Index / Enzymes / Starch / Yeast

 

Bread Ingredients

 

Enzymes

What are enzymes?

Enzymes are proteins which catalyse, or speed up, biological reactions. Enzymes usually have an -ase suffix, eg. lactase, protease, amylase and so on.

Biological reactions are the reactions that occur in living things. For example, when we eat food we need to digest it. Without enzymes in our bodies, digestion could take weeks instead of a matter of hours. Enzymes are specific for a certain reaction, for example, the enzyme lactase will only work in reactions where the chemical lactose is present.

This diagram explains what happens:

Each enzyme, for example lactase, has an active site where the substrate molecules bind to the enzyme. Each enzyme's active site has the right "shape" for a certain substrate molecule but no other. This is like a jigsaw puzzle where only certain pieces fit together.

Enzymes also have temperatures and pH levels which are optimum for them.
The whole process looks like this:

Enzymes in Breadmaking

Several enzyme catalysed reactions occur during breadmaking. First, starch has to be broken down into sugar. The sugar then has to be broken down into simple sugars to allow yeast to react with these sugars during the process called fermentation (rising).

Starch is made up of many glucose units joined together but yeast can't digest starch unless it is broken down into glucose units.

Enzyme digestion of starch can occur in two main ways by damaging starch mechanically, or by gelatinising it.

Damaged starch sounds as if it has been ruined for baking, but this is not true. It simply means that some starch granules have been crushed, broken or chipped during the milling process. In fact, some starch damage is highly desirable in bread flour and 6% damage (of the total quantity of starch present) is considered about right.

Several enzymes are required in dough to convert starch into simple sugars that yeast can feed on. This is a complex process and involves the enzymes alpha and beta amylase. If these enzymes are present they can digest starch and provide the sugars for yeast fermentation.

Starch exists in two different forms - an unbranched chain form called amylose and a branched form called amylopectin. Enzymes that digest starch are called amylases.

There are two importaint enzymes that digest these types of starch alpha-amylase and beta-amylase.

Alpha-amylase

Dough must contain some alpha-amylase to digest the amylopectin part of starch, but if dough contains too much of this enzyme it can completely liquify starch.

Alpha-amylase attacks starch practically anywhere along its chains, producing smaller chains of various lengths. These chains can contain one unit (glucose), two units (maltose) or larger units called dextrins that contain many glucose units. In a dough, beta-amylase can then digest these dextrins into maltose.

Beta-amylase

Cereal grains and flour always have an adequate supply of beta-amylase that can digest amylose completely into sugars. Beta-amylase attacks amylose chains and breaks them into molecules of maltose. Maltose is a disaccharide containing two glucose molecules.

Beta-amylase will also start digesting amylopectin from one end of the molecule but it cannot break the branches so digestion stops whenever it comes to a branch. Therefore, beta-amylase digestion of starch results in a mixture of maltose and larger dextrins. Yeast produces the enzyme maltase to break moltose into glucose molecules that it can ferment.

Once the starch has been broken down into these simple sugars, other enzymes in yeast act upon simple sugars to produce alcohol and carbon dioxide in the bread making step called fermentation. Sucrose (sugar) can't be fermented directly by the yeast enzyme, zymase. One of yeast's other enzymes, invertase, must first digest sucrose into glucose and fructose. The yeast enzyme, zymase, then ferments these sugars.

Starch

Starch belongs to a group of chemical compounds called carbohydrates. They are called this because they contain only carbon, hydrogen and oxygen. Pure dry starch is a white granular powder. Wheat flour contains 70-73% starch and most commonly anywhere between 8 -14.5% protein. If you look at flour under a microscope you can see lots of brick like structures called cells. In each cell you will see a granule of starch surrounded by glassy looking protein. Different types of starch have different structures. Potato starch is oval in shape, wheat starch is oval or round but smaller than potato starch, and maize starch has a "rocky" look.

Structure

Starch is called a complex carbohydrate because it is made up of many sugar molecules linked together. It has two main parts: amylose and amylopectin. Amylose is a straight or linear chain of sugar molecules linked together. Amylopectin is a branched chain of sugars.

Starch is a storage carbohydrate of plants such as cereals (wheat, maize, oats, rice and barley), tubers (potatoes, cassava and taro) and pulses (peas and beans). In whole wheat grains it makes up 60-70% of the grain. It is found in the endosperm which is the part of the grain that white flour is milled from.

Uses of Starch

Starch and the products derived from it are used in the food, brewing, pharmaceutical, paper, textile and adhesive industries.

In the food industry starch is used as a thickener, filler, binder and stabiliser in products such as soups, custard powders, pie fillings, sausages and processed meats, ice cream, sauces and gravies, baby foods, bakery products and baking powder.

The pharmaceutical industry use it in the manufacture of pills. It is used as a filler because it is bland and odourless. The textile industry uses starch for coating the fibre before weaving, and the dye pastes used for printing have starch in them. In the paper industry a starch solution is applied to surfaces of paper to increase the strength of the paper and give it a better finish. Starch also makes a very good adhesive or glue and is used to make cardboard cartons, boxes and containers. The gum used on the back of stamps and on envelope flaps is also made from starch.

