Microwave tempering experiments were conducted on frozen blocks of shrimp (FSB) and the results were used to help determine if microwave tempering of FSB is an improved thawing method over the current, traditional method, water immersion. Results of the microwave tempering experiments were also used to help determine which microwave tempering method amongst those explored by this study is most effective.

Complete thawing of a FSB in a microwave oven was found to be impractical; however, using a combination of microwave tempering followed by water immersion can successfully thaw a FSB. After a microwave tempering experiment was conducted, the final stages of thawing were completed by using the traditional water immersion method. The amount of time to complete the thawing was recorded and is referred to as the additional thawing time. The amount of shrimp cooked during microwave tempering was also recorded and calculated as a percent. The additional thawing time and the percentage of shrimp cooked were used as criteria to compare microwave tempering experiments and also to compare microwave tempering experiments with the current method.

The first set of microwave tempering experiments explored the advantages of freezing a microwave susceptive material within the FSB before microwave tempering. FSBs with susceptors and FSBs without susceptors were tempered in a microwave oven. The FSBs were tempered in a 2450 MHz microwave oven at 255 W for 35 minutes and at 406 W for 22 minutes. The results showed that the addition of susceptors does improve the microwave tempering process. The percentage of cooked shrimp and the additional thawing time was less for FSBs with susceptors than for FSBs without susceptors. The susceptors seem to help distribute the microwave energy more evenly, which reduces runaway heating and in turn reduces the amount of shrimp cooked.

When compared to the current method, microwave tempering with susceptors reduced the total thawing time by 45% while microwave tempering without susceptors reduced the total thawing time by 43%. Both microwave tempering methods, with and without susceptors, are an improvement over the current method. The addition of susceptors does improve the microwave tempering process; however, the improvements are not significant enough to justify its recommendation.

The second set of microwave tempering experiments explored the advantages of pulse microwave tempering. During pulsed microwave tempering the microwave oven was set to a high power level and was turned ON for a period of time and then OFF for a period of time. The ON/OFF pattern was repeated throughout the microwave tempering process. Several pulsed tempering experiments were conducted at a microwave power level of 848 W and at a microwave power level of 993 W. The results showed that there is no significant advantage to using pulsed microwave energy during tempering as opposed to continuous, fixed microwave energy. The results showed that fixed microwave tempering is more effective than pulsed microwave tempering. The percentage of cooked shrimp was lower for fixed experiments than for pulsed experiments and the additional thawing time was slightly less for fixed experiments than for pulsed experiments.

A mathematical model was developed to help predict he temperature profiles of a FSB during microwave tempering. Experimental temperature data were collected at four locations within the FSB during microwave tempering by using four Luxtron Fluoroptic temperature probes and a Luxtron Fluoroptic thermometer. Overall, the temperatures predicted by the model were within 2 oC of the experimental temperatures. After the first 500 seconds or so of microwave tempering, the temperatures predicted by the model were consistently less than the experimental temperatures.

From this study it was determined that the most effective microwave tempering method, amongst those conducted in this study, of a 2.2 kg (5 lb) frozen block of shrimp was accomplished by setting the power output to 255 W and the microwave cooking (tempering) time to 35 minutes. As previously mentioned, the addition of susceptors does improve the process but the improvements are not significant enough to justify its recommendation. Pulse tempering is not an improved method over fixed tempering.