Effect of variety, harvest and storage time, defoliation and nitrogen application on the physical and biochemical properties of potato tubers in relation to bruise susceptibility

Bruising of potato tubers leads to losses of 20% of the UK annual crop. The relationship between bruising, tuber physical and mechanical properties, and composition of phenolic acids, tyrosine and cell wall monosaccharides was explored in this thesis. Three field trials were undertaken and the varieties Lady Rosetta (LR), Maris Piper (MP) and Russet Burbank (RB) were grown in replicate field plots. Field trial 1 was designed to investigate the effect of harvest time and defoliation; field trial 2 was designed to investigate the effect of harvest and storage time and a third field trial was undertaken to investigate the effect of nitrogen application to soil (in variety LR only). Bruising was induced using a falling bolt for severe bruising and bruising index assessment. Weight, specific gravity and oxidative potential were also measured. Cortex and skin mechanical properties were measured using a TA.XT2i Texture Analyser. Phenolic acids, tyrosine and cell wall monosaccharides were analysed chemically using HPLC. The results from the field trials showed that tubers harvested ~ 150 days after planting varied in susceptibility to bruising for MP (11-60%), LR (14-52%) and RB (50-92%). Earlier harvest (98-139 days) showed lower incidence of bruising for MP (0-16.7%) and LR (17-23%) but not always for RB (0-66.7%). Late harvest (180 days) presented high incidence of bruising for all varieties varying from 81-88%. Short storage periods (until January) did not increase bruising significantly. Long storage periods (March) increased incidence of bruising for all varieties, and is associated with higher specific gravity, higher tissue deformability and higher phenolic acid and tyrosine levels. Potato plants defoliated 49 days before harvest showed lower bruising incidence than undefoliated samples, but had significantly (p<0.05) lower weight. Application of nitrogen increased weight of tubers and was associated with higher bruising incidence of LR when tubers were harvested later than 92 days after planting. Tyrosine levels or specific gravity were not always associated with highest bruising incidence. Hot dry conditions during tuber development (observed in field trial 2) was associated with early plant senescence and high tuber bruising incidence. In conclusion, bruising is affected by agricultural and post-harvest practices, and is determined by a number of physical and biochemical factors that vary between variety. The factors determining bruising seem to be dependent upon variety and the maturity of the tubers at harvest. Understanding these factors will help growers manage their crop to optimize quality and minimize waste.