Microgreens: Optimising Seed Density and Exploring the Influence of White Light and White Light Supplemented with UV-A Radiation

Microgreens are gaining prominence for their high nutritional value, rapid growth cycle, and suitability for controlled-environment agriculture (CEA). Among key agronomic factors, seed density critically influences both yield and microbial safety, and it also impacts production cost. This study evaluated: (1) the effects of various seed densities on the yield and microbial load of cress (<i>Lepidium sativum L.</i>), rocket (<i>Eruca sativa</i>), and pea (<i>Pisum sativum L.</i>); and (2) the influence of supplemental UV-A radiation on the biomass, microbial load, and phytochemical profile of pea microgreens. The study found that fresh biomass increased with increasing seed density across all species up to a threshold, achieving maximum yields at 12 seeds/cm² for cress and rocket and 2 seeds/cm² for pea. However, higher seed densities were also associated with increased levels of total aerobic bacteria (TAB), <i>Enterobacteriaceae</i>, and fungi, which could pose an increased risk of microbial hazards concerning food safety, e.g., TAB in cress increasing from 7.04 ± 0.09 to 7.94 ± 0.17 log₁₀CFU/g as seed density increases from 6 to 14 seeds/cm². Initially white light supplemented with UV-A recorded a lower yield (11 g) compared to white light (13 g), but the final biomass was comparable under both lights, with microbial load remaining stable at ~3.8-4.2 log₁₀ CFU/g. A temporary increase in carotenoids exhibited significantly higher levels (2.00± 0.29 µg/mg DW) under white light supplemented with UV-A radiation compared to white light alone (1.48 ± 0.23 µg/mg DW). However, these increases were not maintained throughout the growing period. These results indicate that optimising seed density in these species is vital for balancing productivity and food safety, and continuous UV-A exposure did not lead to sustained higher phytochemical levels or reduced microbial load compared with white light alone.