Progress has been made in the exploitation of yeast as an inert carrier of small molecules. Commercial products consisting of mineral-enriched yeast have been available for some time. Zinc-enriched yeast cells are being used in the brewing industry and selenium-enriched products have been approved for use as nutrient supplements in food. The potential for encapsulating other chemicals inside yeast is less widely known. Materials that can be encapsulated in spent yeast include flavours, pesticides and active pharmaceutical ingredients (APIs).
Yeast is a natural alternative to other encapsulation materials and offers unique performance benefits. A number of encapsulation technologies using yeast have been developed and refined. The use of yeast cells as biocapsules was first considered in the 1970s when Serozym Laboratories discovered that cells (Saccharomyces cerevisiae), treated with a plasmolyzer, could be used to absorb water soluble substances for use in medical, cosmetic and food products. Yeast, widely available in large volumes as a by-product of fermentation processes, was identified as a cost effective material for use in these global industries.
Yeast has a complex structure; it comprises a dense absorbent cytoplasm rich in organelles, lipid membranes and lipid droplets, protected by a 7-12 nm lipid bilayer and heterogeneous cell wall. The surrounding cell wall is typically up to 200 nm thick, rich in beta-glucan, mannoproteins and chitin and provides robust protection to the cell contents as can be seen in Figure 1.
In some cases, materials can be stabilized in yeast at levels as high as 45% by weight of the powdered or granulated end product. There is potential for using waste brewery yeast or spent yeast from ethanol biofuel production with these yeast encapsulation technologies, primarily for use in food systems to improve flavour delivery, flavour stability, appearance and nutritional value. There is also potential for the encapsulation of materials for non-food applications such as pesticides, herbicides, antibacterials, antifungals and essential oils.
Traditionally excess or waste brewery yeast was sold into the malt whisky distilling industry in the UK to supplement distillers yeast and improve the flocculation properties in fermentation vessels. This was summarised in a 1985 guidance document from the Institute of Brewing and the Allied Brewery Traders’ Association. One requirement of this market is the consistent supply of viable yeast. The sale of spent Brewer’s yeast into the food processing industry has been a source of income for brewers in the past, however, the demands for higher quality ingredients at lower cost combined with the increasing costs of supply has reduced the opportunities for the brewer in this area. The supply of segregated and suitably packaged products is required to successfully penetrate the food and feed markets. Slurry, cake, dry powder and granulated products are typically available for supply. However, for food, pet food and many animal feed applications debittering to remove hop components is a prerequisite to improve palatability and may be undertaken by third party processors.
Spent yeast is typically deactivated during processing and live yeast is not required for the absorption of active ingredients to take place. The details of yeast based encapsulation using an aqueous mixing process have been described in new patents and in the scientific literature since the late 1980s.
Each individual cell acts as a sink for hydrophobic molecules and can accumulate material to a high concentration. The presence of droplets found within the 4-7 micron yeast capsule, has been visualized using confocal microscopy and Figure 3 shows essential oil droplets, coloured orange, that have accumulated to around 400 g/kg dry wt.
Absorption efficiency varies from strain to strain but follows a typical pattern of uptake (Figure 4). Little work has been done using Saccharomyces pastorianus as a feedstock but lager strains should perform just as effectively as ale strains.
A high degree of membrane and cell structural integrity remains following the encapsulation process. The intact cell wall and membrane can be clearly seen in Figure 5 following two hours of mixing with Tea Tree oil. Droplets a few tens of nanometers form initially and are distributed throughout the cytoplasm. These typically coalesce later in the process. Some droplets can be observed on the cell wall surface prior to being absorbed into the yeast.
Delivering the goods
One feature of the established process is its restriction to small hydrophobic molecules and principally molecules with octanol/water partition coefficients below Log P 4.0 and molecular weights below 600 Da. It works well, for example with volatile food flavours, some small drug molecules and a limited range of crop protection active ingredients. Recent developments enable the encapsulation of large, to up to 5000 Da, and very hydrophobic compounds in biocapsules for the first time. Potential candidates include active ingredients such as insecticides (e.g. deltramethrin, ivermectin), fungicides (e.g. carboxin, epoxiconazole), molluscicides, (e.g. fentin, methiocarb), nematicides (e.g. carbofuran), rodenticides (e.g. brodifacoum, norbormide), herbicides (e.g. oxasulfuron) and poorly soluble active pharmaceutical ingredients (Class II and Class IV drugs) (e.g. fenofibrate and ketoconazole). This new approach differs from previous biocapsules processes in one key area: the removal of water from the encapsulation process. Water is replaced by a solvent such as dimethylsulphoxide in which many difficult to formulate active ingredients are readily soluble. Dilution with water after the encapsulation process can be used to stabilize the hydrophobic molecules within the cells. Following a period of mixing, the product can be spray dried; alternatively, an aqueous dispersion can be used directly. This moves us into areas where patent licensing is required, costs of processing are high and where there are demands for dedicated or GMP yeast manufacturing. This is a long way from finding an alternative use of spent brewery yeast.
Innovation using yeast biocapsules
In conclusion, using spent yeast as an inert carrier may in some circumstances add value in providing a complex matrix material in which active ingredients can be stabilized as a reservoir or depot. Spent yeast is a potential vehicle to provide a sustained release profile in situ for food flavours or perhaps in the future for crop protection products. The technology is being commercially exploited already so there is clearly life in the old dog yet.
References and further reading
Bishop J., Nelson G., Lamb J. (1998). Microencapsulation in Yeast. Journal of Microencapsulation, 15, (6 ), 761-773.
Dardelle, G., Normand, V., Steenhoudt, M., Bouquerand, P.E., Chevalier, M., Baumgartner, P., 2007. Flavour-encapsulation and flavour-release performances of a commercial yeast-based delivery system. Food Hydrocolloids. 21, 953–960.
Dunlop Ltd 1986 UK Patent GB2162147
Duckham S. C., Burgess, A., Hinds, L. and Echlin, P. (2003) Microencapsulation in yeast cells - Structure and function: A cryo SEM approach. In: Proceedings of the 14th International Symposium on Microencapsulation, September 4-6, 2003, Singapore.
Kilcher, G., Delneri D., Duckham C. and Tirelli N. (2008) Probing (macro)molecular transport through cell walls. Faraday Discuss., 139, 199–212
Mattanovich, D., Sauer, M. and Gasse, B. (2014). Yeast biotechnology: teaching the old dog new tricks. Microbial Cell Factories. 13 (1) :34
Nelson G., Duckham S.C., Crothers M.E.D. (2006). Microencapsulation in yeast cells and Application in Drug Delivery. Polymeric Drug Delivery, ACS Symposium Series 923, Chapter 19, 268-281.
Normand V., Dardelle G., Bouquerand P-E, Nicolas L., Johnston D. J. (2005). Flavour Encapsulation in Yeasts: Limonene used as a Model System for Characterization of the Release Mechanism. Journal of Agricultural & Food Chemistry, 53, 7532-7542.