Background. Microwaves are part of the electromagnetic spectrum, with a frequency ranging between 300 MHz and 300 GHz. Although they encompass a wide range within the electromagnetic spectrum, industrial, medical, and scientific applications for microwaves generally are limited to specific frequencies between 915 MHz and 2.45 GHz (9).
Microwave heating occurs through the interaction between dielectric materials and electromagnetic waves. These interactions occur in two different ways: dipolar motion and ionic interaction. As a result, dielectric properties (i.e., dielectric constant and dielectric loss) play a crucial role in the interaction between materials and microwaves. If the dielectric properties are known for specific materials, the microwave baking process can be controlled. For example, because the dielectric constant indicates the ability of a material to store electrical energy, it can be used to estimate the ability of the material to convert electrical energy to heat (1).
In conventional heating processes, energy is transferred by three different mechanisms: convection, conduction, and radiation. In contrast, microwave energy is directly transferred to the food product through molecular interaction and motion under the electric field. In microwave heating processes, heating occurs due to the conversion of electromagnetic energy to thermal energy. Microwaves penetrate into the material, and heat is generated within the material volume. Thus, microwave heating is known as volumetric heating (16).
Advantages and Disadvantages. Conventional heating requires long processing times, which can negatively affect food quality and texture and degrade the nutritive value of the final product (3). Microwave baking offers several advantages over conventional methods, including a shorter start-up time, precise energy control, smaller space requirement, selective heating, higher nutritive value of the final product, and faster heating (12). One of the important drawbacks of microwave baking, however, is nonuniform heating throughout the sample. Nonuniform heating, which is especially dependent on product shape, dielectric properties, and size, results in hot and cold spots in baked products (3).
Other undesirable outcomes of microwave baking include dense crumb, gummy texture, hard texture, lack of color formation on crust, and high moisture loss. These outcomes are mainly the result of incomplete physicochemical changes and ingredient interactions. The short baking time associated with microwave baking does not provide an opportunity for formation of a solid porous structure, completion of starch gelatinization, denaturation of proteins, and Maillard reaction (12). To overcome these issues, microwave baking can be combined with other heating methods, such as hot air and infrared (IR) processes. Additionally, hydrocolloids can be added to formulations to improve the texture qualities of microwave baked products.
Gluten-free Baked Products: Microwave Versus Conventional Baking Methods
Celiac disease is caused by an intolerance to certain proteins found in wheat (gliadin), rye (secalin), and barley (hordein). These proteins cause damage in the mucosa of the small intestine that results in malabsorption of some nutrients. The only treatment for this disease is the strict removal of these proteins from the diet. Because wheat is the most widely consumed cereal globally and gluten plays a unique and critical role in the development of doughs and batters, removal of wheat from baked products is challenging (18).
Glutenins and gliadins are the two groups of storage proteins present in wheat gluten that are responsible for dough extension and strength. When mixed with water, these proteins begin to interact and form a new polymer network through disulfide bonds or other noncovalent bonds. These bonds provide wheat doughs with a unique viscoelasticity (4).
Rice is the cereal most frequently used to replace wheat in gluten-free products. However, baked products containing rice flour do not have the same characteristics as their wheat flour-based counterparts. To achieve desired properties in terms of dough or batter rheology and end-product quality, additives such as hydrocolloids and proteins are incorporated in formulations containing rice flour. Microwave-assisted baking is another tool that can be used to improve the quality of gluten-free baked products.
Several studies evaluating the quality characteristics of gluten-free rice cakes and breads baked using microwave-assisted or conventional methods have been reported in the literature (Table I). As listed in Table I, different types of hydrocolloids and proteins were added to the formulations to improve the quality of gluten-free cakes and breads. The primary aim of these formulation studies was to improve the viscoelastic properties of the doughs and batters, which promotes entrapment of gas bubbles, helping to achieve the desired end-product texture. In addition, because some gums have a high dielectric property, they can change the dielectric property of a batter or dough and improve microwave baking performance.
In these studies, the moisture loss, color formation, hardness, and volume of microwave and microwave-assisted (microwave–infrared [MW-IR], microwave–hot air) baked products were compared with those of conventionally baked products.
Moisture Loss. In general, moisture loss for products baked in a microwave oven was higher than for conventionally baked products. Microwave baking caused greater interior heating and pressure in products, which resulted in a higher quantity of water vapor being lost from the product (6).
To decrease moisture loss in gluten-free breads, microwaves were combined with hot air. Although hot air promoted crust formation, which acted as a barrier to water loss, breads baked in a combination hot air–microwave oven still had higher rates of moisture loss than did products baked with hot air alone (9).
Similar results were obtained for breads formulated with rice and chestnut flours (5). Although IR energy was focused on the top layer of the product and promoted crust formation, which slowed evaporation of water from the product, crust formation was not enough to decrease moisture loss. Moisture loss for cakes formulated with lentil, chickpea, and pea flours was similar to results for breads baked in an MW-IR oven. However, addition of legume flour did increase fiber content and decrease moisture loss compared with the control sample (8).
Addition of hydrocolloids (xanthan gum and guar gum) to cakes formulated with gluten-free carob bean flour significantly decreased moisture loss and bound water with ‑OH groups. These hydrocolloids have different water retention capacities and interact differently with other ingredients, which caused differences in final product quality (2).
Color Formation. One of the important quality issues encountered in baking with microwaves is the lack of crust formation and browning. In conventional baking, the air in the oven is heated, and the surface of the food is dehydrated. This accelerates formation of crust texture and color resulting from the Maillard browning reaction. However, in microwave baking, air in the oven is not heated; instead, heat is absorbed by the food, and the water inside the food evaporates. Because the air in the oven is cold, during the evaporation process water condenses on the surface of the food, which inhibits increases in surface temperature and, as a consequence, browning and crisping of the crust.
