Sequential decreases or increases in the levels of O2 in controlled atmosphere (CA) were investigated as techniques to improve fruit quality of `McIntosh' apples (Malus ×sylvestris [L.] Mill. var. domestica [Borkh.] Mansf.), a cultivar that tends to soften rapidly in storage. Precooled fruit that were harvested at optimum maturity for long-term storage were placed immediately in different programmed CA regimes. In the first year, CA programs consisted of 1) `standard' CA (SCA; 2.5–3.0% O2 + 2.5% CO2 for the first 30 d, 4.5% CO2 thereafter) at 3 °C for 180 d; 2) low CO2 SCA (2.5–3.0% O2 + 2.5% CO2) at 3 °C for 60 d, transferred to low O2 (LO; 1.5% O2 + 1.5% CO2) at 0 or 3 °C for 60 d, and then to ultralow O2 (ULO; 0.7% O2 + 1.0% CO2) at 0 or 3 °C for 60 d; and 3) ULO at 3 °C for 60 d, transferred to LO at 0 or 3 °C for 60 d, and then to SCA or low CO2 SCA at 0 or 3 °C for 60 d. In the second year, the regimes sequentially decreasing in O2 were compared with continuous ULO and SCA. After removal from storage, apples were held in ambient air at 20 °C for a 1-week ripening period. Fruit firmness was evaluated after 1 and 7 d at 20 °C, whereas the incidence of physiological disorders was assessed only after 7 d. Lowering the temperature while decreasing O2 was the best CA program with significant increased firmness retention during storage and after the 1-week ripening period. Reduced incidence of low O2 injury in decreasing O2 programs and absence of core browning at the lower temperature were also observed.
P. Guy Lévesque, Jennifer R. DeEll and Dennis P. Murr
Jennifer R. DeEll, H.P. Vasantha Rupasinghe and Dennis P. Murr
`Cortland' is an apple cultivar with inherent poor storeability because of excessive vulnerability to the development of superficial scald in long-term storage. The objectives of this investigation were to evaluate the potential of the potent ethylene action inhibitor 1-methylcyclopropene (1-MCP; EthylBloc®) to counteract this constraint and to develop some basic procedures for its exposure. Eight hours after harvest, fruit were exposed to 1.0 mL·L–1 1-MCP for 0, 3, 6, 9, 12, 16, 24, or 48 h at 3, 13, or 23 °C. Following exposure, fruit were placed at 0 to 1 °C in air for 120 days, after which time they were removed to 20 °C and held 7 days for post-storage assessment of ripening and to allow development of physiological disorders. In general, and within our experimental limits, the higher the temperature of 1-MCP exposure the shorter the required exposure time to obtain similar effects. The desired effectiveness of 1-MCP could be achieved by exposing fruit for at least 3 h at 23 °C, for 6 h at 13 °C, or for 9 h at 3 °C. 1-MCP-treated apples were consistently 2 kg firmer than untreated apples. Scald incidence in untreated fruit after 120 days at 0 to 1 °C and 7 days at 20 °C was 100%, whereas 1-MCP reduced scald by 95% in treatments of long enough duration at any particular temperature.
H.P. Vasantha Rupasinghe, Gopinadhan Paliyath and Dennis P. Murr
To decipher the relation between α-farnesene metabolism and the development of superficial scald in apples, trans,trans-α-farnesene synthase, the enzyme that catalyzes the conversion of farnesyl pyrophosphate to α-farnesene, was partially purified from skin tissue of `Delicious' apples (Malus ×domestica Borkh.) and characterized. Total and specific activities of the enzyme were higher in the cytosolic fraction than in membrane fractions. α-Farnesene synthase was purified 70-fold from the cytosolic fraction by ion exchange chromatography and gel permeation, and the native molecular weight was estimated to be 108,000. The enzyme had optimal activity at a pH of 5.6 and absolutely required a divalent metal ion such as Mg2+ or Mn2+ for activity. It exhibited allosteric kinetics, S(0.5) for farnesyl pyrophosphate being 84±18 μmol·L-1, and a Hill coefficient (nH) of 2.9, indicating the number of subunits to be two or three. Enzyme activity was highest between 10 and 20 °C, while 50% of the maximal activity was retained at 0 °C. In vivo α-farnesene synthase activity was minimal at harvest, then increased rapidly during 16 weeks storage in air at 0 °C, and decreased during further storage. Activity of α-farnesene synthase, α-farnesene content, and conjugated triene alcohol (the putative scald-causing oxidation product of α-farnesene) content in skin tissue were not correlated to the inherent nature of scald susceptibility or resistance in 11 apple cultivars tested.
