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Thesis (MScAgric)--University of Stellenbosch, 2001. / ENGLISH ABSTRACT: Potatoes is the fourth most important food source in the world. To provide in the ever increasing demand by the growing world population for this staple food, alternative ways will have to be found to produce a cheaper and a healthier end product. The tubers mainly consist of carbohydrates and starch, although 9% of the dry weight of the tubers consist of proteins. In its natural state the potato tuber is low in sodium and high in potassium and vitamin C. These proteins can provide in all the amino-acid requirements of humans and make it an outstanding balanced diet. The most important factor by which the production and quality of potatoes can be increased, is to provide better propagation material to producers. Minituber production is a relatively new method of producing better quality propagation material for producers. It is, however, very expensive because of the high capital investment needed. By producing more tubers per plant, or more tubers per square meter, the unit price per tuber may be lowered. The system commonly used for minituber production is where in vitro plants are planted in a greenhouse, usually in a vermiculite/peat mixture as medium. As in a hydroponic system, nutrients are added to the irrigation water. As possible strategy to increase the production of mini-tubers, the effect of nutrition was investigated. No significant differences were found in tuber production with small deviations in phosphorous and calcium nutrition. Researchers who reported significant differences in tuber production with nutrition, either corrected extreme shortages or induced it. By subjecting the root and tuber medium of potato plants to a pH-shock (nutrient solution with a pH of 3.5 for 10 hours), large amounts of tubers were initiated. A new production technique was developed where plants were grown in a hydroponic way without any root medium. In this way the right sized tubers could be removed without damaging the plant. Potato plants do not initiate tubers easily where stolons are not subjected to mechanical resistance. This problem was solved with the help of a pHshock, where significantly more tubers were formed per plant. The action of the pH-shock could be explained by a temporary inaccessibility of nutrients to the plant. Both calcium and magnesium absorpsion are hampered at a low pH, but calcium is also poorly translocated in the plant. Considering that a nutrient solution without calcium produced the same results as the pH-shock treatment in this study, one may make the assumption that an induced calcium shortage may be responsible for the low pH effect. The same effect may even be induced by giving the plants clean water for some time. Such a treatment was, however, not tested but should receive attention in further studies. The correct timing for a low pH-shock may need more research. A precise number of days after transplanting can not be prescribed since many factors such as light intensity, daylength and cultivar difference affect plant growth and tuber initiation. Environmental conditions, the physiological age and the biomass of the plant may affect the timing of the shock. A low pH-shock could not at any stage induce tubers in potato plants where unfavorable environmental conditions occurred. It seems as if the plants need to be ready for tuber initiation before the pH-shock can be successfully applied. By removing/harvesting the tubers at the required weight, the burden on the plant may be relieved enabling more tubers to form. It was experienced that the reproductive system of the potato plant is complex with a number of factors affecting it. The new production system has potential, but more developmental work is still needed. / AFRIKAANSE OPSOMMING: Aartappels is die vierde belangrikste voedelsoort in die wereld, Om te kan voorsien aan die ewigdurende verhoogde vraag na hierdie stapelvoedel deur die groeiende wereld-bevolking, sal daar maniere gevind moet word om die eindproduk goedkoper en in 'n gesonder toestand op die mark te plaas. Die knolle bestaan hoofsaaklik uit koolhidrate en stysel, alhoewel 9% van die droe gewig van knolle uit proteiene bestaan. Die aartappelknol is in sy natuurlike toestand laag in natrium en hoog in kalium en vitamien C. Hierdie protei'ene kan in al die aminosuur-behoeftes van mense voorsien wat dit dus 'n uitstekende gebalanseerde dieet maak. Die belangrikste faktor waarmee die produksie en kwaliteit van aartappels verhoog kan word is om beter voortplantingsmateriaal aan produsente te verskaf. Miniknolproduksie is 'n relatief nuwe manier om gesonde en groeikragtige voortplantingsmateriaal vinnig by die produsente te kry. Dit is egter 'n duur metode omdat die kapitaal-investering groot is. Deur meer knoUe per plant, of meer knolle per vierkante meter te produseer kan die eenheidsprys per moer verlaag word. Die sisteem wat algemeen vir miniknolproduksie gebruik word is om in vitro plantjies in 'n kweekhuis, gewoonlik in 'n vermikulietldennebas mengsel te plant. Voedingstowwe word dan soos in 'n hidrokultuurstelsel saam met die besproeiingswater toegedien. As moontlike strategie om die produksie van miniknolle by aartappels te verhoog, is die invloed van voeding daarop ondersoek. Daar is gevind dat daar nie groot verskille in knolproduksie met klein afwykings in fosfaat- en kalsium-voeding veroorsaak is nie. Navorsers wat wel betekenisvolle verskille III knolproduksie met voedingsbehandelings verkry het, het ekstreme tekorte reggestel of gemduseer. Deur die wortel- en knolmedium van aartappelplante aan 'n pH-skok (voedingsoplossing met 'n pH van 3.5 vir 10 ure) bloot te stel, is daar gevind dat 'n groot hoeveelheid knolle geinisieer word. Verder is 'n nuwe produksietegniek ontwikkel waar die plante met 'n suiwer waterkultuur metode geproduseer word, sodat knolle wat oesgereed is verwyder kan word. Aartappelplante inisieer nie maklik knolle onder hierdie omstandighede waar stolons geen meganiese weerstand ervaar nie. Met behulp van die pH-skok het daar betekenisvol meer knolle per plant gevorm. Die werking van die pH-skok kan moontlik aan 'n tydelike ontoeganklikheid van voedingstowwe toegeskryf word. Beide kalsium en magnesium word moeilik by 'n lae pH opgeneem maar kalsium word boonop moeilik deur die plant getranslokeer. Aangesien 'n voedingsoplossing sonder enige kalsium dieselfde resultate as die pHskok gelewer het, lyk dit asof 'n geinduseerde kalsium-gebrek vir die lae pH effek verantwoordelik mag wees. Die skok kan makliker en veiliger toegepas word deur vir 'n tydperk net skoon water aan die plante te gee. So 'n behandeling is egter nie beproef nie maar behoort in opvolgwerk aandag te ontvang. Die optimum tyd waarop so 'n skok toegedien moet word is nog onduidelik. Daar kan nie op 'n eksakte aantal dae na uitplant gewerk word nie, aangesien daar te veel faktore soos ligintensieteit, daglengte en kultivar is wat plante se groei en knolinisiasie kan beinvloed, Omgewingstoestande, die fisiologiese ouderdom van die plant en die hoeveelheid biomassa wat gevorm het mag bepaal wanneer die plante geskok moet word. Waar ongunstige omgewingstoestande geheers het, kon 'n lae-pHskok op geen stadium knolinisiasie induseer nie. Dit wil voorkom asof die plant vir 'n knolinisiasie fase gereed moet wees voordat 'n lae-pH-skok doeltreffend is. Dit verhoog dan bloot die aantal knolle wat per plant ontwikkel. Deur die knolle op die verlangde grootte te pluk kan die las op die plante verlig word sodat meer knolle mag vorm. Daar is ervaar dat die reproduktiewe stelsel van die aartappelplant kompleks is en dat daar baie faktore is wat dit mag beinvloed. Die nuwe produksiestelsel toon potensiaal, alhoewel verdere ontwikkelingswerk nog nodig is.

