Purpose: The purpose of this study was to establish a pretreatment method of shrimp extract applicable to quantify the agar which is used factitiously to increase the weight of shrimp.
Methods: Agar gel (0.2, 0.4, 0.6, 0.8, 1, 2, 3, 4 and 5%) was injected into the head, body and tail part of shrimp from 1 to 5 ml, and the shrimp was frozen and thawed to measure the weight change. The shrimp was minced and extracted by heating under pressure. The added agar in shrimp was quantified by 3,5-dinitrosalicylic acid (DNS) method with pretreatment method applied (33% of a trichloroacetic acid solution; TCA) to eliminate the color of shrimp extract. The sample precipitate was separated at 3000 rpm for 20 min by centrifuge and was heated in a steam bath by adding 3 ml of hot distilled water. The boiling solution was emptied into 50 ml vial, and then was analyzed by DNS method with acid hydrolysis.
Results: There was no loss of agar in pretreatment step, but the agar was not recovered fully through the extraction process. The absorbance at 570 nm of agar by DNS reaction after pretreatment was decreased to 59 to 45% depending on the amount of agar in shrimp, but still it was able to quantify the injected agar at least 10 mg per 500 g of shrimp.
Significance: These results imply that the established pretreatment step for eliminating the color interference prior to the DNS method would be used effectively to detect the fraud of imported seafood and quantify the amount of agar added into them.