Stability of Uric Acid Used as an Indicator of Insect Contamination During Extrusion

Cereal Chem. 73(5):625-627 To determine the stability of uric acid during extrusion, wheat flours butylammonium phosphate and ultraviolet detection. It was found that were spiked with uric acid such that the samples had a uric acid content 62-80% of the uric acid in flour survived extrusion, depending on the of approximately 25 ltg of uric acid per gram of flour. The samples were extrusion conditions. On average, about 72% of the uric acid initially then extruded on a Brabender single-screw extruder at feed moisture present was retained in extruded wheat flour. This high recovery level contents of 28, 30, and 32% (db) and temperatures of 120, 140, and indicates that uric acid is relatively heat stable and could be used as an 160'C. Uric acid in the extrudates was quantified by reversed-phase indicator of insect contamination in extruded products. high-performance liquid chromatography, using ion-pairing with tetraUric acid, excreted by insects as the end product of nitrogen metabolism, has previously been proposed as an alternative to the insect fragment count as an indicator of insect contamination in grains and cereal products (Pachla and Kissinger 1977, Roy and Kvenberg 1981, Galacci 1983, Wehling and Wetzel 1983, Lamkin et al 1991). Good correlations between uric acid content and levels of infestation have been found in whole grain (Wehling et al 1984). However, little is known about the fate of uric acid during thermal processing. In order to use uric acid as a marker to indicate whether extruded products have been produced from insectcontaminated ingredients, it is necessary to examine the stability of uric acid during the extrusion process. It is also important to know the influence of different processing conditions on the retention of uric acid in the final product. Food extrusion is an important processing technique that can utilize a wide variety of raw materials and processing conditions. Food ingredients are subjected to high temperature, pressure, and shearing stress and therefore undergo physical and chemical changes that affect the quality factors of the extruded products. The retention of uric acid in extrudates may be affected by extruder variables and variables related to the raw material. Food extrusion is becoming more popular as a processing technique that can be applied to many raw materials (Altschul and Wilcke 1985), and a number of extrusion applications have been reported by Cheftel (1986). Complete descriptions and advantages of extrusion processing have been written (Harper 1979, Linko et al 1981, Mercier et al 1989). Reviews of the effects of extrusion cooking on nutritional properties have been given by Cheftel (1986) and BjOrck and Asp (1983). They have reported that extrusion affects food product quality factors such as enzyme activity, as well as vitamin and amino acid availability in foods, depending on processing conditions. Important processing parameters in extrusion that affect product quality include moisture content of the feed material, temperature, shear, residence time, screw speed and configuration, and die geometry. Many researchers have investigated the effects of extrusion variables on the physical properties of extrudates made from different materials (Fletcher et al 1985, Lawton et al 1985, Stanley 'Published as Paper no. 11375, Journal Series, Agricultural Research Division, University of Nebraska, Lincoln, NE 68583-0704. 2 Department of Food Science and Technology, 143 Filley Hall, University of Nebraska, Lincoln, NE 68583-0919. Author to whom correspondence should be addressed. Publication no. C-1996-0822-02R. © 1996 American Association of Cereal Chemists, Inc. 1986, Phillips and Falcone 1988). According to many published literature reports (Beetner et al 1976, Seiler et al 1980, Schlude 1987), the moisture content of the feed materials and the barrel temperature are the most important processing variables that affect the final characteristics of the extrudates. There is, however, little literature information on the chemical reactions occurring during extrusion processing and the quality implications related to these changes (Camire et al 1990). Limited information is available about the thermal stability of uric acid. The Handbook of Chemistry and Physics (Anonymous 1991) reports that uric acid decomposes upon heating; however, it does not specify the temperature at which decomposition takes place. The 1-methyl derivative of uric acid, a closely related organic compound, has a melting point of 400'C. This high stability of a uric acid derivative to heat indicates that uric acid may, to a degree, survive cooking temperatures. The objective of this work was to evaluate the survival of uric acid during extrusion using conditions commonly encountered in processing operations. Barrel temperature and feed moisture content were varied within typical ranges. This provided information on the survival of uric acid during thermal processing and indicated whether uric acid survived to a sufficient degree to be useful as a marker of insect contamination in processed foods. MATERIALS AND METHODS Materials Three packages of all purpose wheat flour, each 4.53 kg and bearing different lot numbers, were purchased from local markets and used for sample preparation. Each individual package was used