Browsing by Author "Dixon, Jonathan"

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    Enhancement of aroma and flavour volatiles in apple juice : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Physiology at Massey University
    (Massey University, 1999) Dixon, Jonathan
    Aroma typical to apples develops during ripening and is comprised of a large range of volatile chemical compounds from several chemical classes. Previous research has established that exposing apples to hypoxic conditions induces changes in volatile concentrations; acetaldehyde and ethanol accumulate to high concentrations and after return to aerobic conditions ethyl esters increase and non-ethyl esters decrease. The present study investigated the effect of short term-hypoxic treatments on the enhancement of ethyl esters and decrease in non-ethyl esters with respect to: organoleptic changes in apple aroma induced by exposure to hypoxia; the influence of temperature and time at 0 °C before treatment on the magnitude of enhancement of ethyl esters after exposure to hypoxia; the effect of cultivars and ripeness stage on types and quantities of ethyl esters enhanced after exposure to hypoxia. Brief periods of hypoxia at ambient temperatures have potential for disinfestation treatments or as pre-treatments to maintain fruit quality during extended storage. Volatile compounds were extracted from 20 mL aliquots of apple juice with an equal volume of diethyl ether:n-pentane (2:1 v/v), vigorously stirred for 3-5 seconds, frozen at -18 °C to separate solvent and aqueous phases, concentrated with a fast stream of oxygen free nitrogen (200 mL min-1) to 200 μL and analysed by gas chromatography. Apple juice could be held in ice or air up to 256 minutes without loss of volatile compounds. Loss in solvent washes was 76.5% for octyl acetate and recoveries during concentration of the solvent extract ranged from 2.5% for ethyl acetate to 86.4% for trans-2-hexenal. Solvent extraction was simpler, faster, extracted more compounds, and had better reproducibility than dynamic headspace extracts obtained using Tenax® traps. Nine cultivars of apples, Cox's Orange Pippin, Fuji, Golden Delicious, Granny Smith, Pacific Rose, Red Delicious, Royal Gala, Splendour and Southern Snap were exposed to 100% carbon dioxide for 24 h at 20°C. Apples exposed to hypoxia had concentrations of acetaldehdye, ethanol, ethyl acetate and ethyl esters consistently enhanced while concentrations of acetate esters and aldehydes were depressed. Maximum ethyl ester enhancement occurs within 2 to 3 d after removal from hypoxia. Exposure to hypoxia for 24 h at 20 °C did not affect rates of softening or induce physiological damage. Cultivars varied considerably in response to hypoxic treatment with Cox's Orange Pippin and Golden Delicious having the least and Fuji and Red Delicious the greatest enhancement in ethyl esters. Fruit exposed to hypoxia had larger odour unit scores than control fruit suggesting that such changes in volatile concentration may affect the aroma and/or flavour. Fuji and Royal Gala apples were exposed to 100% CO2 for 24 h, at 10, 15, 20 or 25 °C and maintained at treatment temperature for up to 14 d. Carbon dioxide and ethylene production and firmness were proportional to temperature but were unaffected by exposure to hypoxia. Ethyl esters were enhanced at all temperatures at differential rates according to cultivar. Apples treated and maintained at 10 °C had the greatest overall enhancement of ethyl esters and the least decrease in other esters compared to apples at 15, 20 or 25 °C. This enhancement in volatiles persisted for up to 10 d after removal from hypoxia. Best maintenance of apple quality after treatment with hypoxia is at low temperatures suggesting that apples treated with hypoxia and maintained below 15 °C would have enhanced volatile concentration. Noncooled Fuji and Royal Gala apples at preclimacteric to postclimacteric ripeness stages were exposed to 100% CO2 for 24 h at 20 °C for up to 14 d. A batch of the same fruit were placed at 0 °C, removed to 20 °C and exposed to hypoxia at monthly intervals for up to 5 months. Exposure to hypoxia decreased carbon dioxide production in Fuji apples at the preclimacteric and rising climacteric stages and at the climacteric. Respiration rate, ethylene production and volatile concentration of RG apples were not affected by exposure to hypoxia at any stage of ripeness or period at 0 °C. After exposure to hypoxia Fuji apples had enhanced ethyl esters at the preclimacteric and rising climacteric stages and after being at 0 °C for up to 5 months. Volatile concentrations were lower in apples maintained at 0 °C compared to noncooled apples. Apples at 0 °C had the greatest enhancement of ethyl esters after hypoxia suggesting that exposure to low temperatures did not just slow