Steam sterilization of whey in a bioreactor

I'm trying to design a bioreactor to produce citric acid with whey and Aspergillus Niger.

The first step in the process would be to put whey into the reactor with some dextrose (approx. 10%). Then, this solution should be sterilized, and I was thinking of using batch steam sterilization by direct injection of the steam in the reactor. The sterilization should be done at 121°C.

I'm not sure how to calculate the sterilization time and the amount of steam required for a given reactor volume (approx. 600 m3). I was thinking of calculating it as if I had a 600 m3 autoclave, but I doubt the approximation holds. In that case, the death kinetics would be given by the Arrhenius equation:

$$k = Ae^{\frac{-E}{RT}}$$

I'd need to determine the activation energy, $E$, the pre-exponential factor $A$, or some approximation for $k$. Then, I'd just have to integrate the equations for the heat and hold energy required:

\begin{align} \nabla heat &= ln(\frac{N_0 V_0}{N_1 V_1}) = \int_{t_0}^{t_1} k(t) dt \\ \nabla hold &= ln(\frac{N_1 V_1}{N_2 V_2}) = k(t2_-t_1) dt \end{align}

To figure out $t_1$ and $t_2$.

Am I approaching this problem correctly?

• Do you know the SAL you need? My process engineering handbook goes into this, but only briefly and I want to think about it a bit before I give a wrong answer. – mart May 3 '15 at 20:14
• Sterilization can be achieved both with temperature and pressure. Maybe you should investigate the pressure side of the question as well. – BOB Feb 11 '16 at 19:09

1 Answer

It all depends on the sterilization level you want to achieve. As you say, you would need to know the death kinetics parameters, or equivalently the log-decimal reduction time (D) and how it changes with temperature (Z). As sterilization is a problem that takes place almost everywhere in food industry, a standard sterilisation cycle lasts for 30 min. at 121 degC at the cold spot of your reactor. This ensures that the spores of many Bacillus, Clostridium and other thermophilic microorganisms cannot survive.
For a 600 m3 reactor the easiest is to have a temperature probe inside of the reactor. As how much steam? That depends on how well isolated your reactor is. The best is to make a feedback loop with setpoint 121 degC that would send steam whenever temperature starts to drop. Otherwise, you can try to do the same manually.