Generally AA levels in various apple cultivars are lower under ultra low oxygen ULO condi-
tions than under air in cold storage Haffner et al., 1997. Levels slowly decrease both under CA
and during air storage in ‘Boskoop’ and ‘Golden Delicious’ apples Gerber, 1958; Bohling and
Hansen, 1985, and they slowly decline in pota- toes during storage by as much as 16 of the
original amount after 8 or 9 months Nobile and Woodhill, 1981.
In pears, a relationship was found between AA content and the susceptibility to browning during
experimental storage under various brown core- inducing conditions Veltman et al., 1999. ‘Con-
ference’ pears tend to develop tissue disorders, like brown core, when AA levels drop below a
certain value.
In this paper, we tested and expanded on these results. Pears were monitored in a static system,
and gas conditions and storage temperatures were comparable to those in commercial practice. AA
levels and the susceptibility towards browning in ‘Conference’ and ‘Rocha’ pears were monitored
during CA storage throughout the season. AA and browning values are discussed in relation to
storage conditions, harvest date and growing location.
2. Materials and methods
2
.
1
. Plant material ‘Conference’ pears Pyrus communis L. were
harvested grower
1 in
The Netherlands
province Gelderland on September 3, 10, 16, 24 and October 1, 1997, referred to as picks 1, 2, 3,
4 and 5, respectively. Pick 2 was the optimal harvest date for CA storage based on firmness of
the fruits. ‘Rocha’ pears were picked on August 2, 1997 in Portugal at two locations, Santos
grower 2 and Picarra grower 3.
2
.
2
. Storage and treatment ‘Conference’ pears were precooled at the stor-
age temperature − 1°C for 1 week before CA was applied. For bulk storage, ‘Conference’ pears
were stored in crates about 100 per crate in a static system. Eight crates were placed in a 650-l
container with a water lock, at − 1°C, 2 9 0.1 O
2
and B 0.7 9 0.1 CO
2
standard CA condi- tions for ‘Conference’ fruit or 3 9 0.1 CO
2
. Relative humidity in the containers was 97 – 99.
For short-term experiments Fig. 4, ‘Confer- ence’ pears were kept in 70-l tanks connected to a
flow-through system, at 10°C to maintain a rea- sonable rate of metabolism, and under a series of
O
2
concentrations 1, 2, 7 and 21 with or without 10 9 0.1 CO
2
. Relative humidity in the containers was 97 – 99. N
2
, O
2
and CO
2
were mixed by using Brooks 5850 TR series mass flow
controllers, the flow rate being 500 ml min
− 1
Peppelenbos and Jeksrud, 1998. Before the start of the flow-through experiment the pears pick 2
were stored for 7 months in the static system under standard CA conditions.
‘Rocha’ pears were transported in 3 days to The Netherlands by truck under cooled condi-
tions 2 – 3°C. Before transportation, pears were precooled at − 0.5°C. Altogether ‘Rocha’ pears
had a 1-month cooling period between harvest and application of CA. Bulk storage of ‘Rocha’
pears took place in the same static system as ‘Conference’ pears at − 0.5°C, under 1 9 0.1 or
3 9 0.1 O
2
combined with B 0.7 or 3 9 0.1 CO
2
. The standard CA condition for ‘Rocha’ pears was 3 O
2
and B 0.7 CO
2
. ‘Rocha’ pears were also stored under control conditions 21 O
2
and B 0.7 CO
2
in the same static system. For every harvest date ‘Conference’, grower
‘Rocha’, storage condition, and time both cvs two samples of five pears each were taken. From
each sample internal browning Section 2.3 and AA content Section 2.4 were determined. Deter-
minations for ‘Conference’ pears were performed at t = 0, after 100 days, and after 200 days.
‘Rocha’ pears were judged at t = 0, and after 60, 100, 150, 200, and 270 days after CA had been
applied.
2
.
3
. Browning index Five fruits were cut longitudinally and the area
of the fruit flesh that was affected by brown core was compared to the total area. Pears were di-
vided into four classes: no browning 0, slight browning I, B 30 of the area, moderate
browning II, about 30 – 70 of the area and severe browning III, \ 70 of the area, with
only the cortex fraction just underneath the peel not showing browning. The browning index is
the sum of the browning-score of the five pears divided by 15, and multiplied by 100, and I, II,
and III refer to the number of pears in the various browning classes. A browning index value of 0
means
no browning;
100 means
maximal browning.
Browning index = 100· I + 2II + 3III
3O + I + II + III
2
.
4
. AA measurements A
Waters liquid
chromatography system
HPLC was used: a pump, model 600E, with a Waters 486 tuneable absorbance detector 251
nm, and a Symmetry C-18, 3.9 × 150 mm column, particle size 5 mm, with a Sentry Guard
Column C-18 Waters. Measurements were per- formed at 25°C. Mobile phase: 2.5-g tetrabuty-
lammoniumhydrogensulfate z.s. Merck 818858 and 55 ml methanol p.a. Merck 6009 dissolved
in 942.5 g Milli-Q water Millipore Keijbets and Ebbenhorst-Seller, 1990. Before use, the eluent
was filtered and degassed using a 0.45 mm Mil- lipore filter HVLP 04700. The flow rate was kept
at 1 ml min
− 1
. Analyses were completed within 5.5 min, including a post-column elution time of
about 1 min. As a standard, 177 mM AA Sigma prepared
with Milli-Q water was used stock I. A total of 1-ml of stock I was diluted in 39 ml Milli-Q water
with 5 ml 9.5 wv oxalic acid Merck 100495 and 5 ml methanol vv stock II. A dilution series
from stock II was stored on ice and kept in the dark until injection. Stocks were prepared fresh
every day.
Fruits were peeled, and cut length-wise. With a corkborer diameter 17 mm samples were taken
from the cortex tissue just above the core tissue, at the stalk side, and just adjacent to the middle
vein. Samples of five pears were mixed, immedi- ately frozen in liquid nitrogen, and crushed in a
kitchen mixer. Ten grams of sample were diluted with 5 ml 9.5 wv oxalic acid, 5-ml methanol vv
and 30 ml Milli-Q water. The mixture was directly homogenised with an Ultra Turrax mixer and
filtered through fluted paper Schleicher Schull 59512. Filtration steps were performed at 5°C
in the dark. The filtrate was passed through a unit consisting of a 0.45 mm sterile filter and an acti-
vated Sep-Pak C18 cartridge Waters, and was directly injected in a manual-injector system with
a 20 ml loop. HPLC measurements were per- formed directly after the extraction procedure.
Results were analysed by means of the Millen- nium HPLC manager Waters.