Dan D. MacLean*, Dennis P. Murr and Jennifer R. DeEll
Glutathione S-transferase (GST) is a ubiquitous and constitutive enzyme that is involved in numerous cellular activities including the amelioration of oxidative stresses caused by the presence of xenobiotics and reactive oxygen species. In the present study, a glutathione S-transferase was extracted, purified, and partially characterized from two types of pome fruits. Pear (Pyrus × communis L., cultivar D'Anjou) and apple (Malus × domestica Borkh., cultivar Delicious) fruit were tested. The glutathione S-transferase was extracted using traditional methods, and purified using a combination of ammonium sulfate precipitation, dialysis, and GST-specific affinity chromatography. The GST enzyme was subsequently eluted from the column and concentrated prior to characterization studies. A purified fraction from the column was loaded onto an SDS-PAGE gel, and resulted in a single band with an apparent molecular weight of ≈26 kDa. This band was excised and used for MALDI-TOF/MS peptide mass fingerprint studies, and also served to confirm the apparent mass of the protein (25969 Da). The ExPASy software was used for the peptide mass fingerprint study, where the digest of the GST using trypsin was compared to a theoretical digest of an Arabidopsis GST, and resulted in two peptides of significant mass homology. The purified GST was also tested for enzyme activity using the standard assay substrate of 1-chloro-2,4-dinitrobenzene and reduced glutathione. Total GST protein extracted from `D'Anjou' pear was 0.532 mg·mL-1, while `Delicious' apple contained 0.127 mg·mL-1. The activity of GST enzymes may play a role in minimizing oxidative stress injury in stored pear and apple tissues.
Jennifer R. DeEll, Jennifer T. Ayres and Dennis P. Murr
This study evaluated the effects of 1-methylcyclopropene (1-MCP) on ‘Empire’ and ‘Delicious’ apples (Malus ×domestica) in commercial controlled atmosphere (CA) storage for 12 months and in commercial cold storage for 6 months. Apples were harvested and delivered by growers to a local commercial storage facility. Four different grower lots were chosen for each of three ‘Empire’ and two ‘Delicious’ storage rooms. Fruit were treated with 1-MCP (≈0.8–1.0 ppm) for 24 hours, while control fruit samples were held in a similar nearby storage room. After treatment, control samples were placed with matching 1-MCP-treated samples into either CA (2.5% O2 + 2.5% CO2 at 2.2 °C or 0 °C for ‘Empire’ and ‘Delicious’, respectively) or air storage at 0 to 1 °C. Initial maturity was relatively uniform among the grower lots, with internal ethylene concentration (IEC) averaging less than 1 ppm for ‘Empire’ and 2 to 3 ppm for ‘Delicious’. IEC was lower in apples treated with 1-MCP after air (3 or 6 months) or CA (6, 9, or 12 months) storage, but this effect was reduced after a 14-day ripening period at 22 °C, and was less dramatic in fruit from CA than from air storage. Apples treated with 1-MCP were also firmer than non-treated fruit upon removal from air or CA storage, and this difference became greater with increased poststorage time at 22 °C. 1-MCP-treated apples stored in air had higher soluble solids concentration (SSC), while there was no significant effect of 1-MCP on SSC in fruit held in CA. Core browning developed in ‘Empire’ held in air for 6 months or in CA for 9 or 12 months, and in ‘Delicious’ after 9 or 12 months in CA. 1-MCP decreased the incidence of core browning in ‘Empire’, but increased the incidence in ‘Delicious’. There was no significant effect of 1-MCP on the incidence of internal browning and storage rots, which developed in both cultivars.
Jennifer R. DeEll, Jennifer T. Ayres and Dennis P. Murr
This study evaluated the effects of 1-methylcyclopropene (1-MCP) concentration (1000 vs. 625 ppb) and treatment delays (3, 7, and 10 days after harvest) on the ripening and incidence of storage disorders in ‘McIntosh’ apples from three harvest times in 2004. Apples were stored in air at 0 °C to 1 °C for 3 and 6 months or in controlled atmosphere (CA) storage at 3 °C for 6 and 9 months. Apples treated with 1-MCP and held in air or CA storage were firmer than those not treated, but this difference in firmness was less with later harvests, more delay before 1-MCP treatment, and longer storage time. Apples treated with 1000 ppb 1-MCP were often firmer than those treated with 625 ppb after 6 months of storage and/or 7 days at 22 °C. Ethylene and carbon dioxide (CO2) production were reduced in apples treated with 1-MCP, especially in fruit from the first harvest and those treated 3 days after harvest. Fruit treated with 1000 ppb 1-MCP showed a slower increase in ethylene production than those treated with 625 ppb during 14 days at 22 °C after storage. CO2 production was the lowest in ‘McIntosh’ apples treated with 1000 ppb 1-MCP 3 days after harvest, but fruit treated with 625 ppb also exhibited lower respiration than those not treated. Storage disorders were most prevalent in ‘McIntosh’ apples stored for 6 months in air at 0 °C to 1 °C, whereas fruit from the first harvest treated with 1-MCP 3 days after harvest developed the fewest disorders. 1-MCP reduced the incidence of superficial scald, flesh browning, core browning, and senescent breakdown, while 1-MCP concentration and treatment delay had varying effects. This research has provided the basis for Canadian registration of SmartFreshSM use on apples at 1000 ppb 1-MCP and for the requirement that treatment be within 3 days of harvest.
