Physicochemical, agronomical and microbiological evaluation of alternative growing media for the production of rapini (Brassica rapa L.) microgreens

BACKGROUND: Peat-based mixes and synthetic mats are the main substrates used for microgreens production. However, both are expensive and non-renewable. Recycled fibrous materials may represent low-cost and renewable alternative substrates. Recycled textile-fiber (TF; polyester, cotton and polyurethane traces) and jute-kenaf-fiber (JKF; 85% jute, 15% kenaf-fibers) mats were characterized and compared with peat and Sure to Grow® (Sure to Grow, Beachwood, OH, USA; http://suretogrow.com) (STG; 100% polyethylene-terephthalate) for the production of rapini (Brassica rapa L.; Broccoletto group) microgreens. RESULTS: All substrates had suitable physicochemical properties for the production of microgreens. On average, microgreens fresh yield was 1502 g m⁻² in peat, TF and JKF, and was 13.1% lower with STG. Peat-grown microgreen shoots had a higher concentration of K⁺ and SO₄ ^{2 −}and a two-fold higher NO₃ ⁻ concentration [1959 versus 940 mg kg⁻¹ fresh weight (FW)] than those grown on STG, TF and JKF. At harvest, substrates did not influence microgreens aerobic bacterial populations (log 6.48 CFU g⁻¹ FW). Peat- and JKF-grown microgreens had higher yeast-mould counts than TF- and STG microgreens (log 2.64 versus 1.80 CFU g⁻¹ FW). Peat-grown microgreens had the highest population of Enterobacteriaceae (log 5.46 ± 0.82 CFU g⁻¹) and Escherichia coli (log 1.46 ± 0.15 CFU g⁻¹). Escherichia coli was not detected in microgreens grown on other media. CONCLUSION: TF and JKF may be valid alternatives to peat and STG because both ensured a competitive yield, low nitrate content and a similar or higher microbiological quality.