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Name: Aishwarya I.
Status: Student
Grade: 11
Location: N/A
Country: Australia
Date: Summer 2011


Question:
How does changing the temperature at which orange juice is stored in affect a weak acid such as citric acid? Would it affect the rate it dissociates, increase oxidation or enhance the availability of bacteria/ organisms that can increase acidity?

Please help! Thanks.


Replies:
Aishwarya,

It is possible to consider each factor you mentioned independently. However, since these factors have varying effect on the acidity of a weak acid - especially in a complex solution like orange juice, it is not possible to decide -without actual experimentation- which factor has the strongest effect and which direction acidity will go.

For the case of the "rate of dissociation", this is more appropriately thought of from the point of view of the temperature dependence of the dissociation constant, and of the pH. The applicable equations are: pH =-log([H(+)]); Ka = [H(+)][A(-)] / [HA]; and d(ln(Ka)/dT = delta(H)/RT^2. The upshot of this is that since pH is already a log function, and the change in Ka as a function of temperature is again a natural log function (ln[Ka]), then the pH does not change very much with temperature.

Oxidation, in this case the reaction of oxidizing agents with vitamin C (L-ascorbic acid, not citric acid) is likely increased with temperature. However, I do not know of an equation that quantifies this, especially because there are two opposing factors at work here. Heat, if applied below temperatures of 70deg-C destroys the ascorbic acid oxidase found in orange juice, so heat - in this case - actually preserves ascorbic acid. Furthermore, since oxygen gas (another oxidizing agent) is less soluble in aqueous solutions at higher temperatures, mean that, again, more ascorbic acid is preserved. However, the reaction of oxygen gas and ascorbic acid speeds up as temperature increases, which means that whatever oxygen is dissolved in the juice will work more efficiently. Moreover, at temperatures higher than 70C, the ascorbic acid tends to degrade (bonds are broken).

The bottom line is that it is probably best to actually do several experiments to determine the effect of temperature and heat on fruit juice.

Greg (Roberto Gregorius)
Canisius College


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