Formation of Aldehydes and Alcohols from α-Keto Acids in Strawberries

Studies on Flavor Development in Strawberries (Part VI)

Abstract

Strawberry homogenate (pH6.5) was prepared by blending strawberries with two times as much of 0.2M phosphate buffer (pH7.5) in an ice bath. Five ml of 0.1M α-keto acid-Na was added to 5ml of strawberry homogenate in a 20ml vial vessel, and was sealed with a silicone rubber septum. After 2hr incubation at 25°C, 3ml of vapor sample was taken out through the silicone rubber septum by means of a gas-tight syringe and analyzed by gas chromatography (GC) and GC-mass spectrometry. All α-keto acids examined (e.g., pyruvic, α-keto-n-butyric, α-ketoisovaleric, α-keto-nvaleric, α-ketoisocaproic and α-keto-n-caproic acids) were converted to aldehydes (e.g., acetaldehyde, propionaldehyde, isobutyraldehtde, n-butyraldehyde, iso-valeraldehyde and n-valeraldehyde). The optimum reaction pH was between 6.0 and 6.5. This reaction is assumed to be attributed to pyruvate decarboxylase (2-oxoacid carboxy-lyase; E.C. 4.1.1.1) on the ground that the addition of thiamine pyrophosphate and/or Mg²⁺ enhanced the formation of aldehyde. The addition of alcohol dehydrogenase from strawberry seeds to the reaction mixture together with NADH resulted in the simultaneous production of aldehyde and alcohol from α-keto acid. In this paper, the pathway from α-keto acid to ester in strawberry is briefly discussed based on the all the results obtained in the present examination and from our previous reports.