Microgreens: Optimising Seed Density and Exploring the Influence of White Light and White Light Supplemented with...

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 (Lepidium sativum L.), rocket (Eruca sativa), and pea (Pisum sativum L.); 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), Enterobacteriaceae, 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.