Nigel H. Banks

Reducing gas exchange of fruits with surface coatings

Abstract Surface coatings can increase a fruits skin resistance to gas diffusion, modify its internal atmosphere composition and depress its respiration rate; effects on transpiration tend to be comparatively small. A steady state mathematical model which considered diffusion through cuticle and pores separately was used to investigate two possible mechanisms by which coatings achieve these effects: either by acting as a film wrap or by blocking pores. It was concluded that coatings mainly exert their effects on skin resistance to diffusion of the permanent gases by blocking a greater or lesser proportion of the pores on the fruit surface. Both covering of the cuticle and blocking pores were important for resistance to water vapour diffusion. Coupled with a proposed differential resistance of the skin to oxygen, carbon dioxide and water vapour, this accounts for the observed effects of coatings on internal atmosphere modification, respiration and transpiration rates. Coating treatments which achieved substantial reductions in transpiration rates were associated with the fruit becoming anaerobic. Inherent variability in skin resistance to gas diffusion and fruit respiration rate, and differing proportions of pores blocked by coating, appear to be responsible for the highly variable response of individual fruit to a given coating treatment. Surface coatings may therefore be better suited to treating fruits destined for processing, in which fruit to fruit variability may be less critical, than to fruits destined for the fresh market, in which uniformity of individual fruit quality is of paramount importance.

Relationship between character of skin cover of coated pears and permeance to water vapour and gases

Abstract Pear fruit (Pyrus communis L.) with different skin characteristics (non-lignified: ‘Bartlett’, ‘Doyenne du Comice’ and ‘Packhams Triumph’; or with lignified cells in the skin: ‘Beurre Bosc’) were treated at harvest with a carnauba-based wax emulsion at concentrations of 0, 5, 10, 20, 40, and 100% (v/v) of the commercial formulation to achieve different coating deposits on the skin and assessed for skin permeance at 20°C and 60–70% RH. ‘Bartlett’ had the highest natural skin permeance to water vapour and gases, followed by ‘Bosc’, ‘Comice’ and ‘Packhams’. Small increases in coating deposit on the skin substantially reduced permeance to H2O (P′H2O), O2 (P′O2) and CO2 (P′CO2) in cultivars with non-lignified skin, by improving coverage of cracks in the cuticle and blockage of lenticels. These cultivars also underwent a larger reduction in P′O2 than P′CO2 with increasing coating deposits. While ‘Bartlett’ and ‘Comice’ showed a more variable reduction in P′O2 than in P′CO2 with increases in coating deposit, the inverse was observed for ‘Packhams’. ‘Bosc’, with lignified cells in the skin, had only small changes in P′H2O with waxing. In addition, P′O2 and P′CO2 decreased similarly and more gradually with increasing coating deposits in ‘Bosc’ than in the other cultivars. The epidermis of ‘Bosc’, comprising an irregular layer of lignified cells, seemed to have high P′H2O and low P′O2 and P′CO2. Improving the coating deposit on the skin blocked the lenticels, providing a more variable reduction in P′CO2 than in P′O2. However, this did not effectively cover the lignified cells in the epidermis, providing variable and small changes in P′H2O. These results show that optimisation of surface coatings for pears must take into account differences in the nature of the skin.

