Changes in the composition of beetroot (Beta vulgaris L) during their growth period : a thesis presented in partial fulfilment of the requirements for the degree of the Master of Food Technology at Massey University, Auckland, New Zealand
Beetroot (Beta vulgaris L) is a root vegetable grown widely in the world. The beetroot contains high concentrations of red pigments predominantly betalains and have been reported to be beneficial for health. Beetroot have also been reported to assist with human performance as they are reported to contain high nitrate concentrations which assist with lowering oxygen consumption. Beetroot composition is affected by length of growth time, cultivar and environmental conditions. The aims of this project were to understand the effects of growth time on the accumulation of dry matter, sugars, total phenolic content, betacyanins and betaxanthins, nitrate and nitrite in beetroot. Beetroot were harvested from two plantings, from a commercial grower, with ‘Pablo’ planted in September 2018 and three cultivars, ‘Pablo’, ‘Monty’ and ‘Betty’ planted in December 2018. Medium (size between 5.5 and 8.0 cm in diameter) and large (size >8.0 cm in diameter) ‘Pablo’ beetroot from the September planting were harvested weekly between 110 and 138 days of growth. Medium and large ‘Pablo’, ‘Monty’, and ‘Betty’ were harvested weekly from the December 2018 planting between 70 and 112 days of growth. Key quality and composition parameters of beetroot were monitored: size, weight, dry matter content, percentage of soluble solids, pH, titratable acidity, total phenolic content (TPC), sugar concentration, betacyanin and betaxanthin concentrations, nitrate and nitrite concentrations. The results indicated that the dry matter of beetroot did not change significantly on most harvest dates for the three cultivars grown between day 70 and day 112. The TPC in juice gradually decreased by 42 to 30 % from day 70 to day 98 for the medium beetroot of three cultivars. Sucrose was the only soluble sugar identified in the beetroot juice samples. The sucrose concentration of the juice extracted from the three beetroot cultivars varied between 62.68 and 99.80 g/L over the 70 to 112 day growth period. For medium beetroot of all three cultivars, the betacyanin concentration in the juice fluctuated between 680 and 1544 mg/L while the betaxanthin concentration in juice gradually increased from 431 to 484 mg/L on day 70 to 565 to 763 mg/L on day 105. Overall, the nitrate content in ‘Pablo’ beetroot gradually decreased from 1656 to 618 mg/L for medium and from 2878 to 1002 mg/L for large beetroot between day 91 and day 138. The effect of covering beetroot was investigated but results obtained were inconclusive. After a 21-day postharvest storage in the dark at 4 ± 1°C the beetroot composition did not change. When comparing ‘Pablo’, ‘Monty’ and ‘Betty’ grown between day 70 and day 112, the recommended cultivar and harvest time for obtaining high-nitrate beetroot juice was from large ‘Pablo’ grown for between 84 and 98 days. ‘Monty’ was the largest and heaviest beetroot cultivar, had the lowest titratable acidity and the highest dry matter content. The maximum total phenolic content was from medium ‘Monty’ beetroot grown between 70 and 84 days. The recommended cultivar and harvest time for obtaining high-betalain juice was medium ‘Monty’ harvested between 91 and 105 days of growth. ‘Betty’ was the smallest in the three cultivars, medium ‘Betty’ had higher mean nitrate content (1755 mg/L) than the other two medium cultivars, however, still lower than that from the large ‘Pablo’ beetroot (2195 mg/L). In conclusion, changes in the composition for ‘Pablo’ beetroot from two plantings, ‘Monty’ and ‘Betty’ beetroot from one planting was monitored. After 21 days of postharvest dark storage (4 ± 1°C), the beetroot composition was generally stable. For a further study, the analyse of nitrate and amino acid content in beetroot taproot and leaves during their growth period are recommended to better understand the nitrogen cycle in beetroot. A high-performance-liquid-chromatography technique is recommended to identify and quantify the relative compounds participating in betalain biosynthesis.