Volatile Compounds and Odors in Grain Sorghum Infested with Common Storage Insects

Cereal Chem. 73(6):744-750 Lesser grain borer (Rhyzopertha dominica), red flour beetle (Tribolium flour beetle caused some off-odor. The other three insects caused little or castaneum), rice weevil (Sitophilus oryzae), saw-toothed grain beetle no objectionable odor, even though infestation was heavy. High concen(Oryzaephilus surinamensis), and rusty grain beetle (Cryptolestes fertrations of 2-pentanol and the known aggregation pheromones, dominirugineus) were placed in sorghum (1 kg, 14% mc) for 5, 7, and 10 weeks calure 1 and 2, were consistently present in samples infested with lesser at 27C. Infested samples were analyzed for insect numbers, frass, odor, grain borer. These compounds were only a partial cause of the odor from and volatiles. Volatiles from whole grain at 60C were collected on Tenax lesser grain borer. Several metabolites from lesser grain borer not previabsorbent, thermally desorbed, and analyzed by gas chromatography ously reported were tentatively identified. The presence and absence of using infrared and mass detectors for component identification. Odor was some previously reported insect pheromones and metabolites are disassessed by sensory panels at the Federal Grain Inspection Service and in cussed. our own laboratory. Lesser grain borer caused severe off-odor and red It is generally known that insects often damage grain during storage, but little has been published on the effects of insect infestations on changes in grain odor during storage (Pederson 1992, Pomeranz 1992). According to Smith et al (1971), bread made from flour infested with Tribolium spp. or Oryzaephilus surinamensis (Lin.) exhibited undesirable taste. Quinone compounds were considered most responsible for the problem with Tribolium spp. The odor of grain infested with lesser grain borer has been described as "sweetish, musty" (Pederson 1992). From our experiences with infested grain and from discussions with grain handlers and inspectors, the odor of lesser grain borerinfested grain could also be described as acrid or urinous, with the latter being more applicable to grains with severe infestations. Mites, which are often included with insects in discussions of stored grains and cereal products, have been investigated for odors and volatiles in infested grains (Tuma et al 1990). Tridecane is a major compound produced by mites. The objectives of this study were to: 1) determine which insects are most responsible for causing off-odors in grains, and 2) identify volatile compounds produced by insects that cause or are associated with undesirable odors. This information is needed for development of an objective method for detecting and classifying off-odors in grains. MATERIALS AND METHODS Test 1 Five common grain storage insects from cultures were placed on samples of sorghum (1 kg) from a local grain elevator (14% initial mc) for 5, 7, and 10 weeks at 270C. Each sample was infested with one of the following species: lesser grain borer (LGB), Rhyzopertha dominica (F.); red flour beetle (RFB), Tri'U. S. Grain Marketing Research Laboratory, Grain Marketing and Production Research Center. USDA, Agricultural Research Service, 1515 College Avenue, Manhattan, KS 66502. Mention of firm names or trade products does not constitute endorsement by the U.S. Department of Agriculture over others not mentioned. 2 Corresponding author. E-mail: [email protected] Publication no. C-1996-1004-02R. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. American Association of Cereal Chemists, Inc., 1996. bolium castaneum (Herbst); rice weevil (RW), Sitophilus oryzae (Lin.); saw-toothed grain beetle (STGB), Oryzaephilus surinamensis; and rusty grain beetle (RGB), Cryptolestes ferrugineus (Stephens). Approximately 1,200 g of whole sorghum blended with s10 g of a mixture of flour and brewer's yeast (95:5) was placed in each of 15 1-gal jars fitted with screen-type lids. Three jars were infested with each species. For RW, LGB, and RFB inoculations, s40 insects were added to each jar. For STGB and RGB inoculations, =100 insects were added. The jars were stored in an incubator at 270C. Moisture content was checked weekly. At 5, 7, and 10 weeks, one jar of each species was removed for determination of insect numbers and frass weights. Samples were frozen before testing for odor evaluation and volatiles analysis. Odor was assessed by sensory panels at the Federal Grain Inspection Service (FGIS), Board of Appeals and Review, in Kansas City, MO, and in our own laboratory (GMRL). Odor intensity levels were rated from 0 to 3, with the latter representing highest off-odor intensity. Odor descriptors were also assigned (okay [normal], musty, sour, insect, or commercially objectionable foreign odor [COFO]). Test 2 This test was similar to Test 1, except that 1) it was conducted only with LGB and RFB; 2) samples were stored 3, 5, and 7 weeks; and 3) there was a noninfested control for each storage period. About 1,200 g of sorghum plus flour and yeast (a different lot of sorghum than that used for the first test) was placed in each of 15 1-gal jars. Six jars each were infested with LGB, six with RFB, and three left uninfested. About 40 insects were added to each jar. Jars were then stored at 270C. At 3, 5, and 7 weeks, one of the noninfested jars and two of the infested jars for each insect species were removed for determination of insect numbers and frass weights. Samples were frozen before testing for odors and volatiles. Analysis of Volatiles Volatile compounds in samples from both tests were determined. Each whole-grain sample (30 g) was placed in a 25-ml Tekmar sparger of a Tekmar purge and trap instrument (model LSC 2000) equipped with a sample heater (model 211005) and a capillary interface module (model 2530) (Tekmar Co., Cincinnati, OH). Each grain sample was preheated without gas flow at 600C for 2 min, and then the volatiles from the heated sample were purged with helium at 40 m/min onto a Tenax trap (0.29 g, 60-80 744 CEREAL CHEMISTRY mesh) (Tekmar). After a 10-min sample purge time, a 6-10 min dry purge was performed to remove excess moisture from the Tenax trap. The trap was preheated at 175 0C and the volatiles were desorbed at 200 0C for 4 min. With the capillary interface module, the desorbed volatiles were cryofocused at -130 0C (liquid nitrogen) and the cryofocused zone was heated at 2000C for 0.75 min before initiation of the analytical run. A model 5890 series II gas chromatograph coupled with a model 5965B F1'IR detector (IRD), and a model 5970 mass selective detector (MSD), all from Hewlett Packard Co. (Palo Alto, CA), were used to analyze volatiles collected from the grain samples. A Supelcowax 10 column (30 m x 0.32 mm, i.d., 0.25 pm film thickness) from Supelco Inc. (Bellefonte, PA) was used for analysis. Column head pressure was 90.2 kPa (13.1 psi) at 50 0C, which gave a helium flow rate of about 2.5 ml/min. Oven temperature was held at 500C initially for 2 min, and then increased to 230 0C at a rate of 100C/min. The temperature of the gas chromatography (GC) injector zone under the capillary interface module was maintained at 200 0C. The effluent from the GC column first passed through the IRD and then into the MSD. The transfer line and flow cell temperatures of the IRD were maintained at 2000C. MSD conditions were: direct transfer line temperature, 230 0C; ion source temperature, 250 0C; ionization voltage, 70 eV; mass range, mass/charge 33-230 a.m.u.; scan rate, 1.57 scans/sec; and electron multiplier voltage, 2,200. Compounds were identified by computer matching of experimental infrared spectra and mass spectra of compounds with standard spectra in two IR vapor phase libraries (HP 59963A EPA and HP 59964A flavors and fragrances) and in the HP 59943B Wiley PBM MS database, respectively.