Control of histamine in Rihaakuru : emerging approaches : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy, Massey University, Palmerston North, New Zealand

Abstract

Rihaakuru is a cooked fish paste from the Maldives, consumed as a condiment with rice and other food. The product is unique to the Maldives and there is no information on the composition, characteristics and safety of this product. Histamine contamination has been suspected due to symptoms sometimes seen following consumption. This research established that Rihaakuru is a nutritious and shelf-stable product. Rihaakuru is generally produced from poor quality fish therefore presence of biogenic amines was suspected. This study confirmed that Rihaakuru contained up to ten different biogenic amines, with histamine in excess of 500 ppm. This may cause histamine poisoning with symptoms such as skin rashes, vomiting and fever. The product examined in this study contained a few weak histamine forming bacteria. Most of the histamine is likely to be produced by bacteria in the raw fish. These bacteria are likely to die during the manufacture of Rihaakuru. Histamine in Rihaakuru decreased by 30-70% during storage at - 80°C, 4°C and 30°C for 10 months. This showed that the histamine hazard in Rihaakuru is unlikely to increase and may decrease during long term storage. Traditional control of histamine in food is through refrigeration of raw material. In the case of the fish used to manufacture Rihaakuru, refrigeration is not available or limited. Histamine oxidizing bacteria and enzymes were identified as emerging approaches to degrade pre-formed histamine. Histamine oxidizing bacteria (Lactobacillus sakei [AGR 37, AGR 46, Lb 706] and Vergibacillus halodonitrificans Nai18) tested in this study degraded histamine by 30-50%. The histamine oxidizing enzyme, diamine oxidase (DAO) completely degraded 500 mg/L of histamine at pH 6 and salt 1% in buffer and in the tuna soup used to manufacture Rihaakuru. A regression model was developed that predicted the rate and amount of histamine removal by DAO under varied pH and salt concentration. This model may be used to determine conditions that will reduce histamine in other foods that have similar characteristics to the tuna soup used to manufacture Rihaakuru.