Food Microbiology II- NFSC 278

Industrial Microbiology

Bread and Baker’s Yeast

Bread

• Bread has been produced by man for thousands of years and is one of few foods common to many societies

• The basic concepts of bread and breadmaking have been modified to suit:

- the prevailing quality of raw materials

- the culinary habits

- the nature of the society in which it is consumed

Bread

• Bread is a common food and is a staple food in many geographical areas

• It is eaten everywhere that cereals are available

• Bread types can be defined based on the constituents of the bread, the shape, or a particular market the bread is aiming to satisfy

• Most bread is made from wheat but in Northern Europe and Germany bread containing rye is common

Bread

• The production of bread occurs at differing levels of sophistication from simple earthenware pots and ovens to large computer-based industrial bakeries

• The baking of bread on a large scale in societies demanding high rates of productivity has created an industry to produce yeast for bread-making

• The yeast industry has been estimated to produce approximately 430,000tons of baker’s yeast (on a dry weight basis) per annum

Bread

• Bread in its most simple form uses only flour, yeast, salt and water as ingredients

• Good quality bread can be made using only these ingredients, as is proved by the famous French baguette

• Several other ingredients are commonly used in baking and the ingredients interact with one another and the baking process to produce the texture, crumb structure, crust and flavor expected of any particular variety

Bread

• Bread formulations are a crucial part of determining the bread quality

• These formulations are usually written in terms of the ‘baker’s percentage’

• Flour is assumed to be 100% of the formulation and all other ingredients are related to the amount of flour

• E.g. if sugar level is 25%, this means 25kg of sugar is used per 100 kg of flour, though the dough may finally contain only 11% sugar

Bread Ingredients

• Wheat flour for breadmaking contains 11.0-13.0% protein of which gluten is the most important

• Gluten is responsible for the internal structure of bread and its elasticity and extensibility

• Flour contains a low level of fermentable mono- and disaccharides

Bread Ingredients

• Maltose is released from damaged starch granules during the breadmaking process

• The starch is not simply a source of fermentable CHO but also dilutes gluten to an optimal level, absorbs water and stretches as gas cells in the dough expand and set to form a gas-permeable membrane

Bread Ingredients

• Salt acts as flavor-enhancing

• It reduces the gas production rate of yeast in the dough and strengthen the gluten structure

• The effect on gluten can be desirable or not depending upon the production conditions or strength of the flour

• Salt may be used at levels of 4%

• At low concentration it enhances gas production, at high levels it deteriorates the yeast activity

Bread Ingredients

• Sugar is added at levels from 0-30% depending on preference

• Sucrose is hydrolyzed by the yeast’s extracellular invertase very rapidly

• Yeast will only ferment glucose or fructose during the baking process

• Sucrose reduces the water activity of the dough

• A plain dough has a aw of 0.97 and a 25% sugar dough a aw as low as 0.92

Bread Ingredients

• Bread improvers may consist of oxidizing and reducing agents, pH regulators, emulsifiers and enzymes that produce soft textures, improve shelf life and induce rapid fermentation

• Oxidizing agents make the dough more firm and elastic and allow it to extend more as it is leavened

Bread Ingredients

• Ammonium salts, phosphates and sulphates are often used as they provide nutrients necessary for the growth of yeast

• Salts of fatty acids such as calcium propionate or acids such as vinegar are often used as mold inhibitors at levels up to 300ppm in the finished product

Bread Ingredients

• Yeast and its actions on the other bread ingredients distinguish bread from other bakery products that may contain similar ingredients

The yeast has three functions in breadmaking:

1- It produces gas that expands the dough

2- Modifies the dough rheology

3- Yeast metabolites contribute to the flavor of the finished product

Breadmaking Practices

• The major difference in breadmaking practices is the length of time elapsing between the mixing of the ingredients and the entry of the dough into the oven

The choice of breadmaking process depends upon:

- the quality of the available raw materials (especially flour and yeast)

- the desired rate of production

- the degree of mechanization available and prevailing tradition

• The aim of the breadmaking process is to develop the dough from the initial assemblage of ingredients to a structure that is acceptable as bread

• In the simplest process-the instant process- all ingredients are mixed and after a short time the dough is ready for the oven

• Relatively high levels of yeast are used and the proofing temperatures are in the range 35-40oC so that the rate of gas production can match the development of the dough

Breadmaking Practices

Breadmaking Practices

• Longer processes are divided into the single-stage fermented dough processes and the multistage sponge and dough processes

• Fermented doughs are made in a manner similar to instant doughs except that the development of the dough is not dependent upon chemical developers

• Yeast is used at lower levels than in instant doughs and is allowed to ferment for varying periods

Breadmaking Practices

• In fermented doughs the fermentation temperature is often in the range of 22-30oC

• Higher levels of yeast metabolites develop in this type of bread and some metabolites affect the:

a- rheology of the dough

b- crumb structure

c- texture

d- flavor of the bread

Breadmaking Practices

• Sponge and dough methods are two-stage methods in which most of the water is used in the first stage but a smaller proportion of flour is used

•There are an infinite number of variations to this process as all ingredients may be added in varying proportions to either the sponge (1st stage) or dough (2nd stage)

• The yeast is very active in the sponge state but little growth takes place

Breadmaking Practices

• The final stage of breadmaking before baking is the final proof

• In this stage the dough, which has already been formed into the desired shape is allowed to rise under the action of the yeast

• It is at this stage in particular that the yeast’s gas-producing activity is measured since it determines the proof time required for the bread to rise to the required size or the size of the loaf

Breadmaking Practices

• The proof time is affected by the temperature, the quantity of yeast present and the prior processing steps

• The baking process is critical to achieving the required crumb structure, flavor, and crust color

• Microbiologically it is significant because all yeast is killed at the internal temperature of the bread reaches 100oC