The Golden Rule Of Tempering Uniformity
The golden rule of microwave tempering is that a uniform input will generally equate to a uniform output. Of course, no product is completely uniform and so therefore some variation in output is to be expected. The design of the microwave launcher and applicator cavity also play a role. These variables determine the uniformity of the microwave field into which the input product is placed. It is important to also understand that two types of heating are taking place; both dielectric heating and conventional thermal convection are acting on the product.
Dielectric heating is the effect the microwaves have on the molecules of the product; they cause them to rapidly change molecular alignment which causes friction and creates heat. Then, as the parts of the product more affected by microwaves (those with a higher dielectric loss tangent) heat faster than the other molecules, thermal convection begins to play a role. The (now) warmer parts of the product begin to transfer energy to the surrounding material resulting in a more even final temperature. All things being equal, the longer you leave a product in the microwave, the greater the uniformity of the final temperature.
Though the science behind microwave heating is fixed and outside of our control, the other variables are not and so we turn to those to understand how to better control temperature uniformity.
Let’s talk more about the two major variable categories:
1.) Launcher, Feed and Cavity Design
The design of the applicator cavity and the efficiency of the microwave launcher are both critical to ensuring uniform heating. The launcher design controls the microwave output and ensures that the microwave enters the waveguide and travels to the applicator cavity without disruption. The microwave feed, in the example at right a polarized feed, introduced the microwaves into the cavity and is another point of failure if not tuned properly. Lastly, the cavity design and material handling platform both must be in sync to ensure that the microwave field is strongest on the same plane as the product. At right, the green area’s indicate higher field density and the product is designed to pass through that plane (represented by the red line).
2.) Input Properties
Equipment design is important, but the most influential factor on output temperature control is the uniformity of the input. For instance, if you put a potato in the microwave it will temper or cook very evenly because it’s a homogeneous product of a standard size. However, if you temper a full leg of lamb the joints will temper slightly more quickly because the bone and skin tempers at a different speed than the thicker protein in the thigh.
Ask The Experts
If you’re curious about how to increase the uniformity of your current microwave process, give us a call. Or, if you’re just now considering the technology, consider these questions as you make your implementation and design decisions.
1.) How much uniformity is critical to my process? +/- 1°C, 2°C, 3°C?
2.) How much throughput do I need?
3.) How much time do I have in my process for tempering/cooking/drying?
The answers to these questions will shape both the size and timing of your microwave investment.