The control of the final dough temperature in the production of fer-mented baked products is the foundation of process and product qual-ity control, and vitally important to consistent production and product quality. Because of these considerations it is common to control the final dough temperature at the end of mixing. However, there is no single
‘correct’ dough temperature and each baker must select the one that gives the appropriate dough and product quality consistency required.
The dough temperature at the end of mixing influences many aspects of dough and product quality, including:
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Dough development through chemical actions, e.g. ascorbic acid is temperature-sensitive, and there is less oxidation when dough tem-peratures are lowered.r
Enzymic activity, which proceeds more rapidly when the dough tem-perature is increased.r
Yeast fermentation, which proceeds more rapidly when the dough tem-perature is increased and therefore affects added yeast levels and process timings (see below).r
Dough consistency and rheology, which change with dough tempera-ture, e.g. doughs become softer and less resistant as their temperature rises.r
Dough tolerance to processing delays, e.g. during dividing through the effect of temperature on dough rheology and yeast activity.r
Proof time to constant height, which will be increased with lower dough temperatures.There is a close link between the final dough temperature and the bread-making process that may be used to manufacture the dough. In general, breadmaking processes that employ a period of bulk fermentation (floor-time) require lower dough temperatures (23–26◦C), depending on the length of bulk time. Short fermentation times can accommodate higher
temperatures, while no-time doughmaking processes, e.g. the CBP, typ-ically accommodate temperatures between 28 and 32◦C.
Environmental conditions vary around the world and with season, and so ambient and ingredient temperatures vary within bakeries with time. Since control of final dough temperature is so important to fi-nal product quality, bakers have to modify ingredient temperatures to achieve a consistent dough temperature. In the manufacture of fer-mented products, only two ingredients are used in sufficient quantity to have a significant effect on final dough temperatures: flour and water.
Flour is subject to environmental temperature changes and is a poor conductor of heat so in practice bakers are left only with water as the ingredient with which to adjust dough temperatures.
Control of final dough temperatures is achieved by adjusting the dough water temperature mainly in response to variations in flour tem-perature, along with allowances for heat rise experienced during dough mixing. The latter varies according to the type of mixer being used. If bakers know the typical heat rise experienced by the dough ingredients for a given set of mixing conditions, they can quickly establish a protocol for adjusting final dough temperatures to meet specified limits.
The water temperature required for a given set of mixing conditions and flour temperature can be calculated using the following formula:
Tw= 2(Td− Tr)− Tf
where
Tw= temperature of the water required;
Td = temperature of the dough required;
Tr = temperature rise during mixing;
Tf = temperature of the flour;
The constant 2 allows for flour having approximately half the thermal capacity of water.
For example, using a low-speed mixer and bulk fermentation time if Td = 25◦C
Tr= 0◦C Tf = 20◦C,
then Tw= 2(25 − 0) − 20 = 50 − 20 = 30◦C.
Using a high-speed mixer if Td = 30◦C
Tr= 15◦C Tf = 15◦C,
then Tw= 2(30 − 15) − 15 = 30 − 15 = 15◦C.
The ‘average’ temperature rise for a given set of mixing conditions can be determined by making a series of doughs (probably eight to ten) and recording all ingredient and dough temperatures. By rearranging the equation above, the temperature rise for each mixed dough can be calculated and a mean value for Trderived for general use.
For example, using a spiral mixer if Td = 28◦C
Tf = 18◦C Tw= 20◦C, then
Tr= (2Td− Tf− Tw) 2
= (2× 28) − 18 − 20) 2
= (56− 18 − 20) 2
= 18
= 92◦C.
The precise temperature rise during mixing will be affected to a lesser degree by two other factors:
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The ambient bakery temperature – minor adjustments will be required for cold start-ups, or for low or high ambient temperatures. As a rough guide, the water temperature will need to be changed by 2◦C for every adjustment of 1◦C needed to the dough temperature.r
Heat of hydration from the flour, which occurs with all flours but is nor-mally only a problem with dry flours (less than 14% moisture). It can be calculated from the flour moisture content using a suitable equa-tion (Wheelock and Lancaster, 1970). The effect of heat of hydraequa-tion on temperature rise during mixing is small compared to that which comes from energy transfer, but it can make control of final dough temperature with high-speed mixing more difficult.The temperature rise experienced by the ingredients during dough mix-ing is related directly to the level of energy imparted to the dough (Cauvain and Young, 2006). In some breadmaking processes, such as the CBP, the energy levels are predetermined, while in others they are related to mixing speed and time. In general, the longer the mixing time the more energy is imparted to the dough.