volatile biosynthesis but had an additional effect on volatile biosynthesis. Apple aroma consists of mainly low molecular weight esters produced by esterification of alcohol's by alcohol acyl CoA transferase (AAT) where acyl CoA's are substrates. Increased esterification activity in apples returned to air, following a hypoxic treatment, is due possibly to enhanced AAT activity or to competitive inhibition of other alcohols by ethanol. Concentrations of acetate and ethyl esters from skin disks of Cox's Orange Pippin, Fuji, Golden Delicious, Granny Smith, Pacific Rose, Red Delicious, Royal Gala, Splendour and Southern Snap apples exposed to 100% CO2 for 24 h at 20 °C, were compared to disks from control fruit, after addition of C2 to C6 alcohols, either individually, or as a mixture in equimolar amounts to the disks. Ethanol added as an individual alcohol induced high ethyl acetate concentrations, but when added as part of a mixture, little ethyl acetate was produced indicating substrate preference was for longer chain alcohols. Apple cultivars had four patterns of change in ester production after exposure to hypoxia: increased acetate and ethyl esters; increased acetate esters and decreased or no change in ethyl esters; no change or decreased acetate esters and increased ethyl esters; no change or decreased acetate esters and decreased or no change in ethyl esters, implying that AAT activity is affected differentially by hypoxia. Hypoxia induces changes in capacity to produce esters which last up to 7 d indicating that pre-storage treatments using hypoxia has the potential to change the aroma profile of apples. Juice of Fuji and Royal Gala apples exposed to a brief period of hypoxia (100% CO2 for 24 h at 20 °C) and ripened at 20 °C for up to 8 d, was analysed by taste panels using quantitative descriptive analysis. Hypoxia induced large increases in ethyl esters including ethyl butanoate and ethyl-2-methyl butanoate in Fuji apples but not in Royal Gala apples. There was no difference in average panellist scores for sensory characteristics for Fuji and Royal Gala apples at any sampling time. The lack of difference may have been due to large variation between panellist's assessment of sensory characteristics and/or inability to assess aroma, flavour and sweetness independently. A number of individual volatiles correlated with aroma in juice from apples exposed to hypoxia, including hexan-1-ol, butyl acetate, 2 methyl butyl acetate and propyl butanoate for Fuji; and ethanol, ethyl acetate, propyl acetate and propyl butanoate for Royal Gala. Multivariate analysis indicated that panellists associated increased ethyl esters with off flavour rather than more intense apple aroma. This could have been due to juice from apples exposed to hypoxia having a different apple-like character than control fruit which did not fit the definition of apple aroma used to train panellists. The enhanced ethyl ester concentrations in fruit exposed to hypoxia are probably due to large increases in ethanol concentration that competitively inhibited formation of non-ethyl esters. Golden Delicious and RG did not have enhanced concentration of ethyl esters and/or decreases in acetate ester concentration even though fermentation volatiles were enhanced to high concentrations and ethyl acetate increased to concentrations similar to those found in fruit which had enhanced ethyl esters. The mechanism producing ethyl acetate and ethyl esters in GD and RG was probably different from that in CO, FU, PR, RD, SP, SS cultivars. Therefore, after exposure to hypoxia, additional factors influence changes in volatile concentration other than the increased pool of substrate available for esterification. A possible mechanism by which hypoxia affects ester biosynthesis is that under hypoxic conditions cytoplasmic pH falls below the optimum of 7 to 8, inducing increased ADH activity and synthesis and producing large increases in ethanol concentration. Ester biosynthesis is suppressed during hypoxia leading to increased alcohol and aldehyde concentrations creating a pool of substrates that could be rapidly utilised by AAT on return to aerobic conditions. It is possible that AAT activity or concentration changes are induced by hypoxic conditions. The different capacity of apple cultivars to esterify alcohols from control and hypoxic treated fruit may be due to changes in substrate specificity of either, or both, newly induced ADH and AAT. Exposure to hypoxia consistently caused increases in ethyl esters in several apple cultivars. The practical uses for treatments where apples are exposed to hypoxia for 24 h include: disinfestation treatments, manufacture of apple juice concentrates, enhancement of aroma in apples maintained in long term air or controlled atmosphere storage and as a tool for examining volatile biosynthesis.
