Algae can develop the compounds responsible for these undesirable odours as they grow, depending on growing conditions and the availability of nutrients. Other factors can also play a role in the development of these compounds, such as stagnant water, light intensity, water temperature or pH.
These compounds and their origin have been identified. In particular, the musty or earthy odour is due to a compound, geosmin, produced by the degradation of amino acids. The marine odour is linked to dimethyl sulphite, produced by the degradation of sulphur compounds contained in algae. The production of volatile organic compounds such as aldehydes and ketones is responsible for the herbaceous odour, while the oxidation of lipids is responsible for the rancid odour.
To reduce the impact of these undesirable compounds, 5 main strategies have been identified:
- Prevent their development during cultivation by selecting species and controlling cultivation conditions. Some species, for example, do not produce geosmin.
- Prevent their release by micro-encapsulating the biomass.
- Masking them with other flavours. Vanilla is often used to mask undesirable flavours.
- Prevent their development during the biomass transformation process. Thermal processes used to dry the biomass block the development of these compounds but can lead to the development of other compounds linked to a Maillard reaction, Strecker degradation (conversion of amino acids into aldehydes) or oxidation. Flash processes or the addition of antioxidants can limit these phenomena.
- Eliminate them during the process by adding deodorisation steps.
Various deodorisation processes are described in the literature for eliminating oxides, volatile compounds and free fatty acids. Thermal treatments are often used, but as these compounds have a relatively high boiling point, very high temperatures must be applied, with the risk of degrading the product.
Alternatives include treating the biomass with steam in a vacuum or in the presence of nitrogen to limit the risk of oxidation. The use of solvents such as ethanol or ion exchange resins can also produce interesting results by eliminating free fatty acids.
More recently, deodorisation processes by fermentation have been developed and applied to the manufacture of beer and other seaweed-based drinks, for example, or to the extraction of docosahexaenoic acid (DHA).
A total of 381 deodorisation technologies have been identified in the worldwide patent database, covering both bioprocesses and physicochemical processes. These patents highlight the fact that the new deodorisation strategy adopted by industry involves a search for a balance between thermal and chemical processes to maintain the quality and properties of algae, and that deodorisation of this biomass remains a challenge for the development of new food products.
Referecence: Colonia, B. S. O.; De Melo Pereira, G. V.; Carvalho, J. C. D.; Karp, S. G.; Rodrigues, C.; Soccol, V. T.; Fanka, L. S.; Soccol, C. R. Deodorization of Algae Biomass to Overcome Off-Flavors and Odor Issues for Developing New Food Products: Innovations, Trends, and Applications. Food Chem. Adv. 2023, 2, 100270.