Lorrie M. Friedrich's research while affiliated with University of Florida and other places

Five different methods were used to identify yeast isolates from a variety of citrus juice sources. A total of 99 strains, including reference strains, were identified using a partial sequence of the 26S rRNA gene, restriction pattern analysis of the internal transcribed spacer region (5.8S-ITS), classical methodology, the RapID Yeast Plus system, and API 20C AUX. Twenty-three different species were identified representing 11 different genera. Distribution of the species was considerably different depending on the type of sample. Fourteen different species were identified from pasteurized single-strength orange juice that had been contaminated after pasteurization (PSOJ), while only six species were isolated from fresh-squeezed, unpasteurized orange juice (FSOJ). Among PSOJ isolates, Candida intermedia and Candida parapsilosis were the predominant species. Hanseniaspora occidentalis and Hanseniaspora uvarum represented up to 73% of total FSOJ isolates. Partial sequence of the 26S rRNA gene yielded the best results in terms of correct identification, followed by classical techniques and 5.8S-ITS analysis. The commercial identification kits RapID Yeast Plus system and API 20C AUX were able to correctly identify only 35 and 13% of the isolates, respectively. Six new 5.8S-ITS profiles were described, corresponding to Clavispora lusitaniae, Geotrichum citri-aurantii, H. occidentalis, H. vineae, Pichia fermentans, and Saccharomycopsis crataegensis. With the addition of these new profiles to the existing database, the use of 5.8S-ITS sequence became the best tool for rapid and accurate identification of yeast isolates from orange juice.

Salmonella serovars gaminara, hartford, rubislaw and typhimurium were inoculated into pasteurized orange juice adjusted to pH 3.5, 3.8, 4.1 and 4.4 and incubated at 0 and 4C. Inoculated salmonellae (106 colony-forming units/mL) survived in detectable numbers up to 27 days at pH 3.5, 46 days at pH 3.8, 60 days at pH 4.1, and 73 days at pH 4.4. Lag times before initial cell populations began to decline were directly correlated with pH and ranged from < 1 day at pH 3.5 up to 27 days at pH 4.4. Death rates were inversely correlated with pH. Results suggest that salmonellae which might contaminate orange juice could survive sufficient time to cause illness.

Conventional heat pasteurization of orange juice sometimes results in an off-flavor due to overheating of the juice at the heat-exchange surface. Heating with microwave energy heats the juice uniformly without changing the taste. A domestic microwave oven was modified to serve as a continuous-fluid pasteurizer by the addition of a Teflon coil and an external pump. This was used to assess the heating characteristics of citrus juice and the effects on juice quality. Complete inactivation of bacteria and pectin methylesterase was obtained. There was no adverse effect on juice flavor. A continuous-flow unit using microwave energy will be further developed for use by small-scale processors.