Teses / dissertações sobre o tema "Ripening"

This thesis presents a theoretical approach to Ostwald ripening of droplets in arbitrary dimensions. A mean-field theory is constructed to incorporate screening effects among the competing droplets. The mean-field equations are solved to all orders in the volume fraction to provide analytic expressions for the coarsening rate, the droplet distribution function, and the time-dependent droplet number. These results are in agreement with experiments in three-dimension and with very large scale and extensive numerical studies in both two and three dimensions undertaken in this thesis. The numerical study also provides the time evolution of the structure factors, wherein lengths scale with the average droplet radius. Finally, the mean-field theory is extended to exciton systems and surfactant systems.

Thesis (MSc)--Stellenbosch University, 2002.<br>ENGLISH ABSTRACT: 'Forelle' is one of three blushed pear cultivars produced in South Africa. A mandatory minimum cold storage duration of 12 weeks at -0.5°C to ensure even ripening, prevents 'Forelle' from being marketed earlier. Since earlier marketing can result in premium prices (in excess of 50% morê per carton) research in recent years has been directed at reducing this 12 week cold storage period. Intermittent warming treatments, controlled atmosphere (CA) storage in combination with regular atmosphere (RA) storage intervals, and ethylene treatments, have been tested as alternatives to the 12 week cold requirement. However, none of these treatments delivered a better internal quality in terms of mealiness and astringency. Fruit harvested from the Warm Bokkeveld and Theewaterskloof areas at commercial maturity were stored at -0.5°C for up to 21 and 22 weeks, respectively, to understand the changes in ripening and mealiness of 'Forelle' pears after cold storage (Paper 1). Samples were removed every third weev, placed at 15°C, and maturity factors, total ACC concentration, ethylene production and respiration rates monitored every third day for 12 days. Fruit from the Warm Bokkeveld and the Theewaterskloof areas ripened after 6 and 7 weeks at -0.5°C, respectively. However, after 6 weeks of cold storage followed by 6 days at 15°C, all fruit harvested in the Warm Bokkeveld, and 70 % of the fruit harvested in the Theewaterskloof area, were mealy. With extended storage at -0.5°C (> 15 weeks) the incidence of mealiness declined in fruit from both areas, but never disappeared, when evaluated at 15°C over 12 days. Since harvest maturity affects the incidence of mealiness in other pear cultivars, the effect of harvest maturity on 'Forelle' pears with regard to mealiness development was examined (Paper 2). Fruit were harvested from the Ceres area, in weeks 8 (preoptimum), 10 (optimum), 12 and 14 (both post optimum). Maturity indices, juice content, mealiness, total ACC content, ethylene production and internal ethylene were monitored at harvest, after 6 weeks of storage at -0.5°C and again after 7 days at 15°C. Fruit harvested 2 weeks before commercial harvest (week 8) had the highest total ACC concentration, ethylene production and the potential to ripen, but also developed the highest incidence of mealiness (80%). However, fruit of all harvest maturities (except where contamination with 1-MCP occurred) were mealy. It would appear that factor(s), other than harvest maturity, play a more important role in the initiation of mealiness in 'Forelle' pears. Although ethylene has been shown to shorten the cold requirement of 'Forelle', there are conflicting reports as to its effectiveness in reducing mealiness. Consequently, the aim of the third paper was to evaluate the effect of ethylene on ripening and mealiness of 'Forelle' pears. Fruit harvested from the Elgin area at commercial maturity were stored for 3 weeks at -0.5°C, treated with ethylene (100 Jl L.L·l , 24h, 20°C) and held at 20°C for a further 2 days (without ethylene). Control fruit were held at 20°C for 3 days. Fruit were returned to -0.5°C for a further 3 weeks. After a subsequent 3 days at 20°C, flesh firmness was 4.6 kg in treated fruit compared to 6.1 kg for control fruit. At this point all fruit treated with ethylene were mealy. Control fruit all exhibited mealiness after a further 3 weeks at -0.5°C followed by 7 days at 15°C. Ethylene treatment advanced fruit maturity, but did not prevent or alleviate mealiness. Mealiness is a textural disorder recognized by a dry soft pulp. This has previously been recorded in 'd'Anjou' pears, as a result of storing the fruit for too long at -1.1°C, but has also been the result of a chilling injury in fruit like nectarines, kiwi and persimmon. The role of storage temperature on ripening, and specifically mealiness, of 'Forelle' was thus investigated (Paper 