Krotkov, Wilson, and Street (36) showed that the various stages of acid 
metabolism of McIntosh grown in Canada correspond chronologically very closely 
with those of carbohydrate content and with respiration, They explained that 
during the first 4 to 6 weeks after petal fall the pH of raw apple juice de- 
creases rapidly. It reached a value of 2.8 then slowly rose following a 
straight line for the rest of the ontogeny. They postulated that on account of 
this regularity, once the pH of juice had begun to increase, its magnitude 
represented a better indicator of stage of maturation than its respiration rate 
or either carbohydrate or acid content. Malic acid after reaching a level of 
about 80 percent of the total organic acid content of the fruit remained at 
that level for the rest of the ontogeny. They observed no disappearance of 
organic acids during the climacteric rise in the sugar content of fruits and 
no accumulation in the immediate postclimacteric period when large amounts of 
sugars disappear. They concluded that the variations in sugars are not brought 
about by their appearance from or disappearance into organic acids. 
Comin and Sullivan (9) defined the index number as the ratio of the 
quantity of dissociated hydrogen to the quantity of displaceable hydrogen de- 
termined on a per unit displaceable hydrogen basis by dividing the gram moles 
of ionic hydrogen per liter calculated from the pH by the milliequivalents of 
hydrogen per liter (calculated from titration). Their work indicates that 
apples produced on trees in favorable nutritional balance will be higher in 
active hydrogen and tend to keep better in storage. 
Comin and Sullivan (8) stated that the significance of the dissociation 
of acids index is based on the assumption that pectin, sugar, and acid contents 
at any time in the life history of a fruit are the resultant of certain complex 
equilibria which appear to be controlled, not by the content of any one con- 
stituent but rather by the degree of dissociation of the acid produced. They 
showed that the index number of Rome Beauty apples rose to a maximum occurring 
close to 167 days after full bloom. Most of their data showed a decided and 
consistent rise in index number starting close to 153 days from full bloom re- 
ported as the fore part of the optimum maturity range (155 days) for this 
variety. They concluded that actual physiological maturity can be measured 
closer by index number than by ground color for Rome Beauty apples. 
Eaves and Leefe (17), in reporting a rapid technique for measuring 
titratable acidity, found no significant difference between amount of acid lost 
from apples picked at different dates and stored at 34° F,. for 5 months, Acid 
was positively correlated with potassium of leaves and less positively corre- 
lated with N/K ratio in leaves. 
The results of Eggert, Murphy, and Johnson (19) showed no relationship 
with McIntosh between palatability findings and pressure tests, soluble solids, 
total acid, and soluble solids/acid ratio. 
Pectinic and Cellulosic Changes 
The dissolution of pectin and cellulosic changes in apples with stage 
of maturation has been studied extensively and found to offer value in pre- 
dicting when to pick the fruit. 
Pollard and Kieser (65) described experiments which showed that many of 
the commoner varieties of apple contain appreciable amounts of pectase, the 
level of activity varying according to variety and stage of maturity. 
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