Errol W. Hewett

Factors affecting apple aroma/flavour volatile concentration: A Review

Abstract Typical apple (Malus domestica Borkh.) flavour develops during ripening. Maximum endogenous volatile concentration occurs at the climacteric peak but it is not known whether the volatile biosynthetic enzymes are constitutive or induced during the climacteric. Exposing apples to hypoxic conditions induces accumulation of high concentrations of acetaldehyde and ethanol; after return to air ethyl esters are enhanced and non‐ethyl esters decrease. There are differences in degree of ethyl ester enhancement among cultivars. These may be because of: differential activity or synthesis of alcohol acyl CoA transferase (AAT) or alcohol dehydrogenase (ADH); separate iso‐forms of AAT and ADH each with their own substrate specificity; variation in alcohol precursors in different cultivars; or a combination of all three. Volatile production is greater at higher temperatures in the range from 0 to 30°C but exposure to low temperatures (<3°C) for more than 3 months decreases production. Temperature influences the production of specific volatiles with some compounds only being produced at certain temperatures. It is not known how temperature will affect volatile production after exposure to hypoxia. It is suggested that the enhanced volatile production that occurs in apples following an hypoxic treatment might overcome or reverse the decreases that are induced by low temperatures and controlled atmosphere (CA) storage. The use of hypoxia to enhance volatile concentrations may be a beneficial side effect when such treatments are used for disinfes‐tation purposes. It is possible that given equal efficacy, hypoxia could be either preferred or used as an adjunct to heat treatments to eradicate insects. In addition hypoxic treatment of fresh fruit could induce significant increases in volatile concentrations that could be used in production of high quality essences from apple juice.

POSTHARVEST SOFTENING OF APPLE (MALUS DOMESTICA) FRUIT: A REVIEW

Abstract Postharvest softening of apple (Malus domestica (Borkh.)) fruit is a serious problem for growers in many countries, including New Zealand. To reduce this problem considerable research has been undertaken to determine the biological causes of softening so that this process can be managed or controlled more effectively. This review describes the pattern of softening for harvested apple fruit, and how it is influenced by different preharvest, at‐harvest, and postharvest factors. Information is also given on the likely physiological and biochemical causes of apple softening, such as fruit anatomy and cell packing, modification of the cell wall and membranes, changes in cell turgor, and the role of ethylene and other growth regulators. Despite many softening studies, there is still a poor understanding of what causes firmness variation in the marketplace. Until this understanding is improved, apple producers will continue to struggle to meet market requirements for texture.

Changes in physicochemical attributes of sweet pepper cv. Domino during fruit growth and development

Abstract The effect of harvesting sweet pepper ‘Domino’ fruit at different stages of growth and development on physicochemical attributes was studied. The suitability of fruit fresh weight, diameter and length, weeks from anthesis, total soluble solids (TSS), fruit colour change, firmness, internal CO2 and C2H4 concentrations as well as fruit respiration and ethylene production as maturity indicators were evaluated. Fruits were harvested weekly until fully red ripe. Fruits took 8 weeks after anthesis (WAA) to reach harvest maturity and a further 2–3 weeks to reach full ripeness (11 WAA). Fruits exhibited a sigmoid growth pattern. Hue angle (change in colour from green to red) declined with time while chroma (colour intensity) values increased with fruit maturity, as did TSS. An increase in PiCO2 and PiC2H4 accompanied the decrease in hue angle. The association between fruit ripening and the significant increase in PiC2H4 indicates that ethylene may be responsible for ripening of this cultivar of sweet pepper. Colour change and TSS were reasonable indicators of maturity of sweet pepper fruit complemented with fruit firmness.

Physical change in apple texture with fruit temperature: effects of cultivar and time in storage

