low temperatures. Although it has been difficult to establish the existence of 
the pectin splitting enzymes, pectin methylesterase and pectin polygalacturonase 
in apple tissue, there is evidence of pectin methylesterase activity which re- 
sults in demethylation and increased viscosity of pectin solutions (33, 56). 
Sapozhnikova (58) reports lower activity of pectolytic enzymes in apples more 
suitable for storage. 
The amount of starch in apples after harvest is relatively small and by 
the time the respiration peak is reached a large portion of the starch has 
disappeared (29); therefore changes in starch and activity of starch splitting 
enzymes would not be expected to be involved to any great extent in the ripen- 
ing process. However, Wiley and Stembridge (80) found starch content to be 
closely related to texture in processed apple slices. 
The sugars are important because of their relation to eating quality. 
The principal sugars in apples are fructose, glucose, and sucrose. The presence 
of xylose in small quantities has also been observed (1, 61). The sugar/acid 
ratio has been considered by many to be a measure of ripeness and eating accept- 
ability. The increase in sugar content during ripening and storage was earlier 
presumed to come from the hydrolysis of starch, but the increase has been re- 
ported to be greater than can be accounted for by loss of starch (22, 25). The 
organic acids play a very important role in respiration and are probably much 
more closely connected with the sugar metabolism (28, 41, 42) than was earlier 
considered possible. 
There is no recent evidence to indicate that other carbohydrate frac- 
tions, such as hemicilluloses, pentosans, etc., have any more than a very minor 
relation to the ripening process. Krotkov and Helson (41) have shown a slight 
but steady decrease in hemicellulose content in apples during storage. 
Acids 
Acidity or tartness has long been considered a quality factor in apples, 
Many measurements have been made to relate the total titratable acid to eating 
quality. Wright and Whiteman (83) found that acid content varied considerably 
between varieties and within varieties from year to year. They noted a pro- 
gressive decrease in acidity during storage along with a slight increase in 
total sugars throughout most of the storage period. 
It has been assumed that malic acid, because of its abundance in ripen- 
ing apples, was the most important acid. With the evidence now available that 
the Krebs cycle is operative in apple tissue (20, 35, 43, 52, 72), this assump- 
tion cannot be accepted without question. Accumulations of malic acid may 
simply serve as reservoirs for feeding acids into a complicated cycle of trans- 
formations (28) where minute quantities of certain acids may serve an extremely 
important function in the metabolism of the harvested fruit. 
A number of organic acids besides those involved in the Krebs cycle have 
been identified in apple fruit tissue. Some of the more recent ones include 
quinic (23), shikimic (27), and citramalic (5, 26). 
Phenolic Compounds 
The astringency characteristic in apples is usually considered to be 
caused by the phenolic compounds, more commonly called tannins in earlier years. 
In addition to their effects on flavor they are also responsible for the color 
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