Investigating the discoloration of pink watermelon-flavoured vodka in an aluminium bottle

Introduction

A vodka manufacturer noticed a change in colour or their pink watermelon-flavoured vodka, from pink to colourless. Their similar product, a lime-flavoured vodka, did not experience any change in colour.

One Eighty was supplied with the bottles of both flavours of Vodka, and with samples of the vodka as produced, before discolouration, and after having been in the bottle, with the pink vodka having changed in colour.

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Investigation

Background 

The engineers at One Eighty started off by looking at the composition of the vodka, the additives, and the aluminium bottle that it was packaged in.

Vodka is generally a mixture of water and ethanol with traces of impurities and flavourings. Vodka is produced by the distillation of fermented substances made from fruits or potatoes. Apart from the alcoholic content, vodka can be classified into two groups: clear vodka and flavoured vodka. The latter consists of adding flavourings and other additives to change the taste and colour of the clear vodka respectively.

Vodka additives
Carmoisine (additive in the watermelon flavoured vodka) and has the chemical name disodium salt of 2-(-1 Naphtthylazo Sulphonic Acid)-1-Hydroxy-Naphtalene SulphonicAcid) and its chemical structure is shown in Figure 4. It is originally dark red in colour. When mixed in a water-based medium, Carmoisine gives a bright pink colour (for the watermelon flavoured vodka).

Quinoline yellow is the commercial name for disodium salt of disulphonic acid of 2-(2-Quinoly)-1,3 indandione 5-cyclohexa dienimine) and its general chemical structure is shown in Figure 5.

Packaging
The aluminium bottle used is commercially known as Sport Aluminium Bottle model DFH-600. The aluminium bottle consists of different components, namely an aluminium body, a Polypropolene lid and a Silicone O-ring seal

The next step was to look at the possible reactions that could take place between
(i) The additives and the vodka
Carmoisine and Quinoline under normal atmospheric conditions will not react with the vodka. Ethanol and water present in vodka is regarded as a medium which will interact with the additives to give a pink solution for Carmoisine and a lime green colour for Quinoline. Even though there are chemical interactions between the vodka and the additives, no reactions should occur between them.

(ii) The lacquer and the additives
Under normal conditions, amino resin does not react with ethanol or water. In the presence of the additives Carmoisine, being slight electrically charged (sodium and oxygen ion), can under some conditions (temperature and concentration of other products present) react with the amino resin. Quinoline on the other hand, is neutral and will therefore not react with the lacquer.

Experimental approach

One Eighty used a series of advanced methods, namely:

(i) Material identification through hardness tests and Handheld Spectrographic Analysis
– Testing the aluminium bottle to ascertain whether the supplier’s specification for the bottle is correct
(ii) FTIR (Fourier Transform Infrared Spectroscopy)
– Identifying the various chemical groups that are present in the lime and watermelon flavouring
(iii) Accelerated ageing test
– Placing the aluminium bottles with the watermelon-flavoured vodka in a water bath at elevated temperatures over 72 hours to investigate the long-term reactions that could occur in a short period of time.

What We Found

Results

The results collected could be summarised as follows:
(i) FTIR results of the surface of the aluminium bottle
(ii) FTIR results of the watermelon vodka as produced
(iii) FTIR results of the surface of the lime vodka as produced
(iv) FTIR results of the watermelon vodka as received from the client
(v) FTIR results of the watermelon vodka after accelerated ageing

Conclusion

Conclusions:
1. The additive used to make the watermelon-flavoured vodka pink in colour reacts with the lacquer.
2. The sample provided by the client did contain some residues of Carmoisine which confirms that the additives was added when preparing the watermelon-flavoured vodka
3. Quinoline does not react with the amino resin and hence did not experience any change in colour.
4. The change in colour is solely due to the interactions that occurred between Carmoisine and the amino resin and not because of a higher temperature. (The control sample did not change colour.)

Recommendations:
1. An alternative additive which results in a pink colouration should be used. The additive must be chemically neutral.
2. Ideally, the new additive must contain nitrogen so as to prevent any reactions with the amino resin.
3. If no alternatives can be found, a compatible stabilizer or inhibiter should be added to prevent any reactions between Carmoisine and the amino resin.
4. If the addition of stabilizers or inhibitors is not a viable option, the lacquer should be made of polyester and carbonyl compounds only.

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