Predicting Storage Life of Peeled Onions (Allium cepa L.)

Abstract

Peeled onions (Allium cepa L.) are high value ready-to-use vegetable products that meet the needs of consumers. However, peeled onions have a limited storage life due to sprouting, rooting and mould growth. This study aimed to predict storage life of peeled onions so that to guide the processing and storage within the limit of innate storage potential, therefore, reduce product loss. Physical (firmness, NIR), chemical (dry matter content, soluble solids content, pH values), and physiological (skin properties, sprout length, root length, the development of mouldy base, respiration rate) attributes were measured on four batches of commercially stored onions using both destructive and non-destructive technologies. The measurements were made before and after peeling when onions were equilibrated to five temperatures (2, 4, 10, 15 and 20°C). Data were analysed to quantify the effects of peeling and equilibration temperature on measured values, and to understand correlations among the attributes with respect to estimating initial sprout length at peeling. Peeled onions were then stored in air or in controlled atmospheres (15% O2 & 5% CO2, 10% O2 & 10% CO2 or 5% O2 & 3% CO2) at 2 to 20 °C to monitor sprout, root and mould growth. The results indicated that firmness, respiration rate (rCO2) and internal CO2 concentration measured on intact onions were highly correlated with the values measured after peeling; an increase in onion temperature did not affect measured values of firmness but increased rCO2 and internal CO2 concentration; firmness had the strongest correlation with sprout length and regression models were established to estimate sprout length based on firmness measured using a texture meter or an Aweta™ acoustic firmness sensor (AFS); near-infrared spectroscopy (NIR) had no advantage over firmness measurements in estimating sprout length, but can be used to estimate dry matter content (DM) and soluble solids content (SSC); onions at more advanced sprouting stages tended to be soft with small size, thin skin, low dry matter content (DM), low soluble solids content (SSC) and high rCO2; mathematical models to describe sprout, root and mould growth at different temperatures and atmospheres were established for each batch of peeled onions. The models need further improvement and validation before being used to predict storage life of peeled onions given the limitations of this one-year study.