Onion bulb size differentially affects bulb quality attributes by mediating mineral nutrients and targeted major metabolites during long-term cold storage in commercial packinghouse

Abstract The late-season onion cultivars were stored at 0 °C and 70% relative humidity (RH) for nine months to investigate the effects of cold storage on postharvest physiological quality, mineral nutrients, and targeted major metabolites of onion bulbs of different sizes: small (S), medium (M), large (L), extra-large (XL), and double extra-large (2XL). Among physiological responses, peel color variables, such as peel lightness ( L *), chroma, and hue angle ( h °) were significantly changed, while smallest bulb size onions changed the highest levels of peel L * and h °, compared to the others bulb sizes after long-term cold storage. Moreover, moisture loss during cold storage induced a decrease in bulb firmness and led to an increase in sweetness across all sizes of onion bulbs. Additionally, potassium, magnesium, total carbon, glucose, and lactic acid, free amino acids, such as tyrosine, phenylalanine, lysine, valine, tryptophan, leucine, isoleucine, γ-aminobutyric acid, and aspartic acid, and individual phenolic compounds, particularly quercetin and spiraeoside varied according to the different bulb sizes. Moreover, the targeted metabolomic analysis revealed a more dispersed metabolomic association at harvest. Conversely, after long-term cold storage, the major targeted metabolites, particularly sucrose, fructose, glucose, mineral nutrients, and free amino acids exhibited close interlinkages. The amino acids responses demonstrated that small- and medium-sized onion bulbs maintained higher storage quality than bulbs of other sizes. These findings suggest that long-term cold storage alters onion bulb quality by mediating biochemical parameter profiles and compositions. However, this effect was less significant for smaller onion bulbs.