Questions often asked that can not be answered presently by the literature include:
- When should prawns be cooked in relation to time from harvest?
- Does this have an effect on visual appearance of the cooked prawn, eg. white head etc.?
- Is there a quality and yield difference in cooking in fresh vs salt water?
- Are cooking systems which currently use boiling followed by immersion in cold water the best option. Would a lower temperature cook for a longer period improve the process?
- What is the most effective compromise between sensory acceptability and optimal recovery?
Because prawns vary substantially in size and morphology, and the cooking equipment used by industry varies in power and capacity, at present, it is impossible to determine a time based standard that will optimise yield and quality and still remain broadly applicable.
Less powerful cookers will take much longer to return to the boil than high powered units. Thus the point of returning to the boil is a somewhat arbitrary starting point as slower cookers will expose prawns to cooking temperatures for much longer periods if using a standard time.
The industry needs a simple device which will tell processors when prawns and other crustacea are cooked. This device will measure the internal temperature during the cooking of prawns and will possibly use a cumulative cooking index or formula to determine when the prawns are appropriately cooked. This device would be based on research which determines the time / temperature relationship for enzyme deactivation as the basis for cooking calculations, and would be effective regardless of the type of cooker or maximum temperature during cooking.
Final report
A prawn cooking meter and self-centring thermocouple clip has been successfully developed for monitoring the cooking of prawns. This cooking meter provides a much needed control tool for ensuring reliable and consistent quality required of modern quality assurance programs.
The design of the prawn cooking meter relies upon the fact that enzymes that discolour and soften the flesh of cooked prawns are a major cause of quality loss. Ideally, cooking should destroy these enzymes, but experience shows there is typically not enough control over the cooking step on vessels and in processing factories to bring this about. Simple methods of timing cooking do not take account of the complex factors that can influence the rate of product heating such as size and quantity of prawns, cooker efficiency etc. This results in variable quality product, which can exhibit mushiness and discoloration including black-spot (melanosis) and autolysis.
This problem has been solved by developing a meter (Objective 1) that actually monitors the heat put into a prawn in the cooker and signals the end-point of cooking when the product is cooked enough to destroy the target enzymes, without the over-cooking that might otherwise cause toughness and weight loss. The progress of the cook is tracked by fitting a prawn typical of the batch into a robust clip, also developed in this project. The clip places a temperature sensor in the thermal centre of the tail of the prawn. The cooking end-point used by the meter is calculated from the thermal destruction rates of the enzymes that are achieved at particular temperatures. These were determined in in vitro experiments using extracts from several prawn species.
The use of the meter was tested in a number of confirmatory trials (Objective 2) where the prototype of the meter was used to successfully cook prawns of several species and size, and was, as long as the cooker approached boiling temperatures, independent of the performance of various kinds of prawn cookers used.
The meter was used to monitor a number of alternative cooking techniques (Objective 3). Of these, most interest was in sub-boiling or simmering of prawns. However, after cooking trials, this practice cannot be recommended. It fails to reach the threshold temperatures necessary to denature the enzymes that cause softening, discoloration and black spot. The underlying algorithm in the meter requires the prawns to warm significantly above the temperatures reached by simmering. Perhaps a higher sub-boil temperature can be used, as the final temperature, even in boiling prawns, is typically in the order of 90-95oC. Industry interest in steam tunnels was considered in passing by the project team, but trials were not undertaken. There appears to be no reason why this technology would not be suitable.
After the meter validation trials, ten prototype meters were manufactured and tested by industry with favourable results (Objective 4). These tests used a number of species under commercial conditions with cooperation of processors of both farmed and wild prawns in Queensland and Western Australia. Important feedback was obtained on the design features needed in a commercial model of the meter.
Dissemination of the results to companies participating in the trials was rapid. In addition, two workshops were held for prawn farmers in south and north Queensland (Objective 5). A workshop manual and training video have been prepared. The successful development of the prawn cooking meter has also been widely promoted in trade magazines and general media. Negotiations are in progress with a company wishing to manufacture and market the meters.
The cooking meter was tested on four other types of crustaceans (Objective 6) namely yabbies, redclaw, western rock lobsters and sand crabs. Suitable algorithms have been developed for all four species. Its use proved relatively straightforward for freshwater crayfish, which are of similar size and morphology to prawns, and the same self-centering clip could be used. However, further work is required to develop a practical temperature sensor clip for use with rock lobsters and crabs.
Keywords: processing, cooking. prawn, shrimp, lobster, crustaceans, crayfish, redclaw, temperature; enzyme de-activation, spoilage, meter, monitor.