To overcome this challenge, susceptors were used by Sahin et al. (10) to promote color formation in microwave oven baking. Susceptors absorbed microwave energy and converted it to heat, increasing temperatures in localized areas to 200–260°C and promoting Maillard browning reactions. Color formation was enhanced when susceptors were used at the bottom of the bread during baking in a microwave oven. Increasing the power of the microwave oven increased absorbed heat and led to greater color formation (10).
A combination of IR energy and microwaves is another a promising technology for promoting color formation. In contrast to microwaves, IR energy heats the inside of the oven, and radiation is focused on the top of the product surface, resulting in decreased sogginess, greater dehydration of the surface, and an increase in temperature to a level at which browning reactions accelerate (14).
Color analysis of cakes formulated with lentil, chickpea, and pea flours showed that a combination MW-IR oven produced greater color formation than did a conventional oven due to the lower heat penetration of the halogen lamp (8).
Hardness. Cake quality is evaluated based on quality attributes such as high volume and uniform and soft crumb texture. To produce desirable characteristics, cake batter must be sufficiently viscous to entrap and stabilize gas bubbles until the end of the baking procedure. Starch gelatinization and protein denaturation are major changes that occur during baking, and starch gelatinization is key for development of a porous cake structure (7).
Due to short baking times in microwave ovens, gelatinization of starch may not be completed, and the cake structure may not set at an adequate degree. Combining microwave with IR heating may be a solution for improving completion of starch gelatinization. When cakes were baked in an MW-IR oven, the degree of starch gelatinization significantly improved from 55–78% in microwave baking to 70–90% in MW-IR baking (11).
In another study, cakes formulated with rice and carob bean flours were baked in both conventional and MW-IR ovens. MW-IR baked cakes had higher hardness, which could be explained by two factors. First, starch gelatinization was insufficient due to higher moisture loss and decreased water availability in MW-IR oven baking. Second, cakes with higher specific gravity, which meant a higher amount of air was incorporated in the formulations, had higher hardness when baked in an MW-IR oven due to the lower dielectric characteristic of air. Cake batters with lower dielectric property converted less microwave energy to heat, which inhibited development of the cake structure (19). In the same study, scanning electron microscopy (SEM) analysis of cakes was used to reveal the cause of higher hardness in cakes baked in an MW-IR oven. Images from MW-IR oven baked cakes showed a lack of starch swelling, which indicated starch gelatinization was insufficient.
In general, the hardness of products baked in microwave ovens was expected to be higher than the hardness of products baked in conventional ovens, mainly because of greater moisture loss. This idea was supported by a study of rice bread baked using different methods. Results showed that breads with higher hardness were baked in an MW-IR oven. Moisture loss for breads baked in an MW-IR or conventional oven was recorded as 13–14% and 5.7–8.0%, respectively (19).
Another study also related higher hardness of breads baked in an MW-IR oven with higher moisture loss. Although the degree of gelatinization was more similar for breads baked in a conventional oven (91.92 + 1.27% and 93.87 + 1.13%) and those baked in an MW-IR oven (84.28 + 1.29% to 88.39 + 0.98%), breads baked in a conventional oven had higher hardness due to the longer processing time (5).
Volume. Cake volume is key quality attribute for consumers, and baking method affects the specific volume of a cake. For example, due to internal heat generation and volume expansion during baking, cakes baked in a MW-IR oven had a higher volume than cakes baked using conventional methods (19).
Yildiz et al. (19) studied cakes formulated with carob bean flour that were baked in conventional and MW-IR ovens and found that the higher volume of cakes baked in an MW-IR oven was caused by internal heat generation, which inhibited crust formation due to water condensation on the top surface of the cake, promoting higher cake volume.
Demirkesen et al. (6) studied bread formulated with chestnut flour and baked using different methods. The study revealed that volume was lower for bread that did not contain chestnut flour. This was explained by a lack of fiber, which decreased the ability of the batter to entrap gas bubbles. In addition, increasing both baking time and IR heating promoted immediate crust formation and hindered volume expansion during baking.
Turabi et al. (17) investigated the effects of different hydrocolloid types and baking methods on the quality of rice cakes. Rice cakes formulated with xanthan gum and baked in an MW-IR oven had a higher volume than cakes baked using conventional methods. This result was attributed to the dielectric property of xanthan gum, which enhanced the ability of microwaves to heat the product, gelatinize starch, and promote structure formation. Cakes formulated with xanthan gum and an emulsifier had a higher volume and softer texture when baked in an MW-IR oven than when baked in a conventional oven. Based on this outcome, baking in an MW-IR oven was suggested as an alternative to conventional baking methods (17).
In another study, Turabi et al. (18), in an effort to improve the viscoelastic properties of gluten-free cake batter and baking properties, added different gums to a cake formulation. Cakes formulated with xanthan gum and a xanthan–guar gum blend had higher pore area fractions, higher volume, and increased porosity. These results were explained by the rheological properties of cakes because these gums improved the apparent viscosity of the batters. Adding these hydrocolloids enabled cake batter to easily entrap and stabilize gas bubbles, similar to gluten-containing batters. Further, cakes baked in an MW-IR oven had higher pore area fractions than did cakes baked in a conventional oven (18).
Although microwave baking methods have some advantages over conventional baking methods, they also have some disadvantages. The advantages for baked products include faster heating, time and energy conservation, and greater volume expansion. However, insufficient starch gelatinization during microwave baking and lack of color formation are major disadvantages. As discussed briefly, these handicaps can be minimized by combining microwaves with IR heating. MW-IR combination baking can be used to enhance the color formation and volume of gluten-free baked products and, therefore, can be considered a good alternative for baking gluten-free cakes and breads.