Biological limitations to the production of processed broccoli in Tasmania

An increased pressure from international competitors on the domestic production of processed broccoli has meant that this sector needs to identify ways in which it can reduce the cost of raw produce supplied to local processors. This is best achieved through improvements in net yield and efficiencies that reduce labour costs. This thesis addresses three biological impediments to achieving improvements in these areas: Uneven harvest maturity resulting in numerous harvests; processing inefficiencies introduced by inappropriate head shape; and reduced quality from hollow stems. Investigations undertaken during the course of this work have found that the variation in the meristem diameter during inflorescence initiation explains 50% of the variation in maturity encountered at harvest. This finding highlights the importance of introducing and maintaining seedling uniformity during the transplant and establishment phases of production to minimise variation in maturity at harvest, thus reducing harvest costs. It also provides a basis for future research to focus on techniques to achieve this outcome. The study has also demonstrated the influence of head shape on processing efficiency and net yield. The more compact shape of 'Shamrock' resulted in a superior (low) ratio of the less valuable stem tissue to the more valuable floral tissue when compared to the elongated form associated with 'Marathon'. The architecture of the 'Shamrock' inflorescence also provided higher total floret yield, and more opportunity to manipulate the levels of stem material to suit seasonal requirements. The attributes associated with 'Marathon' resulted in greater processing efficiency, with the comparatively open branch structure of this variety producing more segments within factory specification, and a smaller proportion that required re-dicing. These findings establish the significant impact of head shape on net yield and processing efficiency, providing an additional tool to improve these through varietal selection. Since the early 1930's the development of hollow stems in broccoli and cauliflower has been variously attributed to boron deficiency or to factors associated with plant growth rates. The data presented in this study provides evidence that the development of hollow stem in broccoli is related to growth rate when manipulated by planting density. Evidence is also provided to show that mechanical tissue stresses generated in the stem during inflorescence development can lead to the development of stem fractures. Tissue extensibility tests revealed higher levels of radial, tangential and longitudinal extensibility in the outer pith and vascular / cortex tissues, while the central pith tissues were less extensible in all dimensions. Circumferential tension in the transverse plane was also found to be stored in the vascular cortex region. It is proposed that the differential mechanical properties and capacities for growth across the stem tissues may cause sufficient internal strain to cause the fracture of the less extensible and possibly weaker tissue of the central pith. The findings of this thesis have established a fundamental understanding of the physiological mechanism underlying the development of hollow stem, the importance of the transplant and establishment phases of production on head uniformity, and the significant influence of head shape on net yield and processing efficiency. Application of this knowledge by the Tasmanian broccoli processing industry will assist in improvements in net yield and provide opportunities for efficiencies that reduce labour costs.