FOOD CHEMISTRY

Module 9. Cereals and cereal products

Lesson 23
TYPE OF FLOURS FOR BREAD MAKING AND CONFECTIONERIES AND INFLUENCE OF ADDITIVES

23.1 Introduction

The term baking is usually applied to flour-based foods, for example wheat, oats, maize and sorghum. These grains and flours have a relatively long shelf life. The main purpose of baking is to change the eating quality of the staple and to add variety to the diet. It is also a means of preservation, extending the shelf life by removing moisture and inhibiting enzymes and bacteria. After baking, goods such as breads and pastries have a shelf life of 2 to 5 days and other goods such as biscuits and some cakes have a shelf life of several months so long as they are correctly packaged. During baking food is heated by the hot air in the oven. Moisture at the surface is evaporated by the heat and a dry crust forms. Biscuit production involves slower heating so that moisture is also lost from the inside of the product.

Baked goods are produced from either doughs or batters which are a mixture of flour and water made by mixing, beating, kneading or folding. The processing method depends on the ingredients being used and the product being made. All baking is based around the use of wheat flour, but many other ingredients are also used,each of which has different effects on the final product. The commonest of these are listed here.

23.2 Flour

The word flour refers to the powder obtained from grinding a cereal grain. Flours from different wheat varieties vary in protein content. Flours that are good for bread making (i.e. give a good loaf volume) are obtained from wheat varieties which have high protein contents (12-14%). Good bread making wheat are described as ‘hard’. Hardness is related to the degree of adhesion between starch and protein. Hardness and softness refer to the way in which the endosperm breaks down on milling. In hard wheat, fragmentation of the endosperm tends to occur along the lines of the cell boundaries, whereas the endosperm of soft wheat fractures in a random way. Hard wheat yields coarse, gritty flour and soft wheat give very fine flour. The strength of wheat relates to its baking quality. Hard wheat produces large loaf volume and good crumb structure. Dough of hard flour is more elastic and more resistant to stretching than the dough of soft flour. Bread flour should form good gluten when mixed with water resulting in bread with a good volume when baked. The flour of soft wheat is good for biscuits and cakes. These flours are usually obtained from soft wheat varieties. Their protein content is usually less than 10%. Biscuit flour should make dough having more extensibility, but less resistance than bread dough. They form only a small loaf with coarse crumb structure.

Characteristics of good cake flour

Cake flour is a medium-strength flour ground from soft low-protein wheat of low α-amylase activity and is very fine in structure. The purpose of flour in cakes is to allow an aerated structure to be retained after the cake has been built up. The stability of the final cake depends largely on the presence of uniformly swollen starch granules; hence, the starch granules should be undamaged during milling, be free from adherent protein, and be unattacked by amylolytic enzymes.

The best cakes are obtained from a low-protein flour (7-9%) which is soft and gives tender cakes. For cakes which contain a higher proportion of sugar than normal,the flour must be chlorinated. Good milling can help to achieve these characteristics, but obviously only if the wheat is already of the appropriate quality.

The characteristic and general quality of the flour depends on

• The wheat variety and conditions under which the wheat has grown.
• This affects the quality and quantity of gluten in the grain.
• The milling process
• This determines the degree of separation of the bran and endosperm, as well as the particle size of the flour, an important factor in cake flour.
• Additives and special treatments used by the miller to produce flour mixes with special characteristics.

23.3 Yeast

The particular strain of yeast used in baked products is Saccharomyces cervisiae. After addition to the dough under appropriate conditions of pH 4-6 and temperature 30^oC, the yeast begins to feed on the starch in the mixture, forming sugar, alcohol and carbon dioxide. The bubbles of CO 2 cause the dough to expand ("rise"). The dough must be "kneaded" thoroughly to distribute the bubbles evenly and then left to rise again, usually to about double its original volume. If the mixture is left too long, acid produced by the oxidation of the alcohol causes the product to taste sour.

