Acidity changing during storage

Figure 9. Ascorbic acid changes during storage of whole milk (------),...

Bulk phospholipids are relatively stable on storage (Reblova et al., 1991), but are less stable when dissolved in oils. They slowly change during storage of hpid foods. In unheated food materials, phospholipases are active so that hydrolysis occurs, accompanied by formation of phosphatidic acids. In addition to hydrolytic reactions, oxidation reactions also occur. The oxidation of PUFA, detected in phospholipids, proceeds in a way similar to the oxidation of neutral hpids, only the peroxy free radicals or the hydroperoxides formed from them can react with amine groups in a non-radical way. 

The relation of total sulfur dioxide content and other factors to color changes during storage of apricots is discussed by Nichols and Reed (1931), Nichols ei al. (1938), Chace et al. (1930, 1933), and Sorber (1944). The role of sulfuring in dehydration of cherries and small fruits is discussed by Wiegand et al. (1945), and the effects of methods of sulfuring, dehydration, and temperature of storage on ascorbic acid content and carotene content of dehydrated peaches by Eheart and Sholes (1946), as well as others mentioned previously. 

In some cases it has been found that the physical properties of some alginate fibers change during storage. Calcium-based and alginic acid yarns deteriorate whereas chromium- and beryllium-based yams improve rather than deteriorate during storage under standard conditions. ... 

Other problems occur in the measurement of pH in unbuffered, low ionic strength media such as wet deposition (acid rain) and natural freshwaters (see Airpollution Groundwatermonitoring) (13). In these cases, studies have demonstrated that the principal sources of the measurement errors are associated with the performance of the reference electrode Hquid junction, changes in the sample pH during storage, and the nature of the standards used in caHbration. Considerable care must be exercised in all aspects of the measurement process to assure the quaHty of the pH values on these types of samples. 

Boselli, E., Caboni, M. F., Sabatini, A. G., Marcazzan, G. L., and Lercker, G. (2003). Determination and changes of free amino acids in royal jelly during storage. Apidologie 34, 129-137. 

The restricted shelf life of liquid milk continues to be a problem that is often more influenced by the type of milk being sold rather than the pasteurisation technique. The shelf life of processed milk is determined primarily by the quality of the raw milk from the dairy herd. Increasing cell counts in the milk and a higher concentration of free fatty acids, contribute to rancidity in both liquid milk and milk products. Janzen (1972) reported that the 0-14 day shelf life of pasteurised milk is influenced by the somatic cell concentration in the raw milk and found that after 14 days any observed changes in the flavour and stability of the milk were attributable to microbial activity during storage. 

Yen YH, Shih CH and Chang CH. 2008. Effect of adding ascorbic acid and glucose on the antioxidative properties during storage of dried carrot. Food Chem 107(1) 265—272. 

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