieties of fruit. The data for the active acidity changes in Jonathan were not of a similar nature during the two storage seasons. Soft scald and Jonathan spot formation also differed for the two years.
There appears to be an inverse relationship between the percentage of total acid loss during storage and the susceptibility toward Jonathan spot. A low acid loss during storage usually produced a high percentage of spotted tissue. The total acidity decreased gradually during cold storage in both Grimes and Jonathan. Decrease was more rapid at the higher temperatures. Total acidity decreased more in Grimes during delayed storage at the orchard than thruout the entire cold storage period. The delayed fruit was stored with considerably less total acid than that receiving no such treatment.
During storage two conditions appear to influence the total amount of acid lost: (1) The initial acid content when stored, and (2) the storage temperature. Delayed storage lots with low initial acidity lose their total acid in smaller quantities at 30° and 32°F. than similar lots at 34° and 36°F. The amount of acid loss, then, appears to be regulated largely by a certain minimum temperature level, below which the loss will be relatively little regardless of acid content, above which the loss will be considerably higher even tho the acid content is relatively low at the beginning of the storage period.
The amount of breakdown tends to occur in an inverse proportion to the losses in total acidity during cold storage. A large amount of soggy breakdown is usually accompanied by a smaller amount of acid loss, and, conversely, a low amount of breakdown is associated with a higher acid loss. In order to avoid soggy breakdown, it follows, then, that the rate of acid loss must be fairly constant. This may be accomplished by storing the fruit before the acid content becomes low (directly after picking), or by storing at a temperature sufficiently high to permit acid losses to continue at a more favorable and a more rapid rate.
Altho acid changes appear to be rather definitely associated with breakdown and Jonathan spot susceptibility, other physiological changes occur which may influence these storage disorders. A decided increase in the specific gravity of the juice from Grimes, during delayed storage at the orchard, indicates definite changes in the metabolism of the fruit. Apples delayed for one week showed similar increases in this constant thruout the cold storage period. Little or no increase in specific gravity, however, occurred in the stored fruit which had been delayed at the orchard for two or three weeks. The juice of Jonathan maintained a very uniform constant, irrespective of the delayed storage at the orchard or treatment at the lower temperatures. This different response toward specific gravity and acidity changes in the two varieties may aid in the explanation of the differences in the storage capacity of Grimes and Jonathan. It would appear that the storage capacity of some varieties of apples could be forecast by changes appearing in the specific gravity of their juices.