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- Author or Editor: Dennis P. Murr x
Abstract
Controlled atmosphere (CA) storage prolonged the shelf life of mushrooms (Agaricus bisporus, [Lange] Sing.) if the O2 concentration was 9% or the CO2 concentration was 25 or 50%. Concentrations of 2 to 10% O2 stimulated pileus expansion and stipe elongation with maximal stimulation of growth occurring at 5% O2. Levels of CO2 above 5% markedly inhibited growth, even after air was substituted for the CO2 treatment. Five percent CO2 stimulated stipe elongation and suppressed pileus expansion. Protein degradation, as indicated by protease activity and the level of a-amino N in the tissue, increased during postharvest maturation of mushrooms. As in starving bacteria, it is suggested that the main physiological function of proteolysis in the postharvest maturation of mushrooms is as a source of C and N.
Abstract
Mushrooms [Agaricus bisporus, (Lange) Sing.] stored at 10° and 20°C showed a sigmoid pattern of growth while at 0°C growth was retarded. The postharvest growth exhibited at 10° and 20°C could be related to a decrease in free a-amino N while at 0°C there was a significant increase in the level of free a-amino N during storage. Protease activity in the tissue increased at all 3 temperatures. It is suggested that postharvest maturation of mushrooms is supported by utilization of low molecular weight nitrogenous compounds formed through increased protein degradation. Mushrooms stored at 20°C toughened and matured faster than those stored at 10° or 0°C. Increases in discoloration during storage appeared to be correlated with decreases in total phenols and with increases in o-diphenol oxidase (o-DPO) activity. The relationship of these biochemical changes to postharvest maturation of mushrooms is discussed.
Abstract
Postharvest discoloration of cultivated mushrooms (Agaricus.bisporus [Lange] Sing., ‘tan strain’) was significantly retarded by treatment with succinic acid-2,2-dimethylhydrazide (SADH). The optimum SADH concn was 100 ppm. The effect, however, lasted no longer than 3 days after which time all SADH treatments discolored at rates equal to or greater than controls. The decrease in discoloration was correlated with a decrease in o-diphenol oxidase (o-DPO) activity. Protease activity was higher in SADH treated mushrooms suggesting that reduction in browning was due to degradation of o-DPO rather than direct inhibition of o-DPO by SADH. In vitro SADH competes with proline for quinones produced by enzymatic or non-enzymatic oxidation of diphenols. It is proposed that in vivo SADH exerts a dual effect in reducing mushroom discoloration: first SADH induces degradation of o-DPO through an increase in proteolytic activity, and second it binds to quinones thereby removing intermediates which lead to pigment formation.
Abstract
Storage of mushrooms (Agaricus bisporus, [Lange] Sing.) in 0% O2 reduced discoloration and o-diphenol oxidase (o-DPO) activity for up to 7 days. Levels of O2 above 0% had little or no effect in reducing discoloration and o–DPO activity compared to air controls. Concentrations of CO2 above 5% appeared to increase surface discoloration while markedly inhibiting o-DPO activity. After transfer to air, the effectiveness of the increased CO2 treatments in reducing o-DPO activity in mushrooms depended on storage time in CO2.
In vitro CO2 markedly inhibited o-DPO activity, with 50% inhibition at 25% CO2. Complete inhibition was never attained. The inhibition by CO2 was found to be competitive with respect to catechol and could not be overcome by increasing the O2 concentration above 20%. The action of CO2 in vivo in reducing o-DPO activity could be through a direct competitive inhibition or through the inhibitory effect of CO2 on mushroom maturation.
`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.
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.
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.
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.
Chlorophyll fluorescence, measured using a Plant Productivity Fluorometer Model SF-20 (Richard Brancker Research, Ottawa, Ont.), was evaluated as a rapid and nondestructive technique to detect low O2 and/or high CO2 stress in apples during storage. `Marshall' McIntosh apples were held for 5, 10, 15, 20, or 25 days at 3C in the following four treatments: standard O2 (2.5% to 3%) and low CO2 (<1%); low O2 (1% to 1.5%) and low CO2 (<1%); standard O2 (2.5% to 3%) and standard CO2 (4% to 4.5%); or 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, while 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. Both 1% to 1.5% O2 and 11% to 12% CO2 for 5 days caused chlorophyll fluorescence [Fv = (P – T)/P] of apple fruit to decrease, as compared to those held in standard atmospheres. Additional exposure time did not significantly affect Fv in either the low-O2 (1% to 1.5%) or high-CO2 (11% to 12%) treatment. The results of this study suggest that chlorophyll fluorescence can detect low-O2 and high-CO2 stress in apples, prior to the development of associated physiological disorders.
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.