Abstract Flesh firmness is used to assess apple (Malus domestica Borkh.) quality both before and after low temperature storage. The effect of fruit temperature on apple firmness at different times during postharvest handling is not known. Experiments were conducted to quantify physical change in apple texture readings with change in fruit temperature. ‘Royal Gala’, ‘Granny Smith’, and ‘Pacific Rose™’ apple fruit were stored at 0°C, while ‘Coxs Orange Pippin’ was stored at 3°C. At different times during storage, flesh firmness and cortical tensile strength were measured on fruit at storage temperature, after 24 h at 20°C, or after 24 h at 20°C followed by 24 h at the storage temperature. ‘Royal Gala’, ‘Granny Smith’ and ‘Coxs Orange Pippin’ fruit had higher firmness readings at harvest when measured at 20°C than at 0–3°C, but after 50–100 days at 0–3°C firmness and tensile strength readings were greater when measured at 0–3°C than at 20°C. ‘Pacific Rose™’ had similar firmness and tensile strength readings when measured at 0°C and 20°C. ‘Royal Gala’ and ‘Coxs Orange Pippin’ were measured for firmness at different fruit temperatures at harvest and after storage. The relationship between firmness readings and fruit temperature between 0 and 20°C was linear and positive at harvest and linear and negative for stored fruit. Firmness change with temperature was not affected by orchard or harvest maturity. These results suggest that physical changes in firmness with fruit temperature are common for the cultivars studied, and thus could be used to compare firmness values for fruit from different orchards that were measured at different temperatures.

Ethylene production by Botrytis cinerea

Abstract Ethylene was produced when isolates of the postharvest pathogen Botrytis cinerea Pers.: Fr., derived from fruit of strawberry, blueberry and kiwifruit and leaves of grape and camellia, were grown on a modified Pratts medium containing 35 mM methionine in shaken or static cultures at 22 °C in the dark. Cultures grown on basal media containing glutamate or α-ketoglutarate produced no more ethylene than controls. Optimum growth occurred at pH 3.5 and 4.5 for shake and static cultures, respectively. When B. Cinerea was grown in a methionine-amended basal medium, maximum production of ethylene occurred after 3–4 days of incubation. However, maximum ethylene production per unit dry wt of mycelium (780 μl/g/h) occurred within 48 h of inoculation, after which it declined. That high ethylene production occurs with such small amounts of mycelia suggests a possible role for fungal produced ethylene in B. cinerea pathogenesis of sensitive fruit such as kiwifruit.

Changes in respiration and ethylene production of apples in response to internal and external oxygen partial pressures

Variations in respiration rate and ethylene production of ‘Coxs Orange Pippin’ and ‘Granny Smith’ apples in response to reduced oxygen partial pressures were characterised by studying O2, CO2 and C2H4 partial pressure differences between the internal and external atmospheres (ΔpO2, ΔpCO2 and ΔpC2H4, Pa) of individual fruit maintained in different O2 atmospheres at 20 ± 1°C. ΔpO2 decreased at low O2 levels, reflecting the decreased rate of O2 uptake in low O2 partial pressures. Oxygen uptake relative to that in air (relrO2) approximately followed Michaelis-Menten kinetics, with a half-maximal rate at 3.3 kPa O2 for internal oxygen (piO2) and 10.4 kPa O2 for external oxygen (peO2). An equation was developed to describe aerobic and anaerobic components of CO2 production relative to that in air (relrCO2) as a function of peO2 or piO2. The relationship between rate of C2H4 production relative to that in air (relrC2H4) and piO2 was reasonably described by a Michaelis-Menten type hyperbolic curve, confirming the expectation that small changes in O2 partial pressure would have a much greater effect on relrC2H4 at low piO2 than they do at high piO2. In contrast, presence of the skin as a diffusion barrier resulted in development of an apparent ‘lag phase’ in the relationship between relrC2H4 and peO2 such that it became essentially sigmoidal. These differences are attributed to differences in O2 partial pressure between internal and external atmospheres.

Temperature and ethylene affect induction of rapid softening in 'Granny Smith' and 'Pacific Rose' apple cultivars

The occurrence of rapid softening in apples (Malus domestica Borkh.) at ambient temperatures (10‐20 °C) varies among cultivars, as some cultivars require exposure to low temperatures before autocatalytic ethylene biosynthesis occurs. The influence of time at 0.5 °C on subsequent softening at 20 °C was investigated for the slow softening ‘Granny Smith’ and ‘Pacific Rose™’ apple cultivars. Ethylene (100 ! ll ! 1 for 24 h) was also applied to apples at 20 °C to determine if ethylene treatment could replace the requirement for cold treatment. ‘Granny Smith’ fruit without ethylene or cold treatment softened slowly at 20 °C, while both ethylene- and cold-treated fruit softened more rapidly than non-treated fruit. Non-treated ‘Pacific Rose™’ fruit also softened slowly or not at all at 20 °C, but in contrast to ‘Granny Smith’, did not soften rapidly after either ethylene or cold treatment. The mechanism by which both cold and ethylene treatments initiated rapid softening in ‘Granny Smith’ may be facilitated by ethylene, as these treatments induced maximum internal ethylene concentrations (IEC) that were two to threefold greater than in non-treated fruit at 20 °C. However, rapid softening at 20 °C was often delayed relative to the increase in IEC from basal concentrations for cold-treated ‘Granny Smith’ fruit. This delay for cold-treated ‘Granny Smith’ fruit, and absence of rapid softening in ‘Pacific Rose™’ fruit at 20 °C despite a maximum IEC of more than 100 ! ll ! 1 , suggests that fruit sensitivity to ethylene may be more important than IEC for regulating softening. ‘Pacific Rose™’ may be a genotype of apple with reduced capacity for ethylene biosynthesis and action, and hence softening. The role of ethylene in softening of both cultivars at shelf life temperatures may be clarified by research on changes in ethylene sensitivity that occur during maturation and ripening.

