surfaces. These defects are similar to those re- ported for other minimally processed vegetables
Ahvenainen, 1996. For fresh-cut green onions there are important
additional defects. Due to lack of precision in the cutting process and frequent complete removal of
the compressed stem, growth or extension of the white inner leaf bases may occur. This is also
referred to as ‘telescoping’, and can cause a rapid loss in the overall market quality of the product.
Another defect particular to green onions is leaf curvature due to negative geotropism, which oc-
curs when the product is placed horizontally.
Hruschka 1974 reported that a CA of 1 O
2
+ 5 CO
2
extended the shelf life of intact green onions at 0°C. The atmosphere reduced
yellowing but did not affect curvature or decay incidence. Currently, commercial green onions
may be packaged in films resulting in B 1 O
2
and 8 – 10 CO
2
atmospheres at 5°C Cantwell, unpublished. Heat treatments have been used to
reduce growth phenomena, including sprout de- velopment in potatoes Ranganna et al., 1998
and geotropic curvature in asparagus Paull and Chen, 1999.
This study was undertaken to determine the potential benefits of CAs and heat treatment to
maintain the quality of intact and minimally pro- cessed onions. For the minimally processed
onions, a treatment to effectively control exten- sion growth or ‘telescoping’ is required.
2. Materials and methods
2
.
1
. Plant material Green onions Allium cepa × A. fistulosum
were produced in the Salinas Valley of California under usual production practices Voss and May-
berry, 1999, harvested by pulling the plants within 2 h of undercutting the beds, and brought
to the laboratory by placing in coolers in a water- ice slurry. At the laboratory they were trimmed
leaf tips and roots cut and washed with clean chlorinated water 50 ppm NaOCl, pH 7.0 before
use. In some experiments, commercially prepared onions were harvested into plastic crates in Mexi-
cali, Baja California, shipped overnight in refrig- erated trucks to Salinas, trimmed, washed with
cold chlorinated water and packed in plastic film bags in 0.908 kg units. The bags were placed in
polystyrene-lined carton boxes with gel ice and transported overnight to the laboratory where
they were kept at 0°C until used.
2
.
2
. Pre-storage treatments Onions were prepared in three ways before
storage; 1 intact onions were trimmed and had the roots cut off, but the compressed stem was
intact; 2 cut onions had all roots and 5 mm of the stem plate removed by a razor blade; and 3
heat-treated cut onions had the lower 5 cm of the white stems dipped into 55°C water for 2 min
which was followed by the same cutting treatment received by the second group. We have recently
shown that this is one of several heat treatments effective enough to control ‘telescoping’ of cut
onions Cantwell and Hong, unpublished.
2
.
3
. Storage conditions Two CA experiments were conducted. For the
first CA experiment, freshly harvested trimmed onions with intact stem plates pre-storage treat-
ment group 1 ten onions per replication were placed in glass containers in flow systems with air,
air + 9 CO
2
, 0.1 O
2
, or 0.1 O
2
+ 9 CO
2
for up to 21 days at 0, 5, and 10°C. The atmospheres
were selected based on the analyses of commer- cially packaged green onions. The atmospheres
were prepared as humidified mixtures from gases in cylinders and then distributed to the individual
storage containers by manifolds. Atmosphere compositions were monitored periodically by an
electrochemical oxygen analyzer and an infrared CO
2
gas analyzer, and maintained within 9 5 of the indicated concentrations.
In the second CA experiment, green onions from the three pre-storage treatment groups were
stored up to 21 days at 5°C in glass containers through which the following humidified atmo-
spheres flowed; air, 2 O
2
, and 0.2 O
2
, alone or in combination with 7.5 CO
2
, and air + 15 CO
2
. Product quality was evaluated periodically during storage.
2
.
4
. Quality e6aluations Overall visual quality was scored on a 9 – 1
scale, with reference points of 9, excellent; 7, good; 5, fair; 3, poor; and 1, unusable. A score of
6 was regarded as the limit of marketability. The visual quality assessment included discoloration
and curvature defects but did not include exten- sion or ‘telescoping’ defects in the minimally pro-
cessed onions. Leaf discoloration was scored separately on a 1 – 5 scale, where 1, none; 2, slight;
3, moderate; 4, moderately severe; and 5, severe. Discoloration or loss of fresh green color was
evaluated on the oldest leaf. For curvature, a score of 1 – 5 was used, where 1, none; 2, curva-
ture of stem or leaf up to 15° from the horizontal; 3, 15 – 30°, 4, 30 – 45° and 5, \ 45° from the
horizontal. Inner leaf extension ‘telescoping’ was measured with a vernier caliper as the length to
the nearest 0.1 mm from the cut surface of the white leaf base to the end of the most extended
portion. Root re-growth was measured with a vernier caliper in intact green onions previously
trimmed of roots.
2
.
5
. Compositional changes For analysis of total soluble sugars in the white
stem bases including any extension growth, 5 g of chopped tissue was homogenized with 195 ml
95 ethanol for 2 min. After centrifugation at 3000 rpm for 5 min to precipitate ethanol-insolu-
ble material, 1 ml of the supernatant was diluted 20 times with distilled water. For the assay
Dubois et al., 1956 the reaction mixture con- sisted of 1 ml diluted extract, 0.1 ml of 80
phenol solution and 5 ml concentrated sulfuric acid. Tubes were vortex agitated, let to stand for
10 min, remixed and then held in a water bath at 30°C for 20 min. Absorbance was measured on a
Perkin – Elmer UV – Vis spectrophotometer at a wavelength of 490 nm, and calculations were
based on a glucose standard curve.
Thiosulfinate concentrations in white stem por- tions of the green onions were also measured
spectrophotometrically Riddles et al., 1979; Han et al., 1995. Samples 5 g were homogenized in
10 ml distilled water. After centrifugation at 3000 rpm for 5 min, 375 ml of supernatant was pipetted
into a test tube on ice. An aliquot of 375 ml, distilled water was added to another test tube for
reference. Aliquots of 625 ml 0.8 mM cysteine solution were added to both the sample and refer-
ence test tubes. After shaking, they were held for 10 min at room temperature. Other test tubes at
ambient temperature contained 0.8 ml of 200 mM DTNB 5,5-dithio-bis-2-nitrobenzoic acid. The
200 mM DTNB solution was prepared with 50mM HEPES buffer, pH 7.5. An aliquot of 200 ml of
samplecysteine solution or watercysteine solu- tion was added to the DTNB test tubes. For
blank, only 200 ml of pure water was added to the DTNB tube. After shaking, test tubes stood for
10 min for color development. Absorbance was measured at 412 nm, and thiosulfinate concentra-
tions were calculated according to Han et al. 1995.
2
.
6
. Experimental design and statistical analysis Experiments were conducted as completely ran-
domized designs with three replicates of eight to ten green onions each per treatment. Data were
analyzed as averages 9 S.D. or by two-way analy- sis of variance ANOVA with mean separation
by Duncan’s multiple range test.
3. Results