Starch in Breadmaking and Baking

When starch is heated with water, granules absorb the water, and swell. Eventually they burst and the inner part of the granule spills out to form a thick gel. This is what occurs when you make a gravy or sauce. This process is called gelatinisation. In bread making not as much water is added as when making a sauce or gravy, and gelatinisation isn't completed - the starch granules swell, and many don't burst to form a gel. This forms a network of bloated starch granules all touching at the edges.

Starch also interacts with gluten during baking. The gluten breaks down and gives up water which is quickly taken up by the starch. This makes the gluten set and become rigid, which is why our loaves of bread don't collapse when they come out of the oven.

This scanning electron microscopic view of dough rising shows gluten strands forming two ways, diagonally down and across the photograph. Starch and yeast granules can be seen randomly amongst the gluten. The smallest granules are yeast.

Starch also provides "food" for the yeast to feed on during fermentation. As explained, alpha- and beta-amylase work together to build starch into sugar. It is this sugar that feeds the yeast in fermentation. The yeast produces carbon dioxide which helps the bread dough rise and gives bread its finished texture.

Starch, gluten and the gas from yeast fermentation all work together to produce what we know as bread, with it's crumb and gas bubble texture.

Starch is important for holding water in baked products like cakes. For certain cakes, cake flour treated with chlorine is used. The chlorine alters the starch's properties and the baker can include more sugar and fat (like butter) in the recipe. A soft, low protein wheat flour is usually preferred as less starch damage occurs, which gives better volume and a softer crumb.

Biscuits are high in sugar and fat and low in moisture (water). These factors inhibit starch gelatinisation which therefore does little to contribute to the structure of a finished biscuit.

Handy Hint: To freshen slightly stale bread simply reheat it for a short time in the oven. The starch granules reabsorb water, swell again and produce a "fresh" loaf. If the bread is very stale you could try pouring milk over it first.

 

Yeast

What is it?

Yeast belongs to the fungi family. It is a very small single cell micro-organism. Like all other fungi it doesn't have the power to produce food by photosynthesis. Instead it ferments carbohydrates (sugars) to produce carbon dioxide and alcohol which gives bread it's texture, colour and aroma.

There are several types of yeasts but the important ones for the baking industry are those belonging to the genus Saccharomyces cerevisiae, which means "sugar eating yeast".

History of Yeast

Yeast has been used by man to make bread and alcohol for thousands of years. Evidence of this has been found in ancient Babylonian wall carvings and Egyptian hieroglyphics dating back to 2000 B.C.

The leavening of bread was considered an art form because the ancient peoples didn't understand the process of fermentation. They probably stumbled across leavened bread when a piece of old over-fermented dough full of yeast cells was mixed in with fresh dough and the resulting bread was more palatable than the unleavened bread they had been used to.

In 1676 Anton van Leeuwenhoek, when looking through a microscope, identified that yeast was a cell and that different types of yeast cells could be used for brewing beer or making wine.

In the early days of bread production a piece of dough from yesterday's bake was kept and added to the new day's dough because it was found that the resulting dough was more consistent and fermented faster. The old piece of dough is called the starter or "leaven".

In the 1800's an understanding of the leavening process was fully developed through the work of Louis Pasteur. He discovered that yeast was the orgasnism that caused fermentation.

Since then, many strains of yeast have been isolated and produced. Research and development on yeast is still continuing in an effort to produce strains of yeast that will improve the bread baking process.

What does it do?

Yeast is used for the leavening of bread. Yeast uses the sugars and oxygen in dough to produce more yeast cells and carbon dioxide gas. This is called multiplication. The carbon dioxide makes the dough rise which gives the bread a light and spongy texture. Yeast also works on the gluten network. The by-products of "fermentation", or rising, give bread it's characteristic flavour and aroma. The yeast continues to grow and ferment until the dough reaches around 46°C at which temperature yeast dies.

Fermentation

Yeast uses sugars by breaking them down into carbon dioxide and water. The yeast needs lots of oxygen in order to complete this type of fermentation.

In a bread dough, oxygen supply is limited and the yeast can only achieve partial fermentation and instead of carbon dioxide and water being given off, carbon dioxide and alcohol are produced. This is called alcoholic fermentation.

Production, Growth and Reproduction

To live and grow, yeast needs moisture, warmth, food and nutrients. Commercial yeast is manufactured on an aerated suspension of molasses. Molasses, a form of sugar, provides the food for the yeast so it can reproduce. The molasses is mixed with water and sterilised to kill off unwanted bacteria, clarified by removal of sludge and then held in vats. Once it has been through ths process it is called wort.

Yeast has a phenomenal growth rate and can duplicate itself every 90 minutes by a process called budding. During budding, a mature yeast cell puts out one or more buds, each bud growing bigger and bigger until it finally leaves the mother cell to start a new life on its own as a separate cell.

When conditions are unfavourable for the yeast, for example when no food is or very dry conditions, it doesn't die but goes through a process called sporulation. The yeast spores can then withstand long periods of drought, cold and high temperatures until conditions are right for reproduction and it starts to bud all over again.

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