4). Fruit harvested from the Elgin area at commercial maturity were stored at -0.5°C, 4.0°C and 7.5°C for 0, 3 and 6 weeks. Samples were removed every third week, placed at 15°C, and maturity indices, extractable juice content, mealiness, total ACC content, internal ethylene concentration and ethylene production were monitored on removal and after 7 days. Flesh firmness of the 4°C stored fruit was 0.5 kg lower than fruit stored at -0.5°C, on removal from storage. Fruit stored at 4°C and 7.5°C ripened with little to no mealiness (0 and 8% respectively) in contrast to fruit stored at -0.5°C (70% mealy). Total ACC accumulation and ethylene production were higher for fruit stored at 4°C and 7.5°C than fruit stored at -0.5°C. Storage temperature appears to playa role in the development of mealiness. Although storage temperatures influenced mealiness development, this research should be repeated before this can recommended as a commercial treatment. The underlying mechanism of action in the development of mealiness should be investigated. By examining fruit grown in different climatical areas, harvested at different maturities, treated with exogenous ethylene and stored at different temperatures, this research has helped to a better understanding of the role of factors affecting ripening and development of mealiness in 'Forelle'.<br>AFRIKAANSE OPSOMMING: Forelle' is een van drie blos peer kultivars wat in Suid Afrika geproduseer word. 'n Minimum van 12 weke by -0.5°C opberging, is deur wetgewing vasgestel, om eweredige rypheid van 'Forelle' te verseker. Vroeër bemarking van 'Forelle' kan daartoe lei dat vrugte premium pryse, van 50% meer per karton behaal. Aangesien die vasgestelde koue periode verhoed dat 'Forelle' vroeër bemark kan word, het onlangse navorsing gefokus op vermindering van die vasgestelde opberging van 12 weke by -0.5°C. Afwisselende verwarmmgs behandelings gedurende koue opberging, beheerde atmosfeer (BA) opberging in kombinasie met gewone atmosfeer (GA) opbergings intervalle, en etileen behandelings, is _getoets as alternatiewe vir die 12 weke opbergings vereiste by -0.5°C. Geen van die bogenoemde behandelings kon egter 'n beter interne kwaliteit in terme van meleringheid en frankheid as 'n alternatief verseker nie. Die doel van die eerste proef was om die invloed van koue opberging by -0.5°C op rypwording en melerigheid van 'Forelle' te bepaal. Vrugte is in die Warm Bokkeveld and Theewaterskloof areas geoes tydens kommersiële oesrypheid, waarna hulle vir 21 en 22 weke, respektiewelik, by -0.5°C opgeberg is. Monsters is elke derde week vanaf die -0.5°C koue opberging geneem en by 15°C geplaas, waarna rypheids indeksering, totale ACC konsentrasie, etileen produksie and respirasie tempos elke derde dag vir 12 dae gemonitor is. Vrugte wat in dieWarm Bokkeveld en Theewaterskloof areas geoes is, het rypgeword by 15°C, na 6 and 7 weke by -0.5°C, respektiewelik. Alle vrugte vanaf die Warm Bokkeveld en 70% vrugte vanaf die Theewaterskloof area, het egter na die 6 en 7 weke by -0.5°C, respektiewelik, en 6 dae by 15°C, melerigheid ontwikkel. Vrugte wat vir langer as 15 weke by -0.5°C opgeberg is, het in albei areas 'n afname in melerigheid getoon, maar die defek het nooit heeltemal verdwyn tydens evaluasie by 15°C nie. Oes rypheid affekteer die ontwikkeling van melerigheid in ander peer kultivars. Die tweede proef was dus gefokus op dip. invloed van oes rypheid op 'Forelle' se ontwikkeling van melerigheid. Vrugte is in die Warm Bokkeveld area geoes in week 8 (pre-optimum), 10 (optimum), 12 and 14 (albei post-optimum). Rypheids indekse, uitdrukbare sapinhoud, meleringheid, totale ACC konsentrasie, interne etileen vlakke en etileen produksie is gemonitor, na opberging vir 6 weke by -0.5°C en 7 dae by 15°C. Vrugte wat 2 weke voor kommersiële rypheid geoes is (week 8) het die hoogste totale ACC konsentrasie en etileen produksie gehad, en het reeds die potensiaal gehad om ryp te word. Die hoogste vlakke van melerigheid (80%) is ook in vrugte wat tydens hierdie oes verkry is, waargeneem. Vrugte wat op alle tye geoes is, het melerigheid ontwikkel, behalwe waar kontaminasie met 1-MCP plaasgevind het. Dit wil voorkom asof 'n ander faktor 'n belangriker rol in die ontwikkeling van melerigheid in 'Forelle' speel as oes rypheid. Etileen is bewys om die vereiste koue opberging vir 'Forelle' te kan verkort, maar daar is konflik oor die effektiwiteit van etileen op die verlaging van melerigheid in 'Forelle'. Die doel van die derde proef was dus om die effek van eksterne etileen behandeling op rypheid en melerigheid van 'Forelle' te toets. Vrugte wat in die Elgin area by kommersiële oesrypheid geoes is, is vir 3 weke gestoor by -0.5°C, met etileen behandel (100 Il L.L-1, 24h, 20°C) en vir 'n verdere 2 dae by 20°C gehou (sonder etileen). Kontrole vrugte is