Materials and methods Directory UMM :Data Elmu:jurnal:P:Postharvest Biology and Technology:Vol18.Issue3.Apr2000:

tion to the total pressure as a consequence of Dalton’s Law. Thus, at 10.1 kPa absolute total pressure, the O 2 partial pressure is equivalent to 2.1 vv O 2 at atmospheric pressure, and the same is true for all other gases. The concept of hypobaric storage of fruit was introduced by Burg and Burg 1966. Since then it has been widely investigated Dilley, 1972, 1982; Spalding and Reeder, 1976a,b and reviewed Jamieson, 1980. Hypobaric storage extends the useful life of perishable commodities well beyond that achievable by CA storage at the equivalent O 2 levels. Apples that are preclimacteric at harvest remain preclimacteric during hypobaric storage at 10.1 kPa Dilley, 1972. Internal ethylene con- centration is decreased 10-fold which is well be- low the level required to induce ripening. When this is coupled to a reduction in the rate at which ethylene is produced at the reduced O 2 level equivalent to 2.1, an even further reduc- tion in ethylene concentration may exist within the tissue. Moreover, the concentration of CO 2 produced in fruit metabolism is reduced, and one can assume that the concentration of metabolic products such as ethanol, acetaldehyde and other compounds e.g. a-farnesene which have signifi- cant vapor pressures at fruit storage tempertures would likewise be reduced under hypobaric ven- tilation. a-Farnesene and its oxidation products have been implicated as factors in the superficial scald disorder Anet, 1972; Song and Beaudry, 1996; Wang and Dilley, 1997; Mir et al., 1998; Mir and Beaudry, 1999. Apple fruit do not de- velop scald when stored hypobarically Dilley, 1972, while fruit stored at the equivalent partial pressure of O 2 in CA storage can sometimes de- velop scald. The combined effects of lower pro- duction rates of oxidatively produced metabolites at reduced O 2 levels and enhanced removal of substances with significant vapor pressures under hypobaric storage conditions may at least par- tially explain prevention of scald. Hypobaric storage thus offers an experimental system to elucidate the nature of these substances. It was hypothesized that hypobaric storage controls scald by effectively decreasing the accumulation of scald-related volatile compounds in fruit that may affect fruit metabolism such as ethylene, a -farnesene and its oxidation product MHO. Hy- pobaric storage was investigated to determine ef- fective treatment regimens and to gain insight into the mechanism of scald control.

2. Materials and methods

2 . 1 . Plant materials ‘Law Rome’, ‘Granny Smith’, ‘Red Delicious’, ‘Mutsu’ and ‘Golden Delicious’ apples were har- vested at the preclimacteric stage of maturity from the Michigan State University Clarkesville Experiment Station CHES. 2 . 2 . Hypobaric studies in 1995 96 Apples were placed in two 1270-l vacuum ves- sels in a 1°C storage room. The vessels were maintained hypobarically at 5 kPa pressure of air with a ventilation rate of one vessel void volume change per hour at 97 RH Dilley, 1982. After 2 months of hypobaric storage, fruit were trans- ferred to CA at 1.5 O 2 plus 3 CO 2 , or stored in air at 1°C. After CA or air storage for 6 months, scald index was measured based on the percentage of fruit surface area affected, where no scald = 0, B 25 = 1, 25 – 50 = 2, and \ 50 = 3. The scald index was normalized to 100 by multiplying the values by 1003. Scald index was measured again after an additional 10 months of storage in air. 2 . 3 . Hypobaric studies 1996 97 Fruit were stored hypobarically, or in air or in CA at 1.5 or 3 O 2 with 0 or 3 CO 2 for 8 months at 1°C. The hypobaric fruit were then placed in air for an additional 4 or 10 months. Some fruit stored in CA for 2 months were transferred to air. Scald index was measured as before at various intervals of storage. a-Far- nesene and MHO levels in apples of five cultivars were measured as described below after 7 months of hypobaric storage. Table 1 Effect of pretreatment with hypobaric storage 5 kPa of air followed by controlled atmosphere CA 1.5 O 2 + 3 CO 2 or air storage on scald development in ‘Granny Smith’ and ‘Law Rome’ apples at 1°C in 1995 Scald index a Storage durations months Air ‘Granny Smith’ ‘Law Rome’ Hypobaric CA – 2 0c 6 0c 2 6 10 4c 9b 2 – 6 32b 30a 16 85a – – 2 a Scald index 0 = none; 1 = slight; 2 = moderate; 3 = severe was normalized to 100 by multiplying the values by 1003. Means within columns followed by different letters differ at P50.05. scribed by Wang and Dilley 2000. After a 90- min enclosure, the headspace of a glass jar with about 1.2 kg of fruit was sampled by solid phase microextraction SPME equilibrating the fibre for 4 min to absorb volatiles. The volatiles were measured by gas chromatographymass spec- trometry GCMS as described by Song et al. 1997 to determine volatile productionevolution. At the same time intervals, 14 mg of epicuticular wax sample was removed from five fruit in each treatment and placed in a 2-ml glass vial for 3 h at 20°C to determine the amount of a-farnesene and MHO partitioned in the epicuticular wax by SPMEGCMS as described above. Fruit in paral- lel samples were used for fruit firmness and ripen- ing changes immediately upon removal from storage and again after 7 days at 20°C data not presented.

3. Results