Identification of the Medicinal Off-Flavor Compound Formed from Ascorbic Acid and (E)-Hex-2-enal

Abstract

A test apple beverage made up of apple juice (20%), high-fructose corn syrup (11.5%), citric acid (0.43%), trisodium citrate (0.02%), apple-odor flavor (0.1%), and ascorbic acid (0.02%) was stored at 40 °C and then analyzed for the change of odor in the beverage. Although no thermoacidophilic bacteria (TAB) were detected, a medicinal off-flavor was perceived after the 8 weeks of storage. Model experiments on the ingredients of the test apple beverage revealed that the off-flavor compound had been formed by ascorbic acid and (E)-hex-2-enal. Synthesis and NMR (¹H, ¹³C, HMQC, and HMBC) analyses identified the compound as 6-propylbenzofuran-7-ol. The odor quality, retention index (RI), and mass spectrum of synthetic 6-propylbenzofuran-7-ol were identical with those of the medicinal odor compound from the test apple beverage. Sensory evaluation revealed the recognition thresholds for medicinal odor were 31.4 ppb in water and 24.0 ppb in apple beverage, and the detection thresholds were 19.6 ppb in water and 8.6 ppb in apple beverage, respectively. The quantified concentration of 6-propylbenzofuran-7-ol formed in test apple beverage was 90 ppb, approximately. This concentration was well above the odor threshold, so it was concluded that the compound was the source of the medicinal off-flavor.

Ascorbic acid and white wine production: a review of beneficial versus detrimental impacts

“…Recently, an off‐flavour compound has been reported after the reaction of ascorbic acid with α,β‐unsaturated aldehydes such as (E)‐hex‐2‐enal (Sakamaki et al. ). The compound formed is 6‐propylbenzofuran‐7‐ol and is described as having a medicinal flavour with a detection threshold of 19.6 μg/L in water (not known for wine).…”

Factors Influencing the Formation of Medicinal Off-Flavor from Ascorbic Acid and α,β-Unsaturated Aldehydes

“…In the previous work, the orthonasal odor thresholds of benzofuranol 1 were determined in water and in apple beverage by means of the triangle test. The human recognition thresholds for medicinal odor in that study were 31.4 μg/L in water and 24.0 μg/L in apple beverage, while the detection thresholds were 19.6 μg/L in water and 8.6 μg/L in apple beverage . The concentrations of benzofuranol 1 were higher than the odor thresholds in all model solutions.…”

“…Given the strong likelihood that different conditions would also change the amounts of all three products, that is, benzofuranol 1 , diol 2 , and aldehyde 3 via this multistep mechanism, we decided to quantitate all three compounds in the model solutions by HPLC. The compound 6-propylbenzofuran-7-ol ( 1 ) was first detected from a test apple beverage stored at 40 °C for 8 weeks . For the present experiments the generation of the compounds was accelerated by increasing the temperature to 50–70 °C and shortening the storage period to 1 week.…”

“…In a previous work 6-propylbenzofuran-7-ol ( 1 ) was identified as a medicinal off-flavor compound observed after storage of a test apple beverage at 40 °C for 8 weeks . The main constituents responsible for the formation of the off-flavor compound in the test apple beverage turned out to be ascorbic acid and ( E )-hex-2-enal.…”

“…The compounds identified as products and intermediates of the reaction of ascorbic acid and ( E )-hex-2-enal. 6-Propylbenzofuran-7-ol ( 1 ), 2,3-dihydroxy-6-propylbenzofuran-3,7-diol ( 2 ), 3-(2-furoyl)­hexanal ( 3 ), 1,3,7-trioxa-8-oxo-5,9,12-trihydroxy-10-propyltricyclo-[4.3.2.0 2,6 .0 2,9 ]-dodecane ( 8 ) . Identification procedure: (a) NMR ( 1 H, 13 C, HMQC, and HMBC), (b) HRMS, (c) FT-IR, (d) NMR ( 1 H, 13 C), (e) GC–MS.…”