Hypobaric treatment : an innovative approach to control postharvest diseases of strawberries : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, Palmerston North, New Zealand
Fungal decay is one of the major causes of postharvest losses in strawberries. Conventionally fungicides have been used to control fungal decay. However, fungicides have been questioned as a sustainable and safe method. Pre-storage application of short-term hypobaric treatment is one of the potential non-chemical methods for reducing fungal decay. In this study efficacy of postharvest hypobaric treatments to control natural rot development in strawberries was evaluated and analysed for the possible induction of resistance. Treatment of strawberries with hypobaric pressure (50 kPaa for 4 h at 20 °C) consistently reduced rot development in subsequent storage at 20 or 5 °C. An in vitro study found that the rate of radial growth of colonies of Botrytis cinerea and Rhizopus stolonifer was unaffected by hypobaric treatment. Moreover, post-hypobaric treated strawberries inoculated immediately or 12 h after treatment with corresponding fungi led to significant (P< 0.05) rot reduction during storage at 20 °C. Bio-chemical analysis of strawberries suggested that hypobaric treatment upregulated the activities of defence-related enzymes with phenylalanine ammonia-lyase (PAL) and chitinase peaking 12 h after treatment, while peroxidase (POD) increased immediately after treatment. These results provide evidence that the mechanism of action of hypobaric treatment is induction of the defence system within the fruit rather than a direct effect on fungal viability. Furthermore, the 4 h delay in cooling required for hypobaric treatment did not cause greater rots compared to immediate cooling at-harvest. Therefore treatment is recommended to be applied at low temperature immediately after harvest. Despite these scientific results, a combination of hypobaric treatment and cold storage may not be sufficient for extending commercial storage life of strawberries. For more effective application, probably hypobaric treatment should be regarded as a hurdle technology to be used in conjunction with other physical treatments such as (UV-C, heat and ultrasound) and/or volatile chemical treatments.