Determination of lower oxygen limits for apple fruit

Abstract Knowledge of the lower oxygen limit ( LOL ) is critical for optimising the gaseous storage environment for fruits. The optimum storage atmosphere occurs just above the LOL at which aerobic respiration is at the lowest level which can be achieved without development of anaerobic metabolism. Measures of LOL based on a fruits internal atmosphere, rather than external or package atmospheres, estimate the true LOL as these account more directly for differences in respiration rate and skin permeance of individual fruit. Two measures of LOL were considered: the anaerobic compensation point ( ACP ) and the fermentation threshold ( FT ). The ACP was described in terms of plots of the internal partial pressure of CO 2 versus internal O 2 ( p O 2 i ) and external O 2 ( p O 2 e ) partial pressures. The FT was described in terms of plots of both the respiratory quotient ( RQ ) and ethanol concentration versus p O 2 i and p O 2 e , and occurred at higher p O 2 than the ACP . Mathematical solutions for estimating the ACP and the FT based on the RQ ( FT RQ ) are described. A statistical ‘bootstrap’ procedure is described for estimating the FT based on ethanol concentration ( FT EtOH ) and was also suitable for estimating all other LOLs and their confidence intervals. LOL s were estimated for ‘Coxs Orange Pippin’ apples ( Malus domestica , Borkh) at 24 °C using controlled atmospheres (CA). The steady-state internal partial pressures of O 2 , CO 2 and concentrations of acetaldehyde, ethyl acetate and ethanol were estimated non-invasively by sampling the headspace of 1000 mm 3 glass surface chambers sealed to the equatorial surface of the apples. LOLs estimated on a p O 2 i basis were on average 1.69 kPa, 1.94 kPa and 2.10 kPa p O 2 lower for ACP , FT RQ and FT EtOH respectively than those estimated relative to p O 2 e . The bootstrap 95% confidence limits for internal ACP ( ACP i ) were 0.70 to 0.78 kPa p O 2 i whilst for internal FT RQ ( FT RQ ) the interval was 1.04 to 1.20 kPa p O 2 i and for internal FT EtOH ( FT EtOH ) 0.68 to 0.87 kPa P O 2 i . Bootstrap estimates were similar, though typically higher, than mathematically fitted estimates. Determining LOL s on a steady-state internal atmosphere basis estimates the true LOL more accurately than those estimated from external or package atmospheres, and provides a more mechanistic basis for models used to predict fruit responses to CA. As the FT RQ represents the critical point at which fermentation occurs it was considered the safest estimate of the true LOL for optimising storage atmospheres.

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.

Relationship between water vapour permeance of apples and micro-cracking of the cuticle

Abstract Water vapour permeance is an important determinant of the rate of water loss from fruits. Variation in water vapour permanence of apple fruit may be related to micro-cracking of the cuticle. This study used scanning electron microscopy, confocal and light microscopy to characterise variation in micro-cracking in ‘Braeburn’ apples. While there was considerable variation in cracking around the surface of the fruit as determined by data obtained with the confocal microscope (coefficient of variation=44%), there was no obvious pattern in relation to blush. A model based on diffusion of gases was developed and used to explain the relationship between micro-cracking determined using scanning electron microscope data and the water vapour permeance of whole fruit (r2=48%). This model included terms for intact cuticle, cuticle that was cracked in the outer layer, and boundary layer effects. The model was based upon the effective permeability of air to water vapour (analogous to diffusivity), the depth of the boundary layer (0.5 mm), the depth of cracks in the outer cuticle (8 μm), and the depth of the remaining inner cuticle (8 μm). Permeability of the inner cuticle to water vapour was predicted with the model to be 1.2±0.35 pmol m s−1 m−2 Pa−1 and water vapour permeance of intact cuticle was calculated as 12.3±2.7 nmol s−1 m−2 Pa−1. Cracks were calculated to be 12 times more permeable than intact cuticle. The experimental data and the model presented in this paper provide evidence for, and explain, the major role of cuticular cracks in determining apple fruit water vapour permeance.

The effect of modified atmospheres on the rate of firmness change in 'Braeburn' apples

Abstract Gas exchange rates and softening of ‘Braeburn’ apples were measured under a range of modified atmosphere conditions to characterise their functional relationship. Both gas exchange (measured as CO2 production) and softening were maximally suppressed at around 1–1.5 kPa O2. At these low O2 levels, CO2 had no effect on the rate of softening. Both softening and gas exchange had comparable Km values indicating a common kinetic background. The integrated approach revealed a close quantitative relationship between rate of softening and rate of gas exchange of ‘Braeburn’ apples indicating that fruit softening is directly metabolically linked to gas exchange. Due to the less efficient character of fermentation, the softening of ‘Braeburn’ apples related more closely to respiration than to fermentation.

Effects of coating concentration, ripening stage, water status and fruit temperature on pear susceptibility to friction discolouration

Abstract Pear fruit ( Pyrus communis L.) cv. ‘Duyenne du Comice’ were treated at harvest with different concentrations of a carnauba-based wax emulsion and assessed for susceptibility to friction discolouration (FD) after different periods in cold storage. Susceptibility to FD was low at harvest but increased with storage, especially between 1 and 2 months storage. Concomitantly, higher coating concentrations were required to reduced FD with prolonged storage; coating concentrations of 20 and 40% were necessary to reduce FD for fruit damaged after 1 and 2 months storage, respectively. Increasing coating concentration reduced internal O 2 partial pressure and reduced water loss. The lower internal O 2 level in coated fruit was associated with delayed ripening and reduced fruit susceptibility to FD. Reduced water loss preserved the integrity of skin and reduced susceptibility to FD. Higher levels of FD were observed at temperatures lower or higher than 10–20°C. The physical protection provided by skin coatings in combination with their effects on fruit physiology, presents a useful opportunity to reduce FD after cold storage, if adopted in conjunction with adequate temperature and humidity control during storage and post-storage handling of the fruit.

