**3-4 Heat Release when Extracellular Products are Formed**

When significant amount of product is present, Eq(3-1) will be modified to

where (-DH_{p}) is heat of combustion
per gram of extracellular product(s). Dividing the above by MW of substrate
gives

Dividing the above by gives

Example 3-1

In citric acid fermentation, 2000 kg glucose is fermented over a 50 h period resulting in 1300 kg citric acid and 400 kg biomass. The process required 800 kg O

_{2}. Calculate the total heat released. Heat of combustion of citric acid (C_{6}H_{8}O_{7}) is -1,960 kJ gmol^{-1}.

SolutionThis is a case of extracellular product is formed. Use Eq (3-6) or (3-7) to first obtain heat yield.

The negative value indicates that heat is rejected or being generated due to growth. The interpretation here is similar to that for a chemical reaction. Recall heat of reaction is negative for an exothermic reaction. Ignoring the sign as we label it as heat generated,

Alternatively: we can calculate heat generated from oxygen uptake data.The two methods differ by about 30%, assuming the second method is more accurate! In the first method, it is assumed that all the extracellular products are accounted for in citric acid. It is likely that the cell produces other products that are not included in the energy balance.