Metabolite production of yeasts on a strawberry-agar during storage at 7 degrees C in air and low oxygen atmosphere.
Changes of different quality factors of strawberries have been described until now from a physiological point of view. Possible effects on quality caused by yeast proliferation have not been described. To elucidate the metabolic activity of yeasts (i.e. Debaryomyces melissophilus, Rhodotorula glutinis, Cryptococcus laurentii), isolated from strawberries, they were inoculated on a simulation medium of strawberries (strawberry-agar). Their activity was measured by analysing and correlating microbiological counts, metabolite concentration in the headspace as well as in the medium, and sugar consumption. The isolated yeasts from strawberries could grow on the strawberry-agar, both under air conditions and modified atmosphere (MA) conditions. The maximum count for the yeasts reached 7.5-8.5 log cfu cm(-2) (air conditions) and 5.6-6.4 log cfu cm(-2) (MA conditions). Production or consumption of a number of compounds could be detected when microbial counts reached levels between 4.7 log cfu cm(-2) and 8.5 log cfu cm(-2) depending on species and atmospheric conditions. A range of volatile organic compounds, produced by the yeasts, was detected: acetone, ethyl acetate, ethanol, isopropyl acetate, ethyl butyrate, 1-propanol, 2-methyl-1-propanol, 1-butanol, 2-methyl-1-butanol, 3-methyl-1-butanol, 1-hexanol and hexyl acetate. These compounds are able to influence the sensory properties of strawberries. A simultaneous decrease in sugar concentrations (sucrose, glucose, fructose) was observed. When ethanol reached high concentrations, ethyl acetate and ethyl butyrate were produced. This production can be attributed to a detoxification of ethanol by yeasts. The fermentative metabolism of yeasts during aerobic conditions could be explained by the Crabtree effect. As the detected volatile organic compounds produced by yeasts are also found in fresh strawberries, it can be concluded that these compounds are produced both by microbiological and physiological processes.
Ragaert
P
P
Department of Food Safety and Food Quality, Laboratory of Food Microbiology and Food Preservation, Ghent University, Coupure links 653, B-9000 Ghent, Belgium.
Devlieghere
F
F
Loos
S
S
Dewulf
J
J
Van Langenhove
H
H
Debevere
J
J
D015169:Colony Count, Microbial; D003470:Culture Media / Q000737:chemistry; D005506:Food Contamination / Q000032:analysis*; D005511:Food Handling / Q000379:methods*; D005516:Food Microbiology; D005519:Food Preservation / Q000379:methods*; D031985:Fragaria / Q000382:microbiology*; D008954:Models, Biological; D010100:Oxygen / Q000378:metabolism*; D013649:Taste; D013696:Temperature; D013997:Time Factors; D014835:Volatilization; D015003:Yeasts / Q000254:growth & development / Q000378:metabolism*
D003470:Culture Media; D010100:Oxygen