Renae E. Moran, Jennifer R. DeEll and Dennis P. Murr
Preconditioning, holding fruit at 10, 17.5, or 21 °C temperatures for up to 7 days before placement in cold storage, was inconsistent in its effect on soft scald and soggy breakdown in ‘Honeycrisp’ apples in Maine and Ontario. In Ontario, 4 days of preconditioning at 21 °C increased soft scald in 1 year but had no effect in the next year. Five d of preconditioning at 10 °C reduced soft scald and had no effect on soggy breakdown in 1 year but reduced it the next year. In Maine, 5 days preconditioning at 17.5 °C was effective in reducing soft scald and/or soggy breakdown in 2002 to 2007 when starch index at harvest was 5.9 to 7.2. Seven days of preconditioning at 17.5 °C increased soggy breakdown with an early harvest in two orchards but only in one of two orchards with a later harvest. This same preconditioning had no effect on soft scald with the first harvest but reduced it with the second. In the next year, the same preconditioning treatment increased soft scald and soggy breakdown with an early maturity but had no effect with a later maturity in one orchard but not in fruit from another. Conditions during preconditioning and subsequent cold storage temperatures varied from previous recommendations, and this may be why preconditioning was not consistent in our studies and in some cases increased chilling disorders.
Jennifer R. DeEll, Robert K. Prange and Dennis P. Murr
Chlorophyll fluorescence was evaluated as a rapid and nondestructive technique to detect low-O2 or high-CO2 stress in apples (Malus domestica Borkh.) during storage. `Marshall' McIntosh apples were held for 5, 10, 15, 20, or 25 days at 3C in 1) standard O2 (2.5% to 3%) and low CO2 (<1%), 2) low O2 (1% to 1.5%) and low CO2 (<1%), 3) standard O2 (2.5% to 3%) and standard CO2 (4% to 4.5%), or 4) standard O2 (2.5% to 3%) and high CO2 (11% to 12%). Only 10% of the apples had skin discoloration after 5 days in 1% to 1.5% O2; 80% developed skin discoloration after 20 days in low O2. Small desiccated cavities in the cortex, associated with CO2 injury, developed in 10% of the apples after 20 days in 11% to 12% CO2. Five days in 1% to 1.5% O2 or 11% to 12% CO2 caused variable fluorescence (Fv) of apple fruit to decrease compared to those held in standard atmospheres. Additional exposure did not significantly affect Fv in either the low-O2 (1% to 1.5%) or high-CO2 (11% to 12%) treatment. Our results suggest that chlorophyll fluorescence techniques can detect low-O2 and high-CO2 stress in apples before the development of associated disorders.
R. Andrew Schofield, Jennifer R. DeEll and Dennis P. Murr
Traditional hand compression firmness scores of iceberg lettuce (Lactuca sativa L.) heads were compared with force-deformation data collected from parallel-plate compression tests conducted with a universal testing machine. Sample deformation was measured over a load range of 30 to 40 N. A quadratic response surface best described the relationship between hand firmness scores (1 to 5 scale) and three measurements of sample deformation (mm). Sample deformation was as precise as hand compression in measuring lettuce firmness, and it provided improved reproducibility by eliminating much of the human error. Although adequate for most firm heads, the predictive ability of the statistical model was weak for soft heads (when the hand firmness score was <2), and for heads with inconsistencies in firmness because of uneven leaf distribution. The minimum sample size required to determine accurately the mean firmness score (±0.5 units) of a population of harvested lettuce was ≈20 heads. This may be a disadvantage, since sampling one head requires ≈1.5 minutes. Overall, the instrument-based method measures lettuce firmness as precisely as the hand compression method, and provides a standardized, objective measurement for postharvest researchers when exchanging or reporting firmness data.
R. Andrew Schofield, Jennifer R. DeEll and Dennis P. Murr
Chlorophyll fluorescence responds to a range of environmental stresses that affect horticultural crops. This technique has been used successfully to evaluate the quality of commodities after exposure to a number of postharvest stresses such as chilling, heat, and atmospheric stress. As well, chlorophyll fluorescence measurements have been incorporated as the main characteristics in shelf-life prediction models. Our objective was to evaluate the use of chlorophyll fluorescence measurements at harvest to predict the shelf-life of `Iceberg' lettuce. It was hypothesized that storage potential is influenced by the degree of stress induced by field conditions and that different cultivars, although grown under the same conditions, experience varying degrees of stress that can be detected by fluorescence measurements at harvest, even in the absence of visual differences in quality. The utility of fluorescence measurements was limited by inconsistencies in the development of the heads, such as maturity and leaf formation, and by variation among different areas of the same leaf. Fluorescence data from a homogeneous group of heads revealed that the variation associated with different areas of the same leaf was larger than that associated with measurements from different heads. Also, fluorescence readings from one leaf differed from those taken from any non-adjacent leaves. These sources of variation, along with strong cultivar-dependant differences in the fluorescence signal, were quite large, and hence, any trends in fluorescence measurements related to storage potential were not observed. Therefore, chlorophyll fluorescence at harvest does not appear to be a good predictor of lettuce storability.