The principles of proofing, as an essential step in bread making, have largely remained unchanged for thousands of years. But while the processes and chemical reactions remain the same, the technology behind them has constantly evolved.
R&D efforts have continually been driven by the quest to streamline the process, to adapt it to unusual ingredients and innovative formulations, and to cut costs.
When we talk about things like Ohmic Heating or Ultrasonic Humidifiers, their applications in the proofing process are not immediately apparent. Even though the technologies are not new, and they have been around for a while, somehow they failed to gain traction by themselves. These technologies have only recently started to be considered for real-world applications in bread-making because they come with perks long sought-after by equipment producers: considerable energy savings, small footprint or better product quality.
Ohmic heating (OH), also known as Joule heating or resistive heating, is a heating process based on the passage of an electrical current through a material (in this case – the dough), which is used as an electrical resistance. Its main advantages are rapid uniform heating, no residual heat transfer after shut-off of the current, and a high energy conversion efficiency.
Timothée Gally, a Ph. D. of Oniris University in Nantes, France, alongside his peers, conducted thorough research and published several papers on the topic of Ohmic Heating and its applications for the baking industry.
“Ohmic heating has been used a few times in the past to study the behavior of bread baked under a homogeneous heating process. It had never been studied as a viable baking process directly, despite its known energy yield. Our purpose was to develop a lab prototype able to bake a sandwich bread of the size of what can be found in a supermarket. Unlike the work done by our previous peers, we wanted to study the feasibility of using ohmic heating for industrial purposes,” said Gally.
Proofing Vs. Baking
During baking, swelling and gelatinization of starch occur, which contribute to the fixation of the structure. It was shown that when starch gelatinization occurred, a noticeable decrease appeared in the electrical conductivity of the sample, which translated into a decrease in temperature and efficiency. To compensate, it was necessary either to increase voltage and current, or to add sodium chloride to the starch suspensions in order to maintain the electrical conductivity, and with it, also the temperature.
When gelatinization was completed and while the temperature continued to rise, the granules broke down and leaked amylose, increasing the amount of free water and causing the electrical conductivity to rise again.
This phenomenon is one of the reasons why the researchers decided it would be better to apply this technology to the proofing step rather than to the baking process.
“While studying the baking process under ohmic heating, we had the idea to apply it to the proofing step. Indeed, we know that ohmic heating allows homogeneous heating throughout the product, fast increase of the temperature, and smooth monitoring of the temperature elevation. These 3 factors are what makes the proofing step quite tricky to handle in the baking facilities, as the dough is usually warmer at the edges than at the core, it takes time to warm up, and there is a delay between the time the heater is turned off and the time the temperature reaches an equilibrium. Based on these points, we actually realized that the proofing process was even more promising than the actual cooking step. We could significantly decrease the proofing time (by 45% compared to regular proofing processes) while lowering the energy consumption of the process 10 times over! (though the machines’ size was not accounted for in this comparison)”
It is important to keep in mind that a ¨regular¨ crusted bread could not be obtained using ohmic heating technology, for two reasons:
– The absence (or at least very low) of gradients of temperatures. In order to be created, the crust needs a higher temperature on the outside than on the inside.
– The limit of this technology to be used as a ¨drying¨ process. The crust is an area with very low water content, and ohmic heating does not perform well at low water contents (i.e. low conductivity). The temperature of the bread would then never reach above 100°C, since as soon as all the unbound water was vaporized, the conductivity would be too low for safe ranges of voltages to be used.
Ohmic technology could, however, be used to produce crustless bread (the same kind as the crustless sandwich bread that kids love), or linked to another process to produce the crust (such as infrared heating).
Scaling to Industrial Applications
When asked about the possibility to upscale this innovation for industrial use, Timothée Gally is very optimistic: “We could imagine removable stainless steel plates acting as electrodes inside silicone molds. A PLC would monitor the voltage to keep a steady temperature inside of the dough and keep the gradients of temperature to a minimum. A laser would follow the rise of the dough to measure its expansion and switch to the baking process when the growth ratio reaches the target.
Implementation costs (equipment, expertise and training) might be higher than a conventional process, at least in the beginning, but Gally estimates that these costs would soon be counter-balanced by the massive energy savings and shorter processing times.
Limitations and Differences
Since electrical conductivity is dependant on factors like water and salt content, it’s easy to see how that could limit the formulations which could be used with Ohmic Heating technology.
For example, salt-free formulations would not be suitable for ohmic heating, unless some other non-salt electrolytes (e.g. acids, bases, etc.) were added to the recipe. Also, high fat and high sugar formulations (e.g. pastries) would decrease the effectiveness of ohmic heating, but to a lesser extent.
Gluten-free formulation, however, would not present any more issues for ohmic heating than it already does for the baking process in general, according to Gally.
Electrode gap, current and voltage combinations, and electrical conductivity are all variables that can be fine-tuned to get optimum results. Computer-aided mathematical modeling was used to mimic the process and see the impact of these different parameters on heat transfer and water transfer.
“Our work using X-Ray imagery shows that proofing and baking using ohmic heating leads to a more heterogeneous porosity at the edges than conventional proofing and baking. This result was surprising because OH leads to more homogeneous temperatures inside of the product, and we were expecting the porosity to follow this trend. Our hypothesis is that the absence of crust when using ohmic heating does not prevent the bubbles to expand at the edges, leading to the bigger pores observed on our X-ray images,” Gally explained.
