Thermally processed leeks and their chemical-physical properties

Leeks (Allium ampeloprasum L.) are commonly used as vegetables and might provide various nutrients and bioactive components in our daily diet in fresh and processed form. The aim of the research was to analyze and compare fresh, steam blanched (SB for 1.5 and 3.0 min), convective dried (CV), and microwave-vacuum (MV) dried, and SB followed by MV dried leeks.  The main parameters analyzed and used for the comparison were bioactive components (different types of phenolics), their antioxidant activity, sugars, carotenoids, and organic acids. Moisture, absorption, sorption, and color were analyzed in fresh and processed samples. For the experiment, gravimetric, spectrophotometric, and HPLC methods were used. Fresh leeks have a moisture content of approximately 88% and with the drying process, the moisture was decreased to 9% or lower for maintaining products' microbiological safety. Fresh and CV dried samples have lighter green color but steam blanching and MV processed followed by blanching darkens the sample. Carotenoid content increases with all thermal processes compared with fresh ones. The most common organic acids in the highest concentrations were oxalic, quinic, malonic, malic, and ascorbic acids. Phenolic compound content increases with the thermal processing- lower concentrations were observed in MV dried leeks but pre-processing with SB increases the concentration of phenolics. Using principal component analysis was observed that more organic acids and phenolics were determined in MV processed leeks. Analyzed sugars in leeks decreased with processing. The integrated evaluation showed that SB followed by MV drying is the most suitable method for maintaining leeks' nutritional and biological composition. The processing of leeks helps to improve the availability and increases the concentration in a majority of thermally processed leeks. That might be explained, that different processing and storage factors significantly affect phytochemicals due to weakening the tissues which help to extract them and be more available.