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.

Internal atmosphere composition and skin permeance to gases of pepper fruit

Abstract Characterisation of internal atmosphere composition offers the potential to explain variability in responses of horticultural crops to modified atmosphere treatments and to quantify permeance of fruit skins to the respiratory gases. In this paper, the theoretical basis by which fruit skin permeance can be calculated from other gas exchange variables is presented. Surface chambers close to equilibrium with the fruit’s internal atmosphere were used to monitor internal atmosphere composition of sweet pepper (Capsicum annum, cv. Reflex). Physical equilibration of chamber contents over wounded fruit surface was essentially complete in less than 4 h. However, physiological drift in internal atmosphere composition meant that substantial changes continued to develop over more extensive periods. Removal of cuticle beneath the chamber was shown to be essential for equilibration of chamber contents within physiologically meaningful periods. Samples of atmosphere removed destructively from the fruit cavity consistently contained more O2 but less CO2 than samples similarly removed from the fruit flesh. Levels of CO2 were higher in samples removed directly from the flesh by syringe than in those taken from surface chambers, indicating potential for an effect of the vacuum used to take direct removal samples on sample composition. Permeance of pepper cuticle to CO2 was about ten times greater than that to O2 (∼244 and 24 pmols−1m−2 per Pa, respectively). Removal of cuticle dramatically increased permeance of the fruit surface and hastened equilibration of surface chambers with the fruit’s internal atmosphere. Surface chambers adhered over fruit surface from which the cuticle has been removed would be the most reliable means to assess composition of the atmosphere in immediate contact with the cells of pepper tissue.

Temperature effects on the internal lower oxygen limits of apple fruit

Abstract Exposure of packaged fresh produce in modified atmospheres (MA) to elevated temperatures can cause the partial pressure of oxygen inside the produce ( P O 2 i ) to fall below internal lower O 2 limits ( LOL i s), resulting in fermentation and loss of quality. In this study, two types of LOL i were estimated from steady-state external chamber atmospheres: the anaerobic compensation point ( ACP i ), and the fermentation threshold based on the respiratory quotient ( FT RQ i ) and on ethanol (EtOH) accumulation ( FT EtOH i ). Mean estimates of LOL i s of apple cultivars ‘Coxs Orange Pippin’ and ‘Braeburn’ ranged between 0.5 and 1.0 kPa internal partial pressure of O 2 ( p O 2 i for ACP i ; 0.8 and 1.7 kPa p O 2 i for FT RQ i , and 0.5 and 2.0 kPa p O 2 i for FT EtOH i , for fruit temperatures between 0 ° and 28 °C. Values for ACP i estimated at 32 °C were higher than those between 0 ° and 28 °C. In general, estimates of dissolved O 2 concentration at ACP i and FT RQ i tended to decrease with increasing temperature for ‘Braeburn’ apples, changed little for ‘Coxs Orange Pippin’ apples, but increased again for ACP i at 32 °C. On average, estimates of LOL i s were higher for ‘Braeburn’ than ‘Coxs Orange Pippin’ apples. The effect of temperature on diffusion coefficients and solubility were considered unlikely to contribute significantly to variation in LOL i s except for solubility at higher temperatures, but differences in tissue porosity may have influenced differences in LOL i s between cultivars. Results of the current study indicate temperature effects on LOL i s would not be significant except for MA packages destined for markets with ambient temperatures in excess of 28 °C. On the other hand it seems that to assume that ACP i is independent of temperature may be too simplistic, particularly if fruit are exposed to a very wide range of temperatures. LOL i s were relatively constant compared with lower O 2 limits based on package atmospheres, and can be used to estimate optimum MA package atmospheres that account for fruit within a population with the highest respiration rates and lowest permeance to O 2 .