Humidity is one of the most decisive factors in the baking process directly affecting the quality of the bread.
The use of Ultrasonic Humidification Technology, which generates a cold water mist with small droplets size of 1- 2 μm, provides the opportunity of maintaining high relative humidity levels in the chamber (up to 100%) at low temperatures – which is something current humidifiers cannot do.
Both, the high relative humidity and the small droplet size, avoid the well-known product surface drying and condensation effects in the conventional process.
The ultrasonic humidification for the proofing process was developed by Ungermann System Kälte GmbH and ttz Bremerhaven in 2003. Later this technology was adapted in other processes of the production of baked good (cooling, freezing, etc.). In the first years after developing, the ultrasonic humidification was mainly used in small proofing chambers in craft bakeries.
In the last 10 years, however, the integration of ultrasonic humidification in industrial processes for proofing and also cooling has become more mainstream than ever before.
The Ultrasonic Humidification Technology is based on a climatic chamber for fermentation and cooling with an innovative energy-saving ultrasonic-humidification system for the manufacturing of high-quality bakery products.
The system can be used for the three parts of the fermentation process (direct, retarding and interrupting fermentation) as a single fermentation chamber or as a single cooling unit (especially for the cooling of par-baked and fully baked goods), or as a multifunctional system covering both processes.
The NanoBAK projects were funded by the European Commission under the Seventh Framework Program. The overall aim of the projects was the demonstration of the feasibility of the so-called MicroTec technology in industrial and craft bakeries including all relevant tests. The project partners were two research institutes (ttz Bremerhaven and the Zurich University of Applied Sciences, ZHAW) and 6 business partners.
Within the scope of the NanoBAK project, prototypes of modified automatic proofers, equipped with the MicroTec technology, were installed at the project partners and their performance was tested under realistic conditions. The aim was to find out how actual production parameters in the bakeries (specific products, flour dust, temperature, humidity, hygiene, etc) will not only affect the function of the equipment and the energy consumption but also the product quality. Apart from the expected tests on product quality, lower energy consumption was also on trial.
Rogier Klein Sprokkelhorst, International Sales Manager, Contronics Engineering B.V. walked us through some of the advantages this technology brings:
“Our ultrasonic humidifiers produce an extremely fine, cold mist, consisting of droplets between 1-2 μm diameter. The aerosol is directed through the proofing chamber by an air current. Some of the droplets evaporate immediately, causing humidity to rise evenly and without condensation. The remaining droplets remain suspended and settle on the dough, forming a minuscule layer of moisture. The combined effect results in better dough and dough handling, improved baking times and energy reduction. The bread gains weight (instead of losing it) and the crust formation is optimized resulting in lasting freshness and longer shelf life.
In the cooling process, the cool mist will optimize humidity and can lead to lower temperatures and/or less energy consumption. There is less drying out and reduced weight loss of the product”, Sprokkelhorst explained.
The use of this technology also gives the user a bit more flexibility when dealing with the other crucial parameters of the proofing process. If you add US-humidification you can lower the proofing temperature (3-5°C) to reach the same proofing results. If you want to keep the same temperature then you can shorten the proofing time (ca. 20%) to get the same volume.
Another possibility is to lower the yeast content or the amount of improver.
Dennis Fehner, Area Manager Food Technology, ttz Bremerhaven explained for European Baker & Biscuit what it takes for this technology to be retrofitted to existing plants:
“The ttz is able to make a flow simulation to find out the best way of installation. At first, we simulate the actual system and the air flow in it. So we are able to see how the distribution of air and humidity is existing in the chamber and we can identify critical points of the process. Based on this simulation of the existing and the desired process parameters, we can calculate the needed capacity and the best positions of the aerosol systems. So ttz can scale this technology for every process in every dimension. We already installed systems in chambers ranging from 10sqm up to 1000sqm.”
The costs for an ultrasonic humidification system are no higher than other humidification technologies, while the process is entirely hygienic. The water is cleaned by a reverse osmosis technique, and the air is filtered as well. So there is no danger of contamination in any step of the humidification process. The single most important feature is that the humidity is produced with very low energy-effort. Expensive evaporation of water is no longer required.
Water evaporators (electrical units) are used in conventional processes for the generation of high humidity. The heating of the water and the evaporation process are extremely energy consuming. For example, the evaporation of one liter of water requires 0.8 kWh electricity. The steam, with a temperature of more than 100 °C, is then cooled down to the desired temperature (+36 °C for proofing, even lower for chilling processes). The main part of the steam condensates again which means that the energy used for producing the steam is in fact lost. This is one of the reasons why common proofers hardly achieve a relative humidity of 80-85 % and have a poor energy balance while the formation of condensate is the main factor for critical microbial and hygienic situations. Preliminary tests with the aerosol equipment have shown that energy consumption is only 2.7 % of that needed for a conventional process.
Like in most cases where new technologies emerge to replace traditional processes, there is a certain degree of inertia before they become widely adopted. Bread-making is part of the cultural heritage of so many nations, and so the notion of doing something “the way it’s been done since forever”, still has the power to hold back or at least delay innovations. But the undeniable benefits in terms of energy saving and product quality, coupled with the fierce rivalry in this sector, will surely see these technologies put to use, for the benefit of all.