vir 3 dae by 20°C gehou. Daarna is vrugte weer vir 'n verdere 3 weke by -0.5°C opgeberg. Na 'n verdere 3 dae by 20°C, was vlees fermheid van etileen behandelde vrugte 4.6 kg .n vergelyking met 6.1 kg vir die kontrole vrugte. Alle vrugte wat met etileen behandel is, was melerig op hierdie stadium. Die kontrole vrugte het ook almal melerig geword, maar slegs na 'n verdere 3 weke by -0.5°C en 7 dae by 15°C. Etileen behandeling het dus die rypheid van 'Forelle' bevorder, maar het nie melerigheid voorkom nie. Melerigheid is 'n tekstuur probleem, wat gekenmerk word aan sagte droë pulp. Hierdie tekstuur probleem is voorheen waargeneem in 'd'Anjou' pere wat te lank by -Ll=C opgeberg is, maar is ook kenmerkend by vrugte soos nektariens, kiwi en persimmon wat aan koue skade ly. Lie gevolg is dat die effek van opbergings temperatuur op 'Forelle' rypwording en melerigheid getoets moes word. Vrugte is geoes van die Elgin area tydens kommersiële oesrypheid en gestoor by -0.5°C, 4.0°C en 7.5°C vir 0, 3 en 6 weke. Vrug monsters is geneem, na verwydering van bogenoemde stoor temperature en na 7 dae by 15°C, waarby rypheids indekse, uitdrukbare sapinhoud, meleringheid, totale ACC konsentrasie, interne etileen vlakke en etileen produksie gemonitor is. Vlees fermheid van vrugte wat vir 6 weke by 4°C opgeberg was, was 0.5 kg laer as vrugte wat by -0.5°C opgeberg was. Vrugte wat by 4°C and 7.5°C vir 6 weke gestoor is en by 15°C ryp geword het, het min tot geen melerigheid ontwikkel (0 and 8%, respektiewelik) nie. Vrugte wat by -0.5°C opgeberg was het egter hoë vlakke van melerigheid bereik (70% melerig). Totale ACC akkumulasie en etileen produksie was hoër vir vrugte wat by 4°C en 7.5°C gestoor was, teenoor vrugte wat gestoor was by -0.5°C. Dit wil voorkom asof na-oes opbergings temperature wel 'n rol speel in die ontwikkeling van melerigheid. Alhoewel dit voorkom asof na-oes temperature wel 'n rol speel in die ontwikkeling van melerigheid in 'Forelle' pere, is meer basiese navorsing nodig om die meganisme van werking in melerigheid ontwikkeling te verstaan. Na-oes temperatuur as 'n faktor wat melerigheid kan beïnvloed, moet oor 'n reeks van seisoene nagevors word. Laasgenoemde is van uiterste belang aangesien die invloed van seisoenale variasies op melerigheid nog nie gekwantifiseer is nie. Die navorsing was gefokus daarop om vas te stel watter faktore 'n fisiologiese rol speel in rypwording en melerigheid van 'Forelle' pere. Deurdat vrugte van twee areas met verskillende klimate, vrugte met verskye oesryphede, ekterne etileen behandeling, en vrugte van verskillende opbergings temperature ondersoek is, het hierdie navorsing gehelp om 'n beter begrip te vorm van die rol van hierdie faktore op die rypwording en die ontwikkeling van melerigheid in 'Forelle'.

Various techniques for accelerating mature flavour development in Cheddar cheese were compared. Control cheese ( c) was manufactured by using Streptococcus cremor is AM2, a starter used in normal commercial manufacture. A combination of S. cremoris AM2 and Streptococcus lactis C2 Lac- Prt- mutant was used in the manufacture of test cheeses (M). §._. lactis C2 mutant was grown in glucose broth at 30°c and pH 6 .O for 16 hours, followed by concentration and diafiltration to 1011cfu mL - 1 using microfiltration equipment. The control cheesemilk was inoculated to 6x107 streptococci pe mL with S. cremoris AM2 and the mutant vat cheesemilk to 2x109 per mL with a combination of ~ cremoris AM2 and ~ lactis C2 mutant. The starter population in cheese containing mutant starter was 100 times that in control cheese (1010 compared to 10a). Cheeses were also made with added bacterial neutral proteinase (Neutrase, N) and stored at a0c (a) and 15°c ( 15) for 32 weeks. This resulted in cheese being subjected to the following treatments: ca (control), C15, CNa, CN15, Ma·, M15, MNa, and MN15. Cheddaring times were slightly reduced and milling acidities slightly higher in the vat . containing mutant starter. However the composition of all cheese was satisfactory. Bacteriological counts, proteolysis, rheological properties and flavour development of these cheeses were monitored at regular intervals throughout maturation. The order of the effectiveness of the treatment in accelerating ripening was MN15, >M15,> CN15,> C15,> MNa,> Ma,> CNa,> ca. Cheeses from these treatments attained the characteristics of control cheese stored at a0 c (Ca) for 6 months after 1.4, 1.7, 2.0, 2.6, 2.8, 3.2, 4.3 and 6.0 months respectively. Cheese quality was not adversely affected except for bitterness in CN8 cheese and overmaturity in CN15 cheese late in the storage period. The possible mechanisms and relative merits of the various treatments are discussed with special reference to an "active role" theory of starter bacteria in flavour development.