Each yeast product requires its own approach

Optimal yeast production could be compared to the optimal production of food animals. Natural and genetic selection, fine-tuned nutrition and growing environment all play critical roles in order to get the organism that is intended. Lallemand Animal Nutrition walks All About Feed through one of its numerous yeast plants.

By M. Baulez, M. Castex and L. Dussert, Lallemand Animal Nutrition, France

The final step of Titan yeast processing: Live yeast micro-encapsulating The Titan beads before packaging

Yeast is the micro-organism most used in the food industry due to its exceptional fermentative capacity. But not all yeasts are equal: There are thousands of different yeast strains in nature, each with its own genetic and metabolic characteristics. These specifications will impact the properties and activity of the end product. Hence, each strain used for a specific application in animal feed is carefully selected for its desired outcome, registered and preciously kept in a cell culture collection. Moreover, each yeast strain has its specific optimal growth requirements and the production process is adjusted accordingly. The present article gives an overview of the production process for various yeast products.

Yeast fermentation
The overall process is depicted in Figure 1. All yeast species produced at Lallemand Animal Nutrition belong to the Saccharomyces cerevisiae species. They are both anaerobic and aerobic. For industrial production, all fermentation steps are carried out under aerobic conditions in order to ensure optimal vegetative growth. Multiplication is scaled up stepwise: From a few millilitres of a mother culture of the desired strain to an industrial fermentor that can reach a few hundred m3.
The main challenges of yeast production are to ensure a proper yield (biomass) and optimal performance (metabolic activity) of the end product for users, while preventing the presence of contaminating microbes which could contaminate the whole product. These objectives can be accomplished thanks to meticulously controlled conditions and tight quality control along the process. In particular, all physico-chemical parameters such as medium composition, temperature, pH, nutrient concentration and agitation are strictly controlled during the whole process. These parameters are determinant for the product’s final activity and hence efficacy, and each yeast strain has its own optimal conditions. Hence, for each strain these conditions have been carefully determined in lab-scale fermentors and a dedicated pilot plant before scale up processing. The parameters are thoroughly controlled from seed to final formulation to ensure consistency of the final product. At the end of the fermentation growth step, the resulting biomass of cells is harvested. This is done by centrifugation and/or filtration to separate the cells from the media. Cells are washed to remove any remaining media and finally collected as a yeast cream. According to the nature of the additive produced, the harvested yeast can ­follow different paths.

Production of probiotic yeast
The very definition of a probiotic implies that the yeast cell must be live to express its beneficial effects in the host. One of the challenges of producing probiotic yeast is to ensure that the yeast remains live and active throughout the whole production process, and during further feed processing and storage, until it reaches its site of action: The digestive tract of the host. Probiotic stability is a major issue for feed manufacturers or premixers because of their high sensitivity to several stresses (temperature, pressure, moisture). To overcome these limitations, specific protective technologies have been developed. This is the case of Titan, a patented technology developed by Lallemand specifically for ­probiotic yeast. It combines an optimisation of the fermentation and drying processes with a unique coating technology. Titan enables the probiotic yeast to survive the harsh conditions of feed processing such as pelleting (heat, pressure, humidity), but also the chemical interactions with other aggressive ingredients and minerals.
At Lallemand, probiotic yeast can be produced under two different forms: Either as unprotected active dry yeast, or as Titan protected live yeast. In both cases, following harvest, the yeast cream is extruded and dried on a fluid bed dryer, resulting in particles of active dry yeast. For Titan yeast, fermentation and drying conditions differ and the active dry yeast is using a specific drying process before being coated in a batch fluid bed with a specific, lipid-based solution which has been formulated specifically to optimise yeast metabolic activity in the digestive tract and resistance to manufacturing processes such  as pelleting (Figure 2).

Production of mineral enriched yeast
Thanks to its metabolic properties, yeast can be used to produce e.g. organic selenium. Selenium yeast production process is based on the natural ability of yeast to incorporate mineral selenium from its growth medium into selenomethionine and other seleno compounds, which are easily assimilated by the animals. In this case, a ­specific yeast strain (CNCM R397), selected for its ability to incorporate a large amount of selenium, is used. Selenium is added during critical stages of fermentation for an optimal bioconversion by yeast cells. By carefully monitoring the fermentation, it is possible to optimise selenium incorporation and attain maximal and consistent levels of organic selenium. Harvesting and washing steps permit eliminating the remaining traces of inorganic selenium (more than 98% of the selenium contained in Alkosel is in its organic form). The yeast cream is pasteurised in order to completely inactivate live yeast cells, and spray dried.

Specific yeast fractions
When producing specific yeast fractions (yeast cell walls or specific yeast extracts) for animal nutrition, fermentation conditions and downstream processes are also fundamental and have critical effects on the composition, the structure and ultimately the biological efficacy of the final product. Yeast cell wall product composition is dependent on three variables: The strain used, the fermentation conditions and finally the extraction and purification processes.
Any modification of one of these variables will induce important modifications of the product composition and structure, hence its activity. Indeed, as a protection, yeast adapt their cell wall composition to the fermentation environment (temperature, pH, carbon sources, nutrients concentration such as glucose, or alcohol concentration in the case of distillery/brewery yeasts notably).
Therefore, in order to produce a consistent yeast cell wall product and to guarantee an optimal composition and structure of its active ingredients such as mannans and/or β-(1,3 and 1,6)-glucans, it is necessary to get these variables adjusted and monitored. Contrary to distillery/brewery yeast by-products, the yeast cell wall produced by Lallemand (Agrimos) are based on specific and consistent fermentation conditions. Following harvest, the yeast is processed according to a specific cracking technique allowing to separate the soluble, intracellular components from the cell walls. The latter are dried under well controlled conditions to preserve their structure.

Final steps
Finally, all batches of yeast and yeast derivative products undergo final quality control in a dedicated analytical laboratory in terms of strain identity and purity, and other parameters are monitored including pH, protein, solids and moisture content or more specific parameters depending on the product (yeast cell wall composition, free nucleotides content, amino acid profile, Vitamin B content, etc). For selenium yeast, for example, quality control is crucial to ensure that the final yeast product does contain the guaranteed content of selenium and that most of it comes in an organic form: this is ensured for every batch by routine analysis of Selenomethionine in the final product. For probiotics, viability (live cell count) must be checked and guaranteed throughout the product shelf life (a pre-requisite for probiotics under many national regulations).
Packaging is done under strict conditions using appropriate packaging material. Tight control of temperature, O2 and humidity during packaging ensures product stability and activity, in particular in the case of live yeast. Packaging material is also important as plastics have varying levels of oxygen and moisture permeability. For example, live yeast products are packaged in gas impermeable laminates the same packaging used by the US Military for long-term preservation of sensitive materials like rations.