Glycerol production by four common grape moulds, Aspergillus, Botrytis, Penicillium and Rhizopus : a thesis presented in fulfilment of the requirements for Master of Science in Microbiology at Massey University

Abstract

The production of glycerol by the grape moulds Aspergillus niger, Penicillium italicum, Rhizopus nigricans and Botrytis cinerea growing in juice from Chasselas and Black Hamburg grapes was examined. Juice from both free-run and homogenized whole grapes was filter sterilized and inoculated with single pure cultures of the moulds above. The four juice types were incubated at 25° C for 26 to 29 days. The inoculated juices were incubated in different air relations and during the 26 to 29 day incubation period, samples were taken periodically for the analysis of glycerol, glucose and fructose by HPLC. After 26 to 29 days, the moulds were harvested by filtration so that dry mycelial weights could be obtained.

Large differences in glycerol production were noted among the grape moulds. Under similar conditions of cultivation in Chasselas juice, R. nigricans and B. cinerea produced significantly more glycerol than A. niger and P. italicum. The levels of glycerol never exceeded 0.5g/100mL, whereas all cultures of B. nigricans and li. cinerea exceeded this level after 15 to 18 days of incubation. In Black Hamburg juice glycerol was not detected in cultures of A. niger and P. italicum. The levels of glycerol produced by all the four moulds were lower in Black Hamburg than in Chasselas juice.

Overall more sugar was utilized in Black Hamburg juice than in Chasselas juice under similar conditions. B. cinerea utilized the most total sugar in Chasselas juice than all the other moulds, while R- nigricans utilized the most total sugar in Black Hamburg juice than all the other moulds. In Chasselas juice B. cinerea and R- nigricans displayed a preference for glucose over fructose, while in Black Hamburg juice no preference was evident. The pattern of sugar utilization over the incubation period between Chasselas and Black Hamburg juice was markedly different. In Chasselas juice under most cultivation conditions the four moulds utilized glucose and fructose throughout the incubation period, while in Black Hamburg juice there was rapid utilization during the first three days followed by a reduced rate of sugar utilization in the latter stages of incubation.

The four moulds differed in their production of mycelial dry weight. These differences were most marked in Chasselas juice where ­B. cinerea, depending on air relations, produced five to seven times more mycelial dry weight than R. nigricans and more than twice the mycelial dry weight produced by A. niger and P. italicum. In Black Hamburg juice B. cinerea produced two to three times more mycelial mass than the other three moulds.

At present in the Californian wine industry an HPLC method is currently under investigation, where the level of glycerol in the grape juice is used as an indicator of fungal rot of the grapes. This study has demonstrated that certain grape moulds do not produce the same amount of glycerol and that the level of glycerol is not related to the mycelial growth. Thus this investigation has established that glycerol may not be used as a suitable indicator of fungal rot.