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    Temperature and atmosphere composition influence on colour change of apples : a dissertation presented in partial fulfilment of the requirements for a Masterate of Horticultural Science, Massey University, Palmerston North, New Zealand
    (Massey University, 1993) Dixon, Jonathan
    In apples colour is a major quality parameter used by consumers to determine apple maturity. A full understanding of the nature of the relationship between storage conditions and apple fruit colour change would be of advantage in formulating models to predict how changes to handling systems would affect fruit colour. While much is known in a general way about how environmental conditions affect colour change, little information is available to characterise the nature of the relationships between temperature, oxygen and carbon dioxide. The postharvest change in colour was measured for two export apple cultivars; Cox's Orange Pippin and Granny Smith. Previous research on these and other apple cultivars has determined that colour change is from green to yellow. The colour of Cox's Orange Pippin and Granny Smith apples were measured by subjective and objective methods during experiments to investigate the effect of temperature and atmosphere composition on colour change. The objective methods used were: chlorophyll extraction and colour using a Minolta chromameter. The subjective method was colour matching for Granny Smith using the NZAPMB maturity colour charts. When related to changes in chlorophyll, the principal skin pigment, the colour parameters used had non-linear relationships. Lightness, hue angle and colour chart score all reflect pigment changes occurring as apples change colour from green to yellow. Lightness values were the least variable followed by hue angle then colour chart score. All methods used showed more sensitivity to changes in chlorophyll content when chlorophyll content was low compared to when chlorophyll content was high. The objective measurements were highly correlated with the subjective measurements and the conclusion was that the use of hue angle or lightness to follow colour change in the skin of Granny Smith and Cox's Orange Pippin apples is an accurate indirect measure of chlorophyll and other pigments. The rate constant of colour change (k), measured using a declining exponential function, from green to yellow, at eleven temperatures over two seasons, two harvests per season and several growers was investigated in order to characterise the relationship between yellowing and temperature. All the methods of colour measurement used had the same relationship with temperature which was described by a modified form of the Arrenhius equation. Re-worked published data also fitted the modified Arrenhius equation. The modified Arrenhius equation was used to generate k for the various colour parameters measured (chlorophyll, hue angle, lightness and colour charts score). The value of k, as a function of temperature, increases slowly between 0°C and 6°C (the lag phase), increases exponentially between 6°C and 20°c and reaches a maximum at 25.3°C for Cox's Orange Pippin and 23.5°C for Granny Smith before declining. Pattern of response to temperature was the same for each cultivar although Granny Smith yellowed more slowly than Cox's Orange Pippin. For Cox's Orange Pippin apples more variation was accounted for by differences between growers than years or harvests within a year. For Granny Smith fruit most variation was accounted for by differences between years. Sixteen atmospheres were used each year for Cox's Orange Pippin and Granny Smith apples from one harvest in order to characterise the relationship between yellowing and oxygen or carbon dioxide. Cox's Orange Pippin and Granny Smith apples differ in their response to oxygen. For Cox's Orange Pippin the value of k as a function of oxygen level increased slowly from 0% to 6% and thereafter increased exponentially from 6% to 19%. This function may be sigmoidal as the k values increase slows above 17% oxygen. The relationship for Granny Smith was poorly defined by this function, k values increased slowly as the oxygen level rose. This could be due to a fundamental physiological or biochemical difference between these two cultivars. Each cultivar had a similar response to carbon dioxide, described by a declining exponential function, with the relationship for Granny Smith being better defined than for Cox's Orange Pippin. The relationship of carbon dioxide with colour change was poorly defined as the effects of oxygen on colour change were not removed from the analysis. Oxygen appears to have a greater influence on colour change than carbon dioxide. Atmospheres for Cox's Orange Pippin apples were not scrubbed for carbon dioxide in 1989 but were in 1990. The pattern of response to oxygen in the absence of levels of carbon dioxide above 1% in the atmosphere did not alter the sigmoidal relationship found. This may be evidence that the effect on yellowing by oxygen and carbon dioxide is by separate processes. Ethylene levels in the atmosphere appeared to have little effect on the rate of yellowing in all the atmospheres studied. The carbon dioxide and oxygen functions were combined into a single equation for use as a predictive model. The temperature function, the modified Arrenhius equation, and the atmosphere functions were combined into one equation to which different environmental values were added. The use of such a model and other practical applications for the information gathered for this thesis are discussed and a chart drawn comparing the hue angle, lightness and colour chart score to chlorophyll level.