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Novel Approach for Gluten-Free Bread Baking: Ohmic Heating

11 February 20207 min reading

Scientists bake gluten-free bread using a revolutionary technology. Gluten-free bread is crucial for those with certain allergies and conditions like Celiac disease. However, it's time-consuming to produce. Scientists have successfully made gluten-free bread using a technique called Ohmic heating, in which the bread itself is a conductor for electricity. The test bread had more volume and more uniformity, and was made with less time and energy than traditional methods.

Prof. Dr. Henry Jäger University of Natural Resources and Life Science (BOKU)

In conventional baking, wheat bread is usually produced from doughs with a water content of about 55–60%. After various fermentation periods (around 2 h for pure yeast fermentation, up to several days for sourdough fermentation), the dough is baked in baking ovens (contact heat or convection oven) for approximately 1 h, whereby heat is transferred from the outside to the inside of the dough until it reaches its maximum core temperature of around 100 °C and is then held for drying and crust formation. In contrast, GF bread formulations usually require twice as much water than for wheat bread. Since the protein network is missing, the gas retention is mainly controlled by the starch gelatinization during baking and hence a higher water addition is needed. However, this results in a lower dough viscosity and together with the different gas retention of the starch matrix compared with a gluten network in wheat bread, baking of Gluten-free (GF) bread is challenging when applying conventional technologies.

OH is an emerging technology that has shown many advantages over other heating methods. Heat is distributed in a very rapid and uniform manner, as heating occurs volumetrically and does not rely on conventional heat transfer based on conduction, convection or radiation. Main parameters affecting the heat generation during OH are the electrical field strength and the electrical conductivity of the material. Both determine the electrical current and result in a temperature increase based on the total specific energy input applied and the specific heat capacity of the material. The applied power affects the heating rate. In a heterogeneous material such as food, the electrical conductivity closely depends on certain properties (e.g., composition, pH, concentration, and mobility of ions) and usually increases with temperature, water, and salt content.

Gluten-free (GF) batters usually present several technological challenges that limit the performance during conventional baking and the resulting product quality. Due to the volumetric heating principle and faster heating rates, ohmic heating (OH) may be advantageous compared with conventional baking. Therefore, the potential of using ohmic heating as a novel approach for gluten-free bread baking was explored. In detail, the effect of different OH process parameters (power input, holding time) on the chemical and functional properties (specific volume, crumb firmness and relative elasticity, pore properties, color, starch gelatinization) and digestibility of breads was investigated. Results showed that GF breads could benefit from the uniform rapid heating during processing, as these breads showed superior functional properties (specific volume, 2.86–3.44 cm3/g; relative elasticity, 45.05–56.83%; porosity, 35.17–40.92%) compared with conventional oven-baked GF bread (specific volume, 2.60 cm3/g; relative elasticity, 44.23%; porosity, 37.63%). In order to maximize bread expansion and the OH performance, it was found that the OH process could be improved by applying the electrical energy in three descending power steps: first step with high power input (in this study, 2–6 kW for 15 s), followed by 1 kW for 10 s, and 0.3 kW for 1–30 min. In total, ohmic baking only needed a few minutes to obtain a fully expanded GF bread. The determination of pasting properties and starch digestibility demonstrated that these breads were comparable or even superior to GF breads baked in a conventional baking oven.

Electric shocks are used to heat gluten-free bread from the inside, saving energy and time compared to conventional baking applying heat from the outside. A recent study from the Institute of Food Technology of the University of Natural Resources and Life Sciences (BOKU), Vienna, was just published in Food and Bioprocess Technology. The researchers used a technology called Ohmic heating and adapted it to the production of gluten-free bread. First results show superior quality of the Ohmic bread while saving energy and time during the manufacturing process.

The principle is well known from the light bulb: An electrical current passing through a wire heats it up until it glows. This is due to its electrical resistance and the Ohmic law leading to the dissipation of electrical energy into heat. The resistance of bread dough results in the same effect—it doesn't glow like a wire, but heats up and bakes. The researchers used this smart solution to make gluten-free bread that is challenging to bake conventionally.

Hot flashes "The heat is generated instantaneously within the complete dough," explains Prof. Henry Jäger. "This is the main advantage of the Ohmic heating technology. Conventional baking in the oven requires more time, since the heat needs to penetrate from the outside toward the center of the dough."

This slow heating is a major limitation for the manufacturing of gluten-free bread. Wheat protein, the gluten, which is usually responsible for the dough structure and its expansion, is missing in these products, so starch is substituted. Sufficient heating causes the starch to gelatinize and to contribute to the structure. However, a much larger portion of water is needed in the dough, which results in a lower viscosity and makes it thinner and more liquid. This is challenging for baking.

The team around Prof. Jäger realized that the rapid and uniform heating of the whole dough mass is one of the major advantages obtained from Ohmic heating, particularly benefiting the production of gluten-free bread. "In order to really benefit from these advantages and obtain best results, the optimal process and product characteristics had to be identified," says Prof. Jäger. "Achieving such convincing results and improving the efficiency of the process at the same time was also surprising for us."

Quality The Ohmic bread showed excellent quality characteristics compared to conventionally baked products. The volume of the bread was 10 to 30 percent higher. The crumb was softer and more elastic, and the pores were smaller and more evenly distributed. But the team did not just rely on the physical characteristics of the bread, and looked also into nutritional aspects and digestibility.

"Taking into consideration the short baking time during Ohmic heating, a negative impact on starch digestibility might occur," says Prof. Regine Schönlechner, senior author of the study. However, tests performed by in-vitro methods did not reveal any differences.

The superior quality of the gluten free Ohmic bread was accompanied by savings of energy and time. The first trials indicate savings of around two-thirds compared to the energy needed for conventional baking. Also, the Ohmic baking needs much less time compared to conventional baking. In just a few minutes, the dough is converted into a ready-to-eat, gluten-free bread. Browning and crust formation do not occur, so the bread can be used directly for applications such as toast or tramezzini bread. If a crust is desired, it can be formed afterward in a controlled manner by infrared heating.

Special equipment from the BOKU Core Facility Food & Bio Processing was used by the team for the development and optimization of this promising baking concept. In this study, it was used to identify the exact conditions that result in major benefits from the Ohmic heating to the baking of gluten-free bread. This was achieved by performing trials applying different combinations of electrical power input and duration of different baking steps.

"At the end, the subsequent application of three different process intensities with different holding times proved to be the most suitable option," explains Prof. Jäger. "An initial baking step at two to six kiloWatts for 15 seconds followed by one kiloWatt for 10 seconds and a final baking at 0.3 kiloWatts for five minutes is the recipe for the successful production of gluten-free bread using Ohmic heating."

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