Water


1. Raw materials

1.2. Water

An essential element of any recipe, is water , the importance of which is very often overlooked. A bread dough is roughly 40 % water. In making dough, the consistency depends clearly on the amount of water used in making it. The amount of water needed depends on the quality of the flour and the kind of bread we want to make.

What is the purpose of water in bread making ? Water is needed to form the gluten and give the dough consistency. It is also the solvent or medium for substances like sugar and enzymes that are indispensable for the fermentation. The next essential role is its function in homogenizing all this substances throughout the dough during kneading. The water is also needed for swelling and gelatinisation of the starch. This in its turn improves the easy digestion of the bread. The distribution of the heat through the bread during baking is done by water in the dough. And finally water influences the organoleptic properties of the bread.

Besides the amount of water we are using, its quality plays also an important role. Water is an essential ingredient. In a bakery product all ingredients interact among one another at the molecular and atomic levels to give the final texture, flavour, taste, aroma, character, palatability and mouthfeel. Water is a polar substance and it strongly interacts with other polar ingredients. Based on their interactions with water, water-soluble polar ingredients are hydrophilic, whereas non-polar ingredients are hydrophobic. Substances whose molecules have both polar and non-polar parts are amphiphilic. These substances include proteins, aliphatic acids and some amino acids.

Water and ingredients of baked goods

Baking ingredients are mixtures of many compounds. Each compound has its unique properties, but only the interactions of water with some of the major classes of compounds in the various ingredients will be discussed. These interactions depend on temperature. As the temperature rises. Maillard reactions take place producing various compounds in the final bakery products.

Flour contains mainly starch and proteins. Starches are carbohydrates (i.e. polymers of six-carbon sugars) which have three hydroxyl (-OH) groups that can strongly interact with water molecules. Small starch molecules are soluble in water, whereas large ones are not. Therefore starch is hydrophilic, but the water molecules need time to get into the tightly packed starch granules. Wheat flour and especially rye flour also contain water soluble pentosans, polymers of five carbon sugars which can bind water in a multiple of their own weight. AT high temperatures starch gelatinises. These phenomena are all related to interactions with water.

Flour contains proteins which are polymers of amino acids. All have hydrophilic groups but some also contain hydrophobic groups. Thus proteins may contain hydrophobic segments. When water is added chains of proteins mingle, forming sticky globules.

Sugars are added to the dough to aid the fermentation and to sweeten the product. Brown sugar, white sugar, confectioner's sugar, syrups, honey, maple syrups, lactose etc. contain 12-carbon and 6-carbon sugars, all of which are hydrophilic. The small sugar molecules are much more soluble in water than starch.

Yeast is a single-cell micro-organism. Water helps them convert sugar into carbon dioxide and alcohol in anaerobic conditions and to water an carbon dioxide when oxygen is available. When dry, yeast becomes dormant and they revive when water and food such as sugar and minerals are available at suitable temperatures. Fermentation involves many enzymes which are large protein molecules for catalysing specific reactions. These reactions will not take place if there is no water available.

Salt is a preservative as well as a flavour agent. Baking soda, baking powder and cream of tartar are leavening gents. These and other inorganic ingredients are electrolytes, because their solutions contain positive and negative ions to conduct electricity. These ions attract strongly the polar water molecules. Baking powder contains sodium bicarbonate or potassium bicarbonate and a dry acid such as sodium acid phosphate or sodium aluminium phosphate, which react to give carbon dioxide only in the presence of water. Without water nothing will happen. Inorganic substances affect water in many ways, and minerals dissolved in natural waters affect yeast activity as well as the quality of the dough and the baked product.

We all know that fats are hydrophobic. Oil, butter, margarine, lard, shortening etc., do not mix with water but they have special functions in bakery products. Emulsifiers are long molecules which have parts are hydrophilic and their molecules bridge hydrophilic and hydrophobic molecules in baked goods.

Milk can be regarded as a watery solution of sugars and proteins. About 87 % of whole milk is water. Milk is an aqueous solution of minerals, lactose and proteins with additional fat and protein globules suspended in it. Hydrophobic components dissolve in the milk fat droplets, whereas hydrophilic components dissolve in water. As the pH changes, caseins precipitate, leaving lactose, minerals, water soluble proteins and fat in the whey.

Eggs also contain a variety of components, including proteins, lipids, minerals and 73 % water. A membrane separates the egg white from the yolk. The two have very different chemical compositions, with practically all lipids in the yolk. Egg white is an aqueous solution of proteins, whereas the yolk is an emulsion of lipids, proteins, water and minerals.

Hardness of water

The most important criteria for water is its hardness. This is a measure for the content of calcium and magnesium salts dissolved in the water. Water with a mild hardness is the most useful, because the mineral salts reinforce the gluten network. If the hardness is too high (more then 180 parts ppm or 180 mg per litre) the fermentation slows down because of the too rigid gluten structure. Using more yeast or adding malt to the dough are the best ways to correct this condition. In the opposite case, where water hardness is less then 120 ppm the dough gets sticky. In this case one has to use less water and although the consistency of the dough looks normal, one shouldn't’t forget that such a dough retains less CO2 during rise. Bread gets the right volume, but the crumb structure will be shabby when one uses too soft (with low hardness) water. The softness and the keeping qualities are negatively influenced.

Another important factor is the pH of the water used. Acids are responsible for the flavour and the taste of the bread. The acids, necessary for a good organoleptic experience will be neutralized if the alkalinity becomes larger then a pH of 8. The activity of the yeast and lactic acid bacteria drops if their environment becomes alkaline. The enzymatic activity also suffers from a too high pH. Their optimal pH must be in the range 4.0 to 5.5, which is also excellent for the yeast and the lactic bacteria.

The use of too much water is not a suitable practice. The loaf will stay small and flat, the cells of the crumb will be too large, the crust stays pale and the crumb will be wet and not soft. The other error is too little water. The crust will get tough and the crumb dry.

Water levels key to keeping bread crispy

The key to maintaining a bread crust’s crispiness is in the water content, say scientists whose progress in understanding the process could help prolong the crunch. An article published this month in the Journal of Agricultural and Food Chemistry by researchers from the Netherlands show that both water activity, which determines the direction of water migration, and water content have an effect on the perceived crispiness of bread.

However, the study was able to investigate the water content and water activity separately and found that water content is perhaps more significant than water activity.

“The water content of the crust was found to be decisive for the transition point,” wrote the study’s authors.

“The distribution of the water in samples with a history of high water content is more inhomogeneous, which results in crispy and less crispy regions, thus making them overall crispier than samples with the same water content but higher water activity.”

Discovering techniques to maintain a crust’s crispiness could help extend the shelf life. By modifying these factors, the researchers said, bakers can improve bread ingredients to produce crisper, longer-lasting crusts for bread products, and this could also mean less need for introducing artificial preservatives, which are increasingly unwanted my consumers.

It is already known that a product’s crispiness suffers when moisture is introduced. The article explained: “Water causes hydration, which causes a glass to rubber transition of the amorphous regions of the macromolecules present that were initially in the glassy state.”

The aim of the study was to investigate how water content and water activity contributed to the loss of sensorial crispiness in a bread crust model.

Bread crust samples were tested at different conditions, altering the relative humidity (RH) trajectory (hysteresis effect) to identify the effect water has on the products.

Varying water activities with the same water content, and vice versa, were achieved by either exposing a dry sample to different water vapour pressures or exposing it to 90 per cent air humidity before drying it to meet the desired pressure of water vapour.

The results found that sensorial crispiness could be determined by both the water content and the activity.

Samples with a different water activity but equal water content showed the same amount of crispiness, whereas high water content resulted in a mix of crispy and non-crispy sections, and was therefore perceived as being crispier.

Moisture, moistness and humidity

With regards to moisture there are 3 concepts which are very different but all 3 of them have to do something with the moisture of the product. And of course all 3 are related but nevertheless there are important differences between the 3 of them.

Moistness is the sensation of “soft” or “wet” that you get in your mouth when eating the product. It a sensory characteristic and cannot be measured with chemical or physical methods. It has to be done by sensory evaluation. A product can be “moist” even if it contains little water. Think about oil for instance. Oil gives you a “wet” sensation when you have it in your mouth but it is not water of course. So one has to make a clear difference between moistness and moisture.

The moisture content of a product is the amount of free water present in a product. Water is present under different forms in a product. It can be bound to gums or to the gluten for instance but it is also present as free water because it is not bound to any other ingredient. Obviously the free water content of pure water is 100 %. But think about pasta sauce, there will be a lot of free water. In cakes and breads there is not so much free water because it is bound to the gluten, the pentosans etc. present in the flour or it is bound to special ingredients which are added to the formula.

Bound water is not easily removed and when determining the moisture content of bread one basically determines the free water content or the water which is not chemically bound to the ingredients and that can be evaporated. So the moisture content of a product is the amount of water which can be evaporated by drying the product.

Obviously a product that contains more moisture will be judged as having a “higher degree of moistness”. But a product containing lots of oil can contain little water and still be judged as “moist”.

The water activity expressed as aw-value, is a concept that originally came from the microbiologists. Just think about a packed cake, hermetically sealed, so there is no influence from their air around the pack. After packing the humidity of the product and the humidity of the air around the product inside the packaging will come to equilibrium i.e. there will be no further exchange of moisture between the product and the air surrounding it. The product will stop loosing moisture to the air and the air will stop sucking moisture out of the product. Once the equilibrium is reached, the relative humidity of the air is measured and the result is divided by 100 to get the aw-value. So imagine the relative humidity of the air in the pack is 86 %, then we say that the aw-value of the product is 0,86.

Two factors play an important role in this

Of course in the laboratory the aw-value is measured in standardised conditions.

So there is a relationship between moisture content and aw-value but that relationship is not linear. Water activity is related to moisture content in a non-linear relationship known as a moisture sorption isotherm curve. These isotherms are substance (and substances present in the product) and temperature specific and have to be studied for each product individually. You cannot calculate the aw-value from the moisture content.

Why is the aw-value important? It is important because it has been found that in products with an aw-value lower then 0,9 no bacteria will grow and if the aw-value is lower than 0,75 the product will not get mouldy. That is the reason why some products can have a shelf life of 6 months without getting mouldy.



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Noël Haegens

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