Continuous & Batch Fermentation

Making Useful Products From

Micro-organisms

Fermentation is a process which takes place when micro organisms grow and multiply e.g. in beer, wine, yoghurt and myco-protein production. Fermentation takes place in a container called a 'fermenter'

The fermenter is a vessel which is sterilised to provide a 'clean environment' ensuring that only the desired organism grows. The vessel is set up to provide the best growth conditions for each culture of micro organisms according to its needs, whether it be the manufacture of alcohol as in beer, or simply the growth of myco-protein

Fermenter

Whether producing beer, wine, yoghurt or myco-protein certain conditions are needed.

These include:

•sufficient nutrients

•enough air, and

•constant temperature

To ensure the culture has sufficient food and oxygen the contents of the fermenter must be constantly mixed. These conditions are provided by two different types of fermenter.

Mechanical stirring using a paddle or magnetic stirrer in the fermenter vessel.This method is often used in small fermenters.A pressure cycle fermenter uses the movement of air within the fermenter to effect mixing.

Fermentation either takes place as a 'batch' process where each filling of the fermenter gives a batch of finished product, or a 'continuous' process where nutrients are added and product removed continuously over the period of an extended run.

In myco-protein production the removal of product from the fermenter (called harvesting) only begins when the rate of multiplication matches the rate of harvesting.

Growing Myco-Protein 

Growing ConditionsAs with any controlled growing procedure, the conditions for this fermentation must be carefully monitored. Therefore the fermenter is designed to allow temperature, pH, nutrient, and oxygen levels to be maintained at the levels required for optimum growth. All the nutrients used in the process are of food grade quality or higher, and the water is specially purified before being added to the fermenter.

The ProcessThe fermenter is filled with water and the relevant nutrients added at the exact concentrations. A starter culture of the organism is added and the fermentation process begins (i.e. the cells begin to multiply). This multiplication is allowed to continue until the number of cells in the fermenter (the cell density) is such that some of the contents of the fermenter can be removed without altering the number of cells in the fermenter. This is possible because the cells are multiplying at the same rate as they are being removed. Removal of some of the contents from the fermenter is called the harvesting process.

HarvestingHarvesting takes place by pumping the suspension of cells and liquid removed from the fermenter through a heated vessel which stops growth and renders the cells non-viable. The suspension is then passed to a series of centrifuges where the cells are separated from the liquid residue, and these centrifuged cells are collectively known as myco-protein. The myco-protein is placed in containers and refrigerated ready to be used as the major ingredient in the manufacture of the Quorn™ ™ product range.

Process of Mycoprotein Production

Myco-protein has a high nutritional value, which makes it an ideal food ingredient.

The sole current use for myco-protein is as the major ingredient in the manufacture of the Quorn™ range of meat alternative products. The process involved in their manufacture includes simulating meat product structures by binding the myco-protein cells together with the other ingredients in the recipe. This mimics the muscle fibre / connective tissue interaction in muscle tissue.

This is achieved by mixing the myco-protein with a binder (and all the other ingredients in the product recipe) forming the required shape of the product, and then steaming the shaped material to heat-set the binder. After heating, the products are chilled, packed and distributed for retail sale.

Final Processing

Ethanol Fermenter

Continuous Fermentation of Ethanol

The traditional processes for making alcoholic beverages are batch operations that take days or weeks. Industrial ethanol for solvents and other uses was made from petroleum for several decades because cost was less than for fermentation. When the price of petroleum rose sharply in the 1970's, the economics were better for fermentation ethanol but not for the slow processes. Various improvements such as more energy efficient distillation or drying, recycle of medium that still had valuable nutrients, and better process control apply to either batch or continuous processing. The two types of continuous processing employ some means of retaining yeast cells in the bioreactor. The tower fermentor uses a strain of yeast that flocculates to particles that fall back as the fluid rises to an expanding conical section above the main region for bioreaction..

Fed batch Reactors

Comparison of fed-batch and continuous bioreactors.

With both fed-batch and continuous fermenters it is possible to set and maintain the specific growth rate and substrate concentration at optimal levels.

The major physical difference between a fed-batch and continuous fermenter is that the effluent is not continuously removed and thus washout does not occur.

This provides a fed-batch reactor a number of advantages over continuous reactors:

•A fed batch reactor can be operated in a variety of ways. For example, the reactor can and often must be operated in the following sequence:

•Batch => Fed-batch => Batch

The feed can also be manipulated to maximize product formation.

For example, during a fermentation, the feed composition and feed flow rate can adjusted to match the physiological state of the cells.

Because cells are not removed during the fermentation, fed-batch fermenters are well suited for the production of

compounds produced during very slow or zero growth. •Unlike a continuous fermenter, the feed does not need to contain all the nutrients needed to sustain growth. The feed may contain only a nitrogen source or a metabolic precursor. •Contamination will also not have the same dramatic effect on a fed-batch fermenter as a contaminant will not be able to completely take over the fermenter (unless the contamination occurred during the early stages of the fermentation).