The ability of spores of one type A and one type B strain of Clostridium botulinum to grow and produce toxin in tomato juice was investigated. The type A strain grew at pH 4.9, but not at pH 4.8; the type B strain grew at pH 5.1, but not at pH 5.0. Aspergillus gracilis was inoculated along with C. botulinum spores into pH 4.2 tomato juice; in a nonhermetic unit, a pH gradient developed under the mycelial mat, resulting in C. botulinum growth and toxin production. In a hermetic unit, mold growth was reduced, and no pH gradient was detected; however, C. botulinum growth and low levels of toxin production (<10 50% lethal doses per ml) still occurred and were associated with the mycelial mat. The results of tests to find filterable or dialyzable growth factors were negative. It was demonstrated that for toxin production C. botulinum and the mold had to occupy the same environment. chosphoron sp., and Cladosporium sp. had grown and shifted the pH from <4.6 to >4.6. These are typical cases of metabiosis. This study was carried out to deternine whether C. botulinum growth and toxin production would occur in the presence of Aspergillus sp. in a specific microenvironment within a food system where the pH was less than 4.6. (The results are from a Ph.D. thesis submitted by Theron E. Odlaug to the faculty of the Graduate School of the University of Minnesota.) MATERLS AND METHODS C botulinum spores. Cultures of a C. botulinum type A strain, A16037, and a type B strain, B15580, implicated in outbreaks of botulism where homecanned tomato products were the toxin-carrying vehicle (3, 4), were obtained from the Center for Disease Control in Atlanta, Ga. Spore crops were prepared from these strains by using methods described by Odlaug and Pflug (13). Aspergillus gracilis spores. The strain of mold used in these studies was isolated from a l-quart (ca. 0.95-liter) jar of spoiled stewed tomatoes. The jar was obtained from I. D. Wolf (Department of Food Science and Nutrition, University of Minnesota). The mold was identified as A. gracilis by morphological characteristics, using culture techniques described by Thom and Raper (18). Conidiospore crops were prepared by using potato dextrose agar (Difco). The procedure was the same as that used by Buchanan et al. (1) for growing conidiospores ofAspergillus flavus. A 300-nil flask containing 75 ml of potato dextrose agar was inoculated from the A. gracilis stock culture and incubated at 22°C for 21 days. After incubation the spores were harvested by add-496

The heat destruction characteristics of Clostridium botulinum spores suspended in tomato juice and phosphate buffer were determined by the survivor curve method with aluminum thermal death time tubes. Two type A strains of C. botulinum and a type B strain were evaluated. Strains A16037 and B15580 were implicated in outbreaks of botulism involving home-canned tomato products. Strain A16037 had a higher heat resistance than either 62A or B15580. The mean thermal resistance (D-values) for A16037 in tomato juice (pH 4.2) were: 115.6°C, 0.4 min; 110.0°C, 1.6 min; and 104.40C, 6.0 min. The mean D-values for A16037 in Sorensen 0.067 M phosphate buffer (pH 7) were: 115.6°C, 1.3 min; 110.0°C, 4.4 min; and 104.4°C, 17.6 min. At each test temperature, the D-values were approximately three times higher in buffer than in tomato juice. The zvalue for C. botulinum A16037 spores in tomato juice was 9.4°C, and in buffer the z-value was 9.9°C. The use of aluminum thermal death time tubes in a miniature retort system makes it possible to determine survivor curves for C. botulinum spores at 121.1°C. This is possible because the lag correction factor for the aluminum tubes is only about 0.2 min, making possible heating times as short as 0.5 min. MATERIALS AND METHODS Spores. Cultures of a C. botulinum type A strain, A16037, and a type B strain, B15580, were obtained from the Center for Disease Control in Atlanta, Ga. These strains were implicated in outbreaks of botulism in which home-canned tomato products were the toxin-carrying vehicle (7, 8). The classical 62A strain was obtained from the American Type Cul-23

A variational finite element approach using triangular simplex and 2nd order quadrilateral elements was employed to analyze several problem areas that are of practical importance in the thermal processing of conduction heating products: (1) A method was developed to calculate conversion factors for thermal process design applicable to glass jars filled with conduction heating products. (2) The overshooting of temperatures after steam-off was studied, indicating that not including the contribution of overshooting to sterilization values in process design, can lead to gross overprocessing. (3) A method of correcting sterilization values to account for harmonious fluctuations in retort temperature was developed. (4) The air cooling of cans by natural convection was analyzed.