Acid–base titration endpoint considerations in honey

During the discussion of an article about the determination of YAN by titration with formaldehyde, I was alerted by Dan McFeeley about the possibility of getting incorrect results in the determination of the endpoint due to the action of gluconic acid naturally present in honey.

Gluconic acid (C6H12O7) is a non-corrosive, non-volatile, mild organic acid. Non toxic and readily biodegradable (98% after 2 days), it occurs naturally in plants, fruits, wine (up to 0.25%) and honey (up to 1%).
In the European Parliament and Council Directive No. 95/2/EC, gluconic acid is listed as a generally permitted food additive (E574). The US FDA (Food and Drug Administration) has assigned gluconic acid a GRAS (generally recognized as safe) status and its use in foodstuff is permitted without limitation.

In honey the gluconic acid is produced when glucose is converted in glucono-δ-lactone by the enzyme glucose oxidase in the presence of oxygen. In this reaction is produced Hydrogen Peroxide (H2O2) – the main agent responsible for antibacterial activity in most honeys. The glucono-δ-lactone then spontaneously hydrolyses into gluconic acid. This reaction is pH dependent and W. Franke collected in 1963 some data concerning the relative activity of glucose oxidase at different pH levels and reported 5 and 35% activity at pH=2.0 and 3.0, respectively, based on 100% activity at pH=5.6.

Formula of gluconic acid (A) and glucono-δ-lactone (B)

In honey environments, like mead, Gluconic acid co-exists with its lactone, glucono-δ-lactone. When we titrate a base (sodium hydroxide), the rising pH triggers the reaction and the lactone will hydrolyse to its parent compound, in this case gluconic acid, lowering the pH again and restoring the acid level of the honey. This reaction is very fast, only detected with a pH meter, and is the reason for the unstable endpoint problem in titration.

This reaction is first identified in honey by John W. White in 1958 when he describe a new method of measuring TA in honey considering the reaction lactone / acid (see: Total Acidity (TA) determination in honey). More recently, Dan McFeeley published in Bee Culture magazine (September 2002) a study when he confirm the application of the lactone / acid reaction to mead.