Comparison of Mono- and Di-Saccharides Release in Aqueous Solutions by Raw or Fried Dice of Onion (Allium Cepa L.) Bulbs using Quantitative Nuclear Magnetic Resonance (qNMR)

Although onion bulb tissues, either raw or thermally processed, are widely used as culinary ingredients in homes, in restaurants, and in the food industry, especially for sauces, little is known about the chemical constituents released from such systems. To get a straightforward and fast analysis of sugars released from onion dice soaked in model aqueous solutions, quantitative nuclear magnetic resonance (qNMR) spectroscopy was applied, and the effect of a preliminary thermal processing in oil was investigated. Soaking of raw or fried onion bulb dice at room temperature was followed for 11 d as a model of long-term storage. For the Armstrong cultivar, the extracted dry matter (in milligrams per gram of fresh weight) as well as the content in 3 sugars (glucose, fructose, and sucrose) increased up-to a maximum after about 48 h of soaking. Frying induces no measurable new water-soluble compounds. However, extraction kinetics are different (about 3 times faster with frying). Using additional microscopic studies, a possible extraction mechanism is proposed: compounds from sap-including sugars-would diffuse through conductive tissue channels.