Since 2003, the Pacific white shrimp has been the main shrimp species produced in aquaculture. However, its increased production has raised an already-high demand for aquaculture feeds and the necessary ingredients to manufacture them.
As small pelagic fish become increasingly overfished, commercialising marine-deprived feeds has led to further economic and ecological concerns, and more research into alternative sources of plant and microbial protein.
One such emerging alternative is torula yeast, a single cell protein (SCP) cultured on substrates that comprise a range of industrial waste including molasses, dried citrus pulp or sulphite liquor from the wood, pulp and paper industries.
The amount of substitution in feed depends on the type of yeast and how it's produced. Yeasts are relatively low in methionine but supplementation with synthetic sources of the amino acid could allow yeast to be the only protein source in a diet. Currently, yeasts are still more expensive to use compared to other vegetable-based protein sources, but hopes are high that this could change in future.
Recently, a group of researchers in Mexico, Benigno Fernández-Díaz, Martha Nieto-López and Lucía Elizabeth Cruz-Suárez, examined the use of different proportions of torula yeast and fishmeal in the diets of farmed Pacific white shrimp, headed by Dr Gamboa-Delgado. At the end of a 29-day bioassay, they found that there were no significant differences in the survival rate among treatments. The trial also indicated that torula yeast was suitable enough to replace fishmeal by up to 60 per cent in shrimp diets.
"This particular experiment was part of a series of assays deigned to evaluate different sources of microbial protein in shrimp feeds," explained Dr Gamboa-Delgado.
"We measured some of the traditional nutritional parameters and determined the relative contribution of dietary nitrogen supplied by microbial biomass and fish meal to shrimp growth. The latter was achieved through isotopic methodologies, which in turn require the use of a low number of dietary ingredients per trial to avoid overlapping of isotopic values. So we ran individual experiments to evaluate one or two microbial sources in relation to fishmeal. Yeast is the first group of microorganisms recognized as an effective feed supplement in animal nutrition, so given its good nutritional composition, we were expecting positive results in terms of growth and incorporation of dietary nitrogen into growing muscle tissue."
In the trial, juvenile shrimp from a commercial hatchery in Baja California Sur, Mexico, were placed in two 500L tanks until they had acclimated to local conditions. After being exclusively fed on a crumbled commercial diet for 15 days to establish a known isotopic baseline in shrimp tissue, the shrimp were allocated in experimental units and given one of six diets replacing the dietary nitrogen from fishmeal with increasing levels of torula yeast (produced under the name Uniprot ® (Fermex/Safmex for use in animal nutrition as a protein substitute) at 0, 7.5, 30, 60 and 100 per cent, for 29 days.
The torula yeast contained 41 per cent crude protein and represented one of two protein sources used for diet formulation, the second protein source being fishmeal. From these ingredients six isonitrogenous (40 per cent crude protein) and isoenergetic (4.6kcal/g) experimental diets were formulated.
In order to estimate the relative contributions of dietary nitrogen and dry matter supplied by both torula yeast and fishmeal, nitrogen stable isotope values were measured in ingredients, diets and muscle tissue of the shrimp. The experimental diets were given at 6:00, 9:00, 12:00, 15:00 and 18:00, with feeding rations adjusted in relation to the observed survival and number of sampled shrimp.
On days 0, 4, 8, 15 and 22, one or two shrimps were gathered at random from each tank, killed in ice/water slurry, rinsed with distilled water and their abdominal muscles dissected. The remaining shrimp were harvested for abdominal segments, exoskeleton and hindgut on day 29 (the end of the trial) for isotopic analysis.
Results of the trial showed that the overall survival rate was high at 89 per cent - 100 per cent, and that shrimp reared under the different experimental diets had significantly different mean final weights.
In terms of growth, all the diets that contained a mixture of fishmeal and torula yeast outperformed the control diets (100 per cent fishmeal and 100 per cent torula yeast). In particular, shrimp that were raised on a diet containing 85 per cent fishmeal and 15 per cent torula yeast had significantly higher final mean weights (3,822mg) compared to those raised on a diet containing only fishmeal (2,992mg). The lowest final growth was found in shrimp that were fed only torula yeast (1,873mg).
All the diets had a rapid influence on the isotopic values of muscle tissue (shrimps in all treatments had reached isotopic equilibrium with their diets by day 22) and a relatively similar contribution of dietary nitrogen from both fishmeal and torula yeast showed that both protein sources were nutritionally suitable or that nutrients from both sources complemented each other well. Diets with up to 60 per cent of torula yeast led to a similar growth as diets containing only fishmeal. This indicates that torula yeast improved the final growth of shrimp, and contributed high proportions of dietary nitrogen to growth when replacing up to 60 per cent of dietary fishmeal.
"We achieved the outcome we expected, but in similar experiments, for example using dry biomass of marine microalgae, we found significantly higher contributions of nutrients (dietary nitrogen and carbon) from fish meal than from microalgal biomass," Dr Gamboa-Delgado explained. