It is clear that the rheological properties of biscuit doughs and pastes are very important to both processing of the intermediate and to final
product quality; the level of water and the formation (or not) of gluten play major roles in determining the required rheological character of the dough or paste. There are a number of rheological testing methods avail-able that can be used to assess the biscuit dough and paste properties;
a selection of these, and some brief comments on the effect of water on the readings obtained with each method, are given below. Such testing methods may also be used with bread and fermented doughs though the presence of yeast is a complicating factor since the production of carbon dioxide gas results in progressive change in dough density. Be-cause of this complication it is not the practice to assess the rheological properties of fermented products using full recipe doughs.
Compression–extrusion tests
In this type of test, the dough or paste is held within a container with specified geometry and a force applied. The general principles are il-lustrated in Fig. 3.7a for a closed container and one that has an orifice through which the dough or paste may pass after compression. With the closed container, it is usual to measure the force required to achieve a given penetration depth in the sample; softer doughs or pastes, such as those obtained with higher water levels, require less force to achieve a given penetration. Compression devices are known as penetrometers
(a) (b)
Figure 3.7 General principles of compression extrusion tests for biscuit dough and pastes. (a) Detail of the assembly fitted to the Instron Universal Tester for measuring penetration of paste. (b) Detail of the FIRA-NIRD extruder assembly fitted to the Instron Universal Tester for measuring extrusion of paste.
or compressimeters. Compression measurements commonly relate to the sensory properties (touch or feel) so they are often used by craft bak-ers. For example, Miller (1985) found that the subjective assessment of short biscuit doughs on a seven-point scale from very soft to very tough correlated reasonably well with compressive force measurements.
Compression–extrusion tests may be used to measure the force re-quired to extrude the sample through the orifice, but it is more com-mon to measure the time taken for a given quantity of material to pass through the orifice (see Fig. 3.7b); the higher the water content, the softer the dough and the faster the extrusion time. The forces experienced by the dough or paste during extrusion may be equated roughly to those that might occur when a biscuit passes through a rotary moulder, or when a paste is formed in a die (blocked) or sheeted between pairs of rolls. Hodge and James (1981) used an extrusion test to assess the prop-erties of savoury pie pastry, and Shekara et al. (1986) used a ‘Research’
water absorption meter to evaluate changes in short biscuit dough con-sistency. The ‘Research’ water absorption meter has also been applied to evaluate the water absorption capacity of flours intended for the produc-tion of bread (Kent-Jones and Mitchell, 1962) though it has largely been superseded by the use of the Farinograph (Catterall and Cauvain, 2007).
Recording dough mixers
Included in this category are mixers such as the Farinograph and Mixo-graph, which measure the torque that results from mixing a dough at constant speed. Such instruments are most commonly used in the as-sessment of flour properties (Catterall and Cauvain, 2007), but may also be used with full recipe doughs to make assessments of biscuit dough and paste properties. Changes in dough or paste resistance to mixing are usually recorded on a chart or digitally. Initially there is little resistance to mixing over a given period of time, but the resistance soon increases and this is recorded as a steep rise on the graph; continued mixing re-sults in a rapid fall (the fall is less gradual when assessing flour–water doughs). Hodge and James (1981) used the Farinograph with full recipe savoury pastes and found that paste consistency as measured with this technique was sensitive to added water levels, with higher water levels giving softer doughs with lower Farinograph heights. More fully de-veloped doughs, e.g. puff pastry and crackers, will show Farinograph curves closer in form to the standard flour–water tests in common use.
Load–extension tests
In this case, a prepared dough is subjected to some form of stretch-ing stress. Instruments used with flour–water doughs include the
Extensograph, the Alveograph, the Consistograph and the Dough Bub-ble Inflation device (Dobraszczyk, 1997). Their use with many biscuit doughs and pastes is limited because such products do not have a highly developed gluten structure with reasonable extensional properties.
Load extension tests have been devised by some workers to evalu-ate full recipe pastes. For example, Hodge and James (1981) devised a tensile strength test and applied it to savoury pastes. They used a ‘bone’-shaped (i.e. wider at both ends than the middle) portion of a sheeted paste held in suitable clamps and stretched the sample at a predeter-mined rate. As would be expected, they found that the extensibili-ties of the pastes were low because of limited gluten formation, and the test offered limited information of likely paste behaviour during processing.
Telloke (1991) developed load–extension tests for measuring the prop-erties of base doughs and laminated pastes used in the production of puff pastry, croissant and Danish pastry. Measurements were made of the re-sistance of a suspended sheet of dough or paste to deformation forces and were shown to be highly correlated with dough behaviour dur-ing processdur-ing and final product quality. Increasdur-ing the level of added water in puff pastry doughs decreased the resistance of the dough to deformation but appeared not to significantly affect its elastic recovery.
Telloke concluded that changes in the dough water level affected only the viscous component of the dough. Morgenstern et al. (1996) used a similar method to produce stress–elongation curves for pastry sheets and showed that the ‘pseudoplastic behaviour’ of the dough became less pronounced at higher levels of water addition.
Fundamental tests
Small-scale deformation tests have been developed which apply shear deformation to a sample held between two parallel plates (Menjivar, 1990). Such tests are more readily related to fundamental measurements of rheology and are used when the deformation history of the sample is well defined. A potential drawback of such testing methods is that the size of the stresses and strains applied to the sample are commonly lower than might be experienced during normal processing of doughs and pastes in the bakery. Nevertheless, appropriate instruments have been used in the study of biscuit doughs. For example, Oliver et al. (1995) used a Bohlin VOR-controlled strain rheometer to study the changes in semi-sweet biscuit dough rheology associated with additions of SMS and increases in the added water level. In their study they showed that for a range of wheat varieties increased levels of water addition could be used to produce biscuit dough rheologies similar to those obtained with lower water levels and added SMS. This similarity was achieved even
though water and SMS changed biscuit dough rheology by different mechanisms.