Harvest date and fruit size affect postharvest softening of apple fruit

Summary The influence of harvest date and fruit size on the three phases of softening for Royal Gala and Cox’s Orange Pippin apple cultivars is currently not known. During storage at 0.5 or 3°C, late-harvested fruit had a shorter initial slow softening phase and required less time before the internal ethylene concentration exceeded 1.5 μl–1 than fruit harvested earlier. Rate of rapid phase softening was similar for fruit from each harvest date. Unlike harvest date, fruit size did not consistently affect the harvest indices of fruit from either cultivar. Once in storage at 0.5 or 3°C the softening profiles of different sized fruit from early harvests were similar. However, small fruit softened more slowly than medium and large fruit when harvested at a late stage of maturity. Results in this study indicate that softening of these apple cultivars is not linear in storage, and that both harvest date, and to a lesser extent fruit size, influenced the degree of non-linearity by influencing the duration of the initial slow softening phase. It is also suggested that the initial slow softening phase may be a continuation of the slow softening that occurs in fruit while still attached to the tree.

Relative humidity around the fruit influences the mineral composition and incidence of blossom-end rot in weet pepper fruit

Summary Sweet pepper plants of cultivar Domino were grown using the hydroponic recirculating nutrient film technique (NFT) at an EC of 2 dS m -1 during the autumn season. Two experiments examined the effects of relative humidity (r.h.) around the fruit on the accumulation of Ca, Mg and K on fruit of known age from date of anthesis. The humidity treatments used in the first experiment were on average 30%, 50% and 80% and an ambient humidity achieved by covering sweet pepper fruit with polyethylene bags containing 50 g each CaCl2, NaCl or polyethylene bags without salts. The second experiment used an airflow system to generate average r.h. levels of 20%, 40% and 80%. In both experiments high r.h. levels reduced the accumulation of Ca by the fruit. The r.h. treatments particularly reduced the Ca concentration in the distal end of the fruit, which resulted in a higher incidence of blossom-end rot in experiment I, and with no statistically signficant effect in experiment II. The incidence appears to be related to the Ca concentration as well as the ratio of this element to Mg and K at the distal end. On the other hand, low r.h. promoted the accumulation of Ca in fruit particularly towards fruit maturity. R.h. had little effect on accumulation of Mg and K in sweet pepper fruit and affected fruit mass only at maturity.

Temperature of hypoxic treatment alters volatile composition of juice from ‘Fuji’ and ‘Royal Gala’ apples

Abstract Exposure of apples to hypoxia induces changes in volatile concentrations in extracted juice, potentially affecting aroma quality, but there is little information on how the temperature of hypoxic treatment may affect volatile concentration. ‘Fuji’ (FU) and ‘Royal Gala’ (RG) apples were exposed to hypoxia, 100% CO 2 for 24 h, at 10°C, 15, 20 or 25°C and maintained at treatment temperature for up to 14 days. CO 2 and ethylene production and firmness of intact fruit were proportional to temperature but were unaffected by exposure to hypoxia. Ethyl esters from extracted juice were enhanced at all temperatures at differential rates according to cultivar. Four patterns of total volatile concentration change with temperature after exposure to hypoxia were identified which may be related to changes in rates of volatilisation and volatile production. Apples treated and maintained at 10°C had the greatest overall enhancement of ethyl esters and least decrease in other esters in juice compared to apples treated and maintained at 15, 20 and 25°C. This enhancement in volatiles persisted for up to 10 days after removal from hypoxia. Maintenance of apple quality after treatment with hypoxia was better at low temperatures suggesting that apples treated with hypoxia and maintained below 15°C would have enhanced volatile concentrations compared with untreated apples and apples maintained above 15°C.