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Starch residue samples from two Australian sweet potato varieties (Beauregard and Northern Star) and two Indonesian sweet potato varieties (Bis192 and Bis183), and commercial sample of sweet potato starch residue, were studied for their pectins. Pectins were extracted using 0.1M HCl, 0.05M NaOH, 0.1M HCl/0.75%SHMP, and 0.05M NaOH/0.75% SHMP. Hydrolysis of residual starch in the cell wall of sweet potato using heat stable α-amylase and amyloglucosidase was employed prior to pectin extraction to eliminate starch contamination. Pectins were characterised for yield, moisture, ash, galacturonic acid content (GA), degree of esterification (DE), neutral sugars content and molecular weight (MW). Selected pectins were also characterised for their dispersion and gel properties. Pectin with the highest yield was compared to commercial low methoxyl pectin (GRINSTED??YF 450) for its application in milk pudding. The pectin characteristics are dependent on variety and extraction process; however, the extraction methods were dominant. Yields were between 7 and 30 % of the cell wall. Ash varied from 2.9 to 6.8%. NaOH/SHMP extraction gave higher yields and ash contents. Moisture was between 5.5 and 6.7. GA varied from 27 to 80% with the highest found in Bis192 extracted using NaOH/SHMP. DE varied between traceable and 57%. HCl extraction gave higher DE, while NaOH/SHMP caused demethylation. Neutral sugars varied from 10 to 25% where galactose was the predominant sugar, followed by arabinose, rhamnose, glucose and xylose. Pectin MW was very heterogeneous. HCl extraction gave higher MW than the others. Dispersion of sweet potato pectins displayed shear-thinning. Consistency coefficient (K) increased, while flow behaviour index (n) decreased with increasing pectin concentration. HCl-extracted pectins formed stiff sugar-gels at concentrations higher than 2%. The optimum Ca2+ concentration for NaOH/SHMP-extracted pectin gels was 20 mg/g pectin. Pectin with the highest yield and stiffness (Norther Star extracted using NaOH containing SHMP) was used for milk pudding preparation. This pudding had comparable texture to pudding prepared from commercial low methoxyl citrus pectin (GRINSTED??YF 450). Overall, this study demonstrated that pectin from sweet potato starch residue is mainly low methoxyl pectin with possible chemical structure of rhamnogalacturonan I (RGI) that has potential for food applications.

Graduation date: 1998

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