Because of the relatively high energy level which may be imparted to the dough, considerable increases in temperature are encountered dur-ing mixdur-ing, so it will be necessary to reduce the temperature of the water
by chilling it. The water temperatures required can be very low, espe-cially with high-speed or controlled-energy mixing, and often approach 0◦C. In some cases crushed ice may be added to the mixer in order to try and achieve the necessary temperature control (Anon, 1997a), but it should be noted that this will affect the hydration and dissolution processes that normally take place in the dough and may affect dough development. Indeed Campos et al. (1996) found that a uniform mix-ture of ice and flour that was warmed later gave a homogenous but undeveloped dough.
Where the control of dough temperatures can be a problem (in sum-mer months, in hotter climates and with stronger flours), crushed ice can be used to keep dough temperatures at the required level. Using crushed ice or slush for breadmaking is more economical than cooling flour, though the latter approach is encountered in some parts of the world. The cooling capacity of ice is at least four times greater than that of cold water. The ice must be in a form that is easily dispersed and can quickly melt to take up the heat energy in the dough (Anon, 1997b).
The following formulae can be used to determine the quantity of ice which must replace a portion of the recipe-added water to obtain the required water temperature to control the final dough temperature. The heat to be removed to cool the added water must be balanced against the heat required to convert the ice to water and then to heat that melted ice to the required water temperature. In other words a ‘heat balance’ is achieved. The calculation procedures using metric standards follow.
Wi= weight of ice
Ww= required weight of recipe water Tt= temperature of tap water in◦C Tr= required water temperature in◦C
Heat, Q1,to be removed from added water= (Ww− Wi)× (Tt− Tr)× 4.186
Specific heat capacity of water= 4.186 J/kg/◦C
Heat, Q2, needed to melt ice and heat resulting water to the required water temperature= Wi× 334.6 + Wi× Tr× 4.186
Latent heat of ice= 334.6 kJ/kg For heat balance,
Q1= Q2 (2.1)
(Ww− Wi)× (Tt− Tr)× 4.186 = Wi× 334.6 + Wi× Tr× 4.186.
Example: 40 kg of water is required for a dough mix. The temperature of tap water is 20◦C. The required temperature of water for the dough is 10◦C. Calculate how much of the added water would need to be ice.
Cooling (40 kg – weight of ice) of water from 20 to 10◦C requires the heat Q1to be removed.
Q1 = (40 − Wi)× (20 − 10) × 4.186
= (40 − Wi)× 41.86.
This is the heat ‘available’ to melt Wikg ice, and to heat that ice water to 10◦C:
Q2 = Wi× 334.6 + Wi× (10 − 0) × 4.186
= Wi(334.6 + 41.86).
Using heat balance equation (2.1), (40− Wi)× 41.86 = Wi(334.6 + 41.86) 41.86 × 40 − 41.86 Wi= 376.46 Wi
418.32 Wi= 1674.4 Wi= 4.
Of the 40 kg of water required for the recipe, 4 kg should be added as ice and 36 kg added as tap water at 20◦C.
The application of cooling jackets to mixers provides another means of reducing or restricting the temperature rise during mixing (French and Fisher, 1981). The coolant may be chilled water or some other suitable refrigerant, such as glycol, circulating through the jacket. In general, the contact time between the dough and bowl surfaces during mixing is short and so the effect of the cooling jacket on the temperature rise is limited. Very low refrigerant temperatures may lead to the formation of ice on the upper, inner surfaces of the mixing bowl, which may lead to problems of controlling ejection of the dough from the mixer. The higher energy levels needed for the development of doughs made with strong North American flours, up to 20 Wh/kg (Tweedy of Burnley Ltd, 1982), require the use of a suitable cooling jacket.
In its simplest form, the control of the water temperature can be achieved by blending together quantities of hot and cold water until the required water temperature is achieved. Automatic metering of wa-ter levels at the appropriate temperatures can be achieved in virtually any bakery. The water meters concerned often contain a microprocessor capable of adjusting water temperatures to meet specified final dough temperatures, with the algorithms in the program taking into account mixing energy (or time), batch size, ingredient temperatures and even ambient bakery conditions.
In some cases, automatic systems are available which are capable of assessing the consistency of the dough part-way through the mixing process, usually via the stresses experienced by the mixer motor, with an allowance for the addition of extra water to optimise dough consistency
if required. In such cases, the water level added initially is slightly lower than the optimum level to allow for the addition of extra water later during the mixing process (Russell Eggitt, 1975). Should subsequent doughs become too soft then a reduction in initial added water has to be made. In high-speed mixing processes, e.g. the CBP, the mixing time is very short, so assessment of consistency must be made quickly and there may not be sufficient time to fully disperse any additional water uniformly throughout the dough. With spiral mixing, the slower mixing and longer timescale make the addition of extra water more practical (Ahlert and Gerbel, 1993; Gerbel and Ahlert, 1994).