Optimal Conditions for Yeast Fermentation

water jug ferment

The optimum condition for yeast fermentation are as follows:
1) Absence of oxygen.
2) Temperature range of 20° to 30° C
3) pH range of 4 to 4.5
4) Minimum water activity of 0.85
5) Sugar content of less than 40%

The conditions are explained by Fermented Fruit and Vegetables, A Global Perspective, by Mike Battcock and Sue Azam-Ali:

Most yeasts require an abundance of oxygen for growth, therefore by controlling the supply of oxygen, their growth can be checked. In addition to oxygen, they require a basic substrate such as sugar. Some yeasts can ferment sugars to alcohol and carbon dioxide in the absence of air but require oxygen for growth. They produce ethyl alcohol and carbon dioxide from simple sugars such as glucose and fructose.

C6H12O6 = 2C2H5OH + 2CO2

Glucose      =yeast=    ethyl alcohol + carbon dioxide

In conditions of excess oxygen (and in the presence of acetobacter) the alcohol can be oxidised to form acetic acid. This is undesirable if the end product is a fruit alcohol, but is a technique employed for the production of fruit vinegars.

Yeasts are active in a very broad temperature range – from 0 to 50° C, with an optimum temperature range of 20° to 30° C.

The optimum pH for most micro-organisms is near the neutral point (pH 7.0). Moulds and yeasts are usually acid tolerant and are therefore associated with the spoilage of acidic foods. Yeasts can grow in a pH range of 4 to 4.5 and moulds can grow from pH 2 to 8.5, but favour an acid pH (Mountney and Gould, 1988).

water jug ferment

In terms of water requirements, yeasts are intermediate between bacteria and moulds. Bacteria have the highest demands for water, while moulds have the least need. Normal yeasts require a minimum water activity of 0.85 or a relative humidity of 88%.

Yeasts are fairly tolerant of high concentrations of sugar and grow well in solutions containing 40% sugar. At concentrations higher than this, only a certain group of yeasts – the osmophilic type – can survive. There are only a few yeasts that can tolerate sugar concentrations of 65-70% and these grow very slowly in these conditions (Board, 1983). Some yeasts – for example the Debaromyces – can tolerate high salt concentrations. Another group which can tolerate high salt concentrations and low water activity is Zygosaccharomyces rouxii, which is associated with fermentations in which salting is an integral part of the process.

Fermentation Process Can Increase Vitamin Content

bottled tuba

Heat treatment may reduce the amount of available nutrients in foods, especially the Vitamins.  Some of them are destroyed during processing. Nutrients can be added back through fortification. The popular fortified products are rice, noodles, fruit juice and canned goods.

On the other hand, fermentation can do a reverse effect. It may increase the level of nutrients in final product. I read this information from Fermented Fruit and Vegetables, A Global Perspective, by Mike Battcock and Sue Azam-Ali

bottled tuba

Fermentation processes can result in increased levels of vitamins in the final product. Saccharomyces cerevisiae is able to concentrate large quantities of thiamin, nicotinic acid and biotin and thus form enriched products.

Sorghum beer in Southern Africa contains relatively high levels of riboflavin and nicotinic acid, which are important for people consuming a high maize diet. Pellagra (a vitamin deficiency disease associated with high maize diets) is unusual in communities in which sorghum beer is consumed. Even children benefit from consuming the dregs which contain relatively little alcohol but are rich in vitamins.

Palm wine in West Africa is high in vitamin B12, which is very important for people with low meat intake, and who subsist primarily on a vegetarian diet.

Pulque (a fermented plant sap) is an important source of vitamins for the economically deprived in Mexico. The fermentation process involved in Pulque production increases its vitamin content. For instance the vitamin content (milligrams of vitamins per 100g of product) of pulque increases from 5 to 29 for thiamine, 54 to 515 for niacin and 18 to 33 for riboflavin (Steinkraus, 1992) during fermentation.

Idli (a lactic acid bacteria fermented product consumed in India) is high in thiamine and riboflavin.

How to Make Sauerkraut (Fermented Cabbage)

cabbage

Basic Principles:

1. Selected heads of cabbage are core-shredded and soaked in tap water with 2.5% (by weight) salt concentration and allowed to ferment. During the initial stages of fermentation, there is a rapid evolution of gas caused by Leuconostoc mesenteroides; this process imparts much of the pleasant flavour to the product.

2.The next stage involves Lactobacillus cucumeris fermentation, resulting in an increase of lactic acid;

3. Finally after approximately 5 days at 20-24°C, the third stage, involving a further group of lactic acid bacteria such as Leuconostoc pentoaceticus, which yields more lactic acid combined with acetic acid, ethyl alcohol, carbon dioxide, and mannitol.

4. The fermentation process ends when the lactic acid production is approximately 1-2%. This can be tested by titration of the acid with a 0.1 N sodium hydroxide (NaOH) solution, using phenolphthalein (0.1% w/v) as colour indicator (i.e., 2-5 drops are added to the acid solution; colour will change from clear to pink and persists for 30 seconds). After the fermentation process, either the tank is sealed to exclude air or the product is then packed into glass jars or canned.

cabbage

Specific Instructions:

1. Select good, mature cabbages; remove external leaves; wash remaining heads well.
2. With a sharp knife cut the heads into four sections, removing the hearts. Slice two and a half kilos of cabbage into fine strips approximately 2 to 3 cm long.
3. Put above cabbage in pot or plastic container and mix well, adding two tablespoons of salt. Let stand for 15 minutes or more, while preparing another batch of cabbage. The quantity of salt added must be in accordance with the amount of cabbage used for proper fermentation. While the cabbage is in repose, the salt works to reduce the lot size, extract the juice, and soften the cabbage. This will prevent breakage of strips during packaging.
4. The cabbage is packed into clean wide-mouth 4 L glass or plastic jars.
5. Eliminate air bubbles from the cabbage by pressing hard with hand. This allows juices to penetrate the tissues and holes formed between strips. Soft pressing is recommended to avoid breaking the finer strips.
6. Place plastic bag full of water on top of the cabbage to prevent air from penetrating the container and the cabbage. Close the jars tightly. After approximately 24 hours of fermentation, the juices should have completely covered the cabbage. Otherwise, add a brine solution composed of 25 g of salt per L of water until all cabbage strips are covered. The presence of bubbles is an indication that fermentation is in progress. This process lasts from 5 to 6 weeks or until the bubbles disappear from the solution, after which the fermented cabbage is heated in a pot until boiling.
7. Pack cabbage into sterile jars and cover with hot juice, leaving a head space of 2.5 cm below the jar’s rim.
8. Place lids on each jar and sterilize the jars in a boiling water bath for 15 and 20 minutes for 0.5 L and 1 L jars, respectively.

an excerpt from handling and preservation of fruits and vegetables by combined methods for rural areas