Digital imaging for monitoring of dough leavening

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When the mixture of flour and water becomes a leavened dough under the action of yeasts and/or other microorganisms, the gluten viscoelastic net is able to trap and retain the gas bubbles that are generated by microbiological metabolism (transformation of cereal flour carbohydrates in ethyl alcohol, carbon dioxide and other products). The last part of the leavening (also called “proofing”) is a specific phase of rest of the dough, in which its maximum expansion takes place. At this stage it is essential to achieve a specific volume of the dough in order to have a finished product with the desired characteristics (size, lightness, softness of the crumb, chewing texture, etc.). The final volume is determined by the number and size of gas bubbles that constitute the leavening dough alveolation. Some of the factors determining ease of bubbles formation, their magnification and their stability are here reported:

– Flour characteristics, and in particular its “strength”, have a crucial role in allowing a marked leavening: only flours which gluten is plentiful and of good quality (called “strong flours”) will give a net able to trap a rich gas development within the dough without the bubbles breaking the structure, with loss of volume and decline in quality of the final product (1,2)

– Some ingredients or additives, such as ascorbic acid, cysteine, and some enzymes, have stabilizing effect on gas bubbles (3).

– The appropriate incorporation of air during kneading is a long known factor capable of favouring the abundance of initial sites of bubbles nucleation: during the leavening these sites will then be able to expand (4).

– Temperature (T) and relative humidity (RH) are obviously two physical parameters of paramount importance in determining the output of leavening: these parameters must therefore be accurately set inside the leavening cells in order to reach the desired final volume.

– Finally, other factors of lesser importance might be: amount of fat present in the dough, mineral composition of the water used, pressure applied by kneading machines, any vacuum applied during the leavening, etc.

Monitoring the density of dough during fermentation is a good approach to understand and control the production consistency of leavened foods. Dough density obviously decreases during its rising, along with its increase in volume. The measurements of these parameters, however, are not absolute but depend on the method used to detect them (5). The density of the dough at the end of leavening is also related to the size of the loaf (6). Therefore, monitor dough density is not as simple as it might seem, especially due to often irregular shapes and surfaces of loafs. In the past, the calculation of the surface area was carried out with FEM (Finite Element Method), based on a computer software (7). More recently, CVS (Computer Vision System) has been used to more accurately estimate surface area and volume of various foods(8,9,10). Both techniques are non-destructive. A method of digital imaging has recently been developed to follow dough expansion during the whole leavening process (11): compared to previous methods, it has the advantages of being very cheap and able to measure dough density in a dynamic way and in the real conditions of T and RH. In such study the images acquisition was carried out every 5 minutes, using a CVS constituted by a PC coupled to a digital camera. Samples of bread dough were prepared by mixing flour (100%), water (62.5%), sugar (1.5%), salt (1.5%) and yeast (1.5%); such mixture was then divided into pieces of 85 g each. Leavening lasted 90 minutes and was performed at 3 different T (25, 30 and 35° C) and 3 different RH (65, 75 and 85%), thus giving rise to many different parameters combinations. In all the various T and RH combinations, a decrease in dough density in conjunction with the leavening and with the increase in volume was observed. In the first minutes of leavening there were no significant changes of the height of the dough; on the contrary, after the first 10 minutes a steady and significant increase in this height was observed, and it lasted up to 50-60 minutes from start of leavening (depending on T and RH condition). After that phase, the doughs height still increased, but to a lesser speed, until the end of leavening (90 minutes). The initial dough densities ranged from 1.025 to 1.097 g/cm3, while the final ones were obviously characterized by greater differences, due to the incidence of the different experimental conditions (T and RH), and ranged from 0.417 to 0.521 g/cm3. With higher T and RH, the speed of dough density reduction was higher, in particular in the first part of leavening: this means that the desired volume is reached more rapidly by setting high T and RH parameters inside the leavening cells. In fact, higher T during leavening onset increases the microbial metabolism and consequently the production of carbon dioxide, which forms the alveolation; higher T also decrease the solubility of that gas inside the dough, making easier the formation of bubbles and their magnification. However, once 40°C are reached, there is no further positive effects on the dough volume and the speed of it increase, because of the thermal stress on yeasts. Regarding RH, its effect is secondary compared to that of T, but RH has an important influence in softening the dough surface, reducing the tension and thus facilitating the expansion of the dough itself. For this reason, with higher RH values within the leavening cells, dough expansion occurs more easily and higher final volumes are reached. In conclusion, digital imaging can be considered a valid method for monitoring dough leavening in a simple and effective way, in order to follow and understand its characteristics and modifications during the leavening, evaluating the effects of parameters such as T and RH, but also the influence of different ingredients choice, and/or of their quantity inside the dough.

Bibliography

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2)  Campbell G. et al., Measurement of dynamic dough density and effect of surfactants and flour type on aeration during mixing and gas retention during proofing. Cereal Chemistry, 78:3 (2001) 272-277

3)  Gan Z. et al., Gas cell stabilization and gas retention in wheat bread dough. Journal of Cereal Science, 21 (1995) 215-230

4)  Baker J.C. and Mize M.D., The origin of the gas cell in bread dough. Cereal chemistry, 18 (1941) 19-34

5)  Elmehdi H.M. et al., Evaluating dough density changes during fermentation by different techniques. Cereal Chemistry, 84:3 (2007) 250-252.

6)    Ktenioudaki A. et al., Monitoring the dynamic density of wheat dough during fermentation. Journal of Food Engineering, 95 (2009) 332-338.

7)  Cleland et al., Prediction of rates of freezing, thawing or cooling in solids of arbitrary shape using the finite element method. International Journal of Refrigeration, 7:1 (1984) 6-13.

8)  Zheng et al., Estimating shrinkage of large cooked beef joints during air-blast cooling by computer vision. Journal of Food Engineering, 72:1 (2006) 56-62.

9)  Du C.J. and Sun D.W., Estimating the surface area and volume of ellipsoidal ham using computer vision. Journal of Food Engineering, 73:3 (2006) 260-268.

10)Stabilov et al., Image processing method to determine surface area and volume of axisymmetric agricultural products. International Journal of Food Properties, 5:3 (2002) 641-653.

11)Soleimani Pour-Damanab A.R. et al., Monitoring the dynamic density of dough during fermentation using digital imaging method. Journal of Food Engineering, 107 (2011) 8-13.