Review: Indole as a quality indicator in shrimps and prawns

Shrimps are an important food commodity in Europe. Quality control of imported shrimp is essential due to technological and hygienic problems in tropical and sub-tropical areas where the shrimps are cultivated in large scale. In this paper the processing of shrimp is described and microbial formation of indole is discussed. The effect of various process parameters on the presence of indole in shrimps is reviewed in relation to the FDA action levels for indole in shrimps of 25 mu g/100 g and 50 mu g/100 g, respectively. Various analytical procedures for the measurement of indole are evaluated. Temperature abuse is an important critical process parameter for the formation of indole. A number of bacteria has been identified as in-dole positive but only a few (Escherichia coli, Proteus (P.) vulgaris and P. morganii) were detected on shrimp. High indole levels in frozen shrimp can be considered an indicator of poor product quality prior to freezing and thus of hygienic practices far away from Good Hygienic Practice (GHP). High indole levels (> 10 mu g/100 g) are reached at the end of the shelf-life of shrimps when stored at elevated temperature levels. However, a low content of indole does not necessarily imply a good quality. Taking into account the results from the limited storage studies the FDA action level for indole is therefore at least debatable. High pressure liquid chromatography (HPLC) with fluorescence detection seems to be the most effective analytical method for the determination of indole