23.4 Chemical Leavening Agent

The most important chemical agent used in leavening is baking powder. Baking powder is essentially a mixture of NaHCO₃ and a weak solid acid or acid salt. When mixture dissolves in water and the temperature is raised, CO₂ is released according to this equation:

NaHCO₃ + H ⁺ (from the acid) → Na ⁺ + H₂O + CO₂

The most common acids used are potassium hydrogen tartrate, tartaric acid, acid calcium phosphate .

When baking powder is used rather than baking soda alone, the by-products are less alkaline than Na₂CO₃ and thus they have no undesirable effects on the taste of the product. The type of acid used in the baking powder affects the rate of CO₂ production,which in turn affects the product. In baking, the rate at which CO₂ is produced and the continuity of CO₂ production are both important.

Baking powder is a very widely used ingredient in cooking and baking. Self-raising flour has also become popular in recent years. This is merely high grade flour to which baking soda and a suitable acid (such as cream of tartar) have already been added,

Baking soda (sodium bicarbonate) is also used as leavening agent. It has the property of releasing CO₂ when it is heated:

2NaHCO ₃ → Na₂CO₃ + H₂O + CO₂

When used on its own, only half the available CO₂ is released and, more seriously, the sodium carbonate producedis strongly alkaline and gives the baked product a bitter, "soapy"taste and a yellow colour. During digestion of such products the Na₂CO₃ reacts with the HCl in the stomach to produce the other half of the available CO₂ . Forthe above reasons, it must be stressed that sodium bicarbonate is very rarely used on its own, but generally mixed with some acidic material. To avoid an imbalance between the acidic and basic materials, i.e. an incorrect pH, baking powder is more commonly used.

23.5 Fat

Fat has a number of functions in baking. Fat weakens or 'shortens' a dough by weakening its gluten network, resulting in the baked product being softer, breaking easily and having a more tender mouthfeel. Fat can trap air during beating and mixing, producing a batter that consists of masses of tiny air bubbles trapped within droplets of fat. This is very important in cake baking in which it is these air bubbles that expand during baking forming a light, airy structure. In puff pastry fats which are soft over a wide temperature range are used. These can be spread between pastry layers and will separate them during cooking giving a layered pastry. Usually the fats used should have a bland flavour to prevent them from changing the flavour of the finished product, but occasionally fats are chosen on the basis of their flavour, e.g. using butter for particular baked goods and lard for meat pie pastry. In addition,the fat chosen needs to be able to form an emulsion with the other ingredients in the batter or dough.

23.6 Sugar

Sugar is most commonly thought of as a sweetener, but in baked goods it is also involved in several other processes. Sugar undergoes a series of complex browning reactions above 160^oC and the products of these form the brown crust of many baked goods. The reactions are known as Maillard reactions, and are essentially amino acid-catalysed caramelisation reactions in which a sugar aldehyde or ketone is converted to an unsaturated aldehyde or ketone.

In addition, 0.5-0.75% w/w of sugar increases the rate of fermentation for fermented goods (i.e. breads) by giving the yeast more sugar to work on. In non-fermented goods such as biscuits, large quantities of sugar can be added.This improves the keeping quality of the biscuits as well as sweetening them.

23.7 Ascorbic acid (Vitamin C)

Addition of a small amount (up to 100ppm) of Vitamin C also shortens the time needed fort he dough to mature. This is because ascorbic acid catalyses the gluten cross linking reactions to form a more extensible, elastic, strong network.

23.8 Egg

Eggs and their products constitute important ingredients in a wide range of bakery products. They improve the physical and organoleptic properties of the products in which they are used. Beaten egg white is used, like baking powder, to give the dough a light, airy texture. Eggs can be used as emulsifiers, moisteners (instead of simply adding water) and, nutritionally,as a source of fat and all the essential amino acids. Eggs improve the cell structure of the product, maintain it during the baking process, and reduce the moisture loss from the baked product.

23.9 Salt

Salt is added to enhance the flavour of cakes and breads and to "toughen up" the soft mixtureof fat and sugar. Salt has also are tarding effect on yeast fermentation. In its absence fermentation takes place very rapidly and the dough is too sticky to knead properly. The result is the coarse texture of the baked bread.