Studies on macadamia nut quality

Du Preez, Alwyn Britz (2015-04)

Thesis (MSc)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: The South African macadamia industry is centred in the sub-tropical regions of South Africa with 40% of the plantings in the Lowveld. Growers receive higher pay-outs for high kernel recovery and unblemished (not discoloured) whole kernels. It is known that the same cultivar in the Lowveld region, produces nuts that differ in kernel recovery, whole kernel recovery and kernel discolouration. Therefore to develop optimal management strategies to maximize productivity and profitability of macadamias, factors that influence kernel recovery, whole kernel recovery and kernel discolouration needed to be investigated. The fruit structures are formed the first 90 days after anthesis and the fruit continues to grow until 12 to 15 weeks after anthesis until the shell hardens. Climate, soil moisture, cross-pollination and nutrition influence this process which determines the shell thickness and kernel size which in turn both determine kernel recovery. A large set of historical data from different regions were used to establish and isolate possible factors involved in kernel recovery, whole kernel recovery and kernel discolouration. These differed between the six regions over two seasons. High kernel recovery was associated with high orchard altitude, good cross-pollination, high crop load (high yield), early season harvesting and processing of nut-in-shell (NIS), high leaf boron concentrations in Nov., water management using deficit irrigation and low daily maximum relative humidity during the nut growth stage (Oct. to middle Jan.). High whole kernel recovery was associated with high kernel recovery, early season harvesting and processing of NIS, Bungay curing system of NIS compared to ambient air, low vapour pressure deficit during the nut maturation period (middle Jan. to harvest), elevated leaf boron and copper concentrations and low manganese leaf concentrations in Nov. High crop load, no cross pollination, low leaf nitrogen and zinc and high leaf potassium concentrations in Nov. were associated with low kernel discolouration. In order to develop possible orchard practices that increase kernel recovery, whole kernel recovery and decrease kernel discolouration, two irrigation trials and one kaolin trial were conducted. In the two irrigation trials, water stress was induced over two growing seasons (2011-2013) by applying different levels of irrigation at different phenological stages. Kernel recovery was not affected by any of the treatments, but water stress could not be applied continuously due to frequent high rainfall. Moderate water stress did not influence yield, only trees that were over watered during a drier premature nut drop stage during the 2011/12 season increased yield, although it could not be repeated the following season during a wetter premature nut drop stage. In the kaolin trial, the efficacy of kaolin foliar application was evaluated to reduce heat stress. Kaolin applications did not affect kernel recovery, nut yield or quality. Temperature during the study was not continuously high (>30 ⁰C), thus heat stress could not be mitigated. We did however establish that up to five layers of foliar applied kaolin did not significantly reduce individual leaf photosynthesis.

AFRIKAANSE OPSOMMING: Die makadamia-industrie in Suid-Afrika, is gesentreer in die sub-tropiese streke van die land met 40% van die aanplantings in die Laeveld. Produsente ontvang hoër uitbetalings vir neute wat 'n hoë uitkraak (kernherwinning) persentasie lewer asook ongeskonde (nie verkleurde), heel-kern neute. Daar is gevind dat dieselfde kultivar verskil ten opsigte van kernherwinning, heel-kernherwinning en kernverkleuring in die Laeveld. Om 'n optimale bestuurstrategie te ontwikkel en so maksimale opbrengs en wins te verkry, moes die faktore wat kernherwinning, heel-kernherwinning en kernverkleuring beïnvloed ondersoek word. Die eerste 90 dae na blom word die vrugstrukture gevorm en vrugte groei tot en met 12 tot 15 weke na volblom totdat die dop verhard. Klimaat, grondvog, kruisbestuiwing en voedingstowwe beïnvloed die prosesse wat dopdikte en kerngrootte beïnvloed en wat beide kernherwinning bepaal. 'n Groot stel historiese data vanaf verskillende streke is gebruik om die moontlike faktore wat kernherwinning, heel-kernherwinning en kernverkleuring beïnvloed te bepaal. Hierdie aspekte het verskil in die ses streke oor twee seisoene. Hoë kernherwinning was geassosieer met hoër liggende boorde (hoogte bo see spieël), goeie kruisbestuiwing, hoë oeslading, vroeë seisoen oes en prosessering van neut-in-dop, hoë boor blaarkonsentrasie in Nov., waterbestuur met onthoudingsbesproeïng en lae daaglikse maksimum relatiewe humiditeit gedurende die neut-groei-stadium (Okt. tot middel Jan.). Hoë heel-kernherwinning was geassosieer met hoë kernherwinning, vroeë seisoen oes en prosessering van neut-in-dop, Bungay droging, lae waterdampdruk tekort gedurende die neut-rypwording stadium (Jan. tot oes), hoë boor en koper blaarkonsentrasies en lae mangaan blaarkonsentrasie in Nov. Hoë oeslading, geen kruisbestuiwing, lae stikstof, sink en hoë kalium blaarkonsentrasies in Nov. was geassosieer met lae kernverkleuring. Twee besproeïngsproewe en een kaolienproef is uitgevoer om moontlike boord praktyke te ontwikkel wat kernherwinning en heel-kernherwinning verhoog en kernverkleuring verlaag. In die twee besproeïngsproewe is watertekorte aangewend oor twee seisoene (2011-2013) deur verskillende hoeveelhede te besproei gedurende verskillende fenologiese stadiums. Kernherwinning was nie geaffekteer deur die behandelings nie, maar dit moet ingedagte gehou word dat watertekorte nie deurlopend toegepas kon word nie as gevolg van hoë reënval. Gematigde watertekorte het nie die opbrengs beïnvloed nie, slegs bome wat oor-besproei was in ’n droeër voor-rypwording-neut-val stadium in die 2011/12 seisoen het 'n verhoging in opbrengs getoon, maar dit kon nie herhaal word die daaropvolgende natter seisoen nie. In die kaolienproef, is die gebruik van kaolien blaarbespuiting geëvalueer om hitte stress te verminder. Kaolienbespuitings het geen effek op kernherwinning, neut opbrengs of neutkwaliteit gehad nie. Temperature gedurende die studie was nie deurlopend hoog nie (>30 0C) en dus kon hitte stress nie gemanipuleer word nie. Daar is wel vasgestel dat tot vyf lae kaolien nie enkel-blaar fotosintese verminder het nie.

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