Fluids, such as water, have no particular shape of their own and flow under the influence of gravity to fill the space of a container. When held in a suitable container and stirred the fluid is pushed away by the front of the stirrer but flows to fill in the spaces behind as the stirrer moves on.
In addition to noting the ability of the fluid to flow back into the shape of the container we would observe a resistance to the movement of the stirrer through the fluid. This frictional force which is encountered when moving through a liquid is described as its viscosity; the more viscous the fluid, the greater the frictional force observed. This type of viscosity is the one most commonly quoted in the technical and scientific literature and is known as shear viscosity (Menjivar, 1990).
Fluids have a measurable viscosity that varies with temperature; in general the higher the temperature, the less viscous the fluid. In bat-ters, the presence of dissolved materials and suspended particulates, and even the numbers of air bubbles present, affect the viscosity of the system (Cauvain and Cyster, 1996). The shape, size, molecular weight distributions, inter-particle charges and concentrations all play their part in determining the batter viscosity. The key role played by water, either as added water or through its presence in other ingredients, has been discussed above. In most batter systems, the higher the water level, the less viscous the batter and the more readily it flows. The ability of batters to flow, particularly under the influence of heat, has a major impact on baked product quality as discussed in Chapter 4.
Batter viscosity may be measured either by assessing its ability to flow from a container under the influence of gravity or more commonly by using some form of recording mixing device, i.e. by measuring shear viscosity. When the viscosity of fluid is measured on its ability to flow under the influence of gravity, it is usual to measure differences based on the behaviour of the fluid with time, while shear viscosity is measured in terms of stress and strain rates (Menjivar, 1990).
Pritchard et al. (1975) used a standard Ford cup in their study of the influence of ingredients on the properties of wafer sheets. The technique they used was based on the time taken for a known volume of batter to drain from a cylindrical cup with a cone at the bottom and pierced by a hole (see Fig. 3.5). The time taken for the cup to drain is taken as an
Figure 3.5 Principle of assessing wafer batter viscosity with a Ford cup.
indication of the batter viscosity: the longer the time, the more viscous the batter. While related to the fundamental properties that control bat-ter viscosity this technique does not offer a fundamental measurement and so is not often seen quoted in the literature. However, the technique is particularly useful in a production environment because it is relatively simple to use, it is not sensitive to plant conditions, and it is inexpensive.
In such cases, a trained operator can compare the flow time for a given batter with that expected from the standard batter and adjust the vis-cosity of the batter accordingly to remain within specification, usually by adjusting water levels. The correlation between Ford cup flow times and some wafer properties can be very good.
A more popular way of assessing batter viscosity is by using a record-ing viscometer, such as the Brookfield viscometer. With this instrument, the stress placed on the batter during stirring with different strain rates may be recorded. The strain rates that are applied may be adjusted to more closely resemble those to which the batter will be subjected dur-ing processdur-ing. In the same way as would apply with the Ford cup technique, it is usual to establish a batter ‘viscosity’ profile from the Brookfield, which delivers a standard batter with appropriate process-ing qualities, and to assess test batters against that profile. Thus while more fundamental measurements of viscosity may be obtained with such viscometers, it will still be necessary to ‘interpret’ the data for a given process or product. This is particularly true of some of the other viscometers that may be encountered, e.g. Brabender Amylograph and Rapid Visco-Analyser, which have the additional facility of being able to apply heat to the batter according to a predetermined time–temperature profile. Such instruments have found particular use for the evaluation of the properties of flour–water slurries as a means of assessing and pre-dicting flour performance during baking, but are not commonly used with full recipe batters (Faridi, 1990), in part because of the complicating impact of the action of aerating agents.
Formation and processing of biscuit and cookie doughs
The product groups covered in this section are those that may fit the general descriptors of hard-dough semi-sweet (e.g. Marie in the UK), rotary-moulded short-dough (e.g. Digestive in the UK) and wire-cut cookies. Crackers and other forms of laminated biscuits are discussed below. The individual groups may be distinguished from one another according to the degree to which gluten development occurs, or is desirable, as well as on the basis of the type of equipment used in their production. The key elements of the groups are summarised in Table 3.5.
Table 3.5 Key characteristics of biscuit and cookie doughs.
Type Gluten developmenta Equipment
Crackers Modest Sheeter, laminator and cutter
Hard-dough, semi-sweet Some to modest Sheeter and cutter
Short-dough Limited Rotary moulder
Cookies Limited Wire-cut
aBy comparison with bread doughs.
The role of water in the production of biscuit and cookie doughs is very similar to that in fermented doughs and batters, with respect to the dispersion and hydration of ingredients. In biscuit- and cookie-making, high levels of added water at the doughmaking stage are avoided be-cause most of it must be baked out in the oven in order to produce the crisp eating properties and long shelf-life that characterise biscuits (Cauvain and Young, 2006). Typically, baked biscuit moisture contents fall well below 10% (see Chapter 4). This requirement limits the amount of water that is added to the other ingredients and the degree to which the softer dough types are mixed. In the case of short-dough and cookie dough, gluten development needs to be limited so that the forming pro-cesses for individual pieces can be easily accomplished, and changes in biscuit shape (e.g. shrinkage) after forming and during baking can be minimised. This limited gluten development comes from water ad-ditions which are significantly lower than seen with fermented dough recipes (typically less than 15% flour weight), from the effects of sugars and other soluble materials on water activity, and through additions of fat.
In addition to the effects of recipe ingredients and their levels, the mix-ing method may be modified in an attempt to limit gluten formation. The most common variation is called ‘creaming’ because all the ingredients, except the flour and any nuts, fruits or chocolate pieces, are first mixed together. In this mixing stage, the sugars and other materials dissolve in the recipe water and the resulting solution becomes dispersed in the fat. The final mixture has a creamy-white colour and a soft consistency.
The flour is blended through the cream mixture, giving a soft dough that lacks significant gluten formation because it is difficult for the flour proteins to become hydrated. However, protein hydration and gluten formation are possible, as shown by the fact that extended mixing when the flour has been added will lead to a much firmer, tougher mixture, which is more difficult to process, and to loss of biscuit shape because of shrinkage on baking.
Hard-dough, semi-sweet biscuits require a degree of gluten forma-tion and so added water levels tend to be a little higher (typically 20–
25% flour weight), and fat and sugar levels somewhat lower. An all-in
mixing process tends to be used which also encourages gluten forma-tion. Modification of the dough rheological character may also be un-dertaken through the addition of a reducing agent, commonly sodium metabisulphite (SMS), or a suitable proteolytic enzyme. If it is not pos-sible to modify the dough rheological properties through the addition of a reducing agent, extra water may be used to give a softer, more machinable dough.
The important effects on biscuit processing which arise from varying added water levels have been studied by a number of workers. Gaines (1982) evaluated the consistency and stickiness of sugar-snap cookie doughs made with four different water levels. His data suggested a two-phase (initial and time-dependent) requirement for water by flour and sugar. In freshly mixed doughs, variations in water levels had ap-proximately equal effects on dough consistency and stickiness, while in doughs that had been rested for 1 h, changes in the water level had twice the effect on dough consistency as on dough stickiness.