Fresh and processed apple products : vacuum infiltration, texture and quality : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Science at Massey University
Apple slice texture and quality is affected by a diverse array of preharvest, postharvest and processing factors. The study described in this thesis had two primary objectives: 1) to investigate factors that influence the effectiveness of the vacuum infiltration process and thereby identify ways to enhance infiltration in difficult-to-infiltrate fruit. 2) to ascertain the effects of a range of pre- and post- harvest factors including cultivar, temperature, edible surface coatings and calcium treatments on fresh and processed apple texture and quality. Vacuum infiltration is used to replace the 8-36% of tissue volume made up by occluded gases in the commercial production of solid-pack canned apple slices. This removal: reduces textural degradation caused by thermal expansion of these gases; prevents can corrosion and off-flavour development caused by residual oxygen; and ensures that relative density of the tissue is increased sufficiently to achieve prescribed can fill weights. Vacuum infiltration is often incomplete for fruit produced in cold growing seasons and also with immature fruit. In this study, level of infiltration achieved in apple slices was affected by pre-condition of the tissue (eg. maturity, porosity, whole fruit density) and by variables that relate directly to the vacuum infiltration process (eg. vacuum time, absorption time, solution temperature). Infiltration was enhanced in fruit taken from later harvests and in fruit pre-stored for a short period at 20 °C. Key aspects of the vacuum infiltration process were investigated and the relationships between vacuum time, absorption time, and slice relative density were characterised. Reduced vacuum levels were detrimental to liquid impregnation. To maximise infiltration in 'Braeburn' fruit required: high vacuum levels (preferably > 95 kPa), vacuum times of approx 2 min, and absorption times > 6 min. Infiltration was enhanced by heating the infiltrating solution. The texture and quality of solid-pack canned apple slices is to a large extent determined by the quality of the raw product. 'Braeburn', 'Fuji' and 'Granny Smith' apples varied quite markedly in terms of textural quality, storage potential, tolerance of ambient temperatures and ultimately in their response to processing. In general, fresh and processed apple texture declined with increasing fresh fruit storage temperature and duration. Application of edible surface coatings enhanced texture and reduced free-juice content of canned slices. The level of benefit achieved varied considerably with cultivar and storage temperature and, to a more limited extent, grower line and coating concentration. Calcium application during the pre- or post-harvest phases had little effect on processed slice texture, but in some cases free-juice volume was reduced. The interrelationships between the variables under study are discussed and a conceptual model presented that describes the effects of key postharvest variables on fresh and processed fruit texture.