Gideon Ashworth investigates the source of Sulphur Dioxide identified in microalgae and the need for further research in this field.

The EU wide recall by Nutrisure Ltd of Organic Chlorella Power (Chlorella vulgaris) Batch O.CH-DF-160330 (BBE Mar 2019) in April 2018 due to the presence of sulphites¹ posed questions on the origins of the allergen Sulphur Dioxide (SO₂) within the microalgae.

When the recall notice was first posted, one logical explanation pointed towards whether this could be a false positive reported by the laboratory that carried out the analysis. Extensive further testing (circa 70 samples) of multiple batches of Chlorella and also Spirulina from suppliers in different continents (Europe, Asia and North America) was therefore carried out. Samples were sent to two separate UKAS accredited laboratories for SO₂ analysis, both used the optimised Monier-Williams method. The results returned positive readings at levels above the permitted limit of <10ppm for 100% of tested Spirulina and Chlorella products, across both organic and conventional lines (Table 1).

These results were alarming; not only is the presence of SO₂ and/or sulphites a breach of Organic regulations but sulphites are also a listed allergen and therefore must be declared on pack/ at point of bulk sale. Any allergen must be segregated during storage, transportation and further processing.

Another explanation for the positive result could have been that the producer had deliberately added the SO₂ either during the production process to increase yields in the growing pools or during postharvest processing for increased durability and shelf life. However, the product already has a 36-month shelf life so there seems to be no benefit in using additives to extend this. Sulphur dioxide addition had not been declared by the supplier during the rigorous approval process, nor by any other supplier or competitor in the marketplace. An alternative explanation is that the SO₂ could have been present as a contaminant in the water used for growing the algae, however investigations did not show evidence of this.

Since Chlorella powder has been traded for decades now, it is unclear why no previous complaints relating to SO₂ content of algal food products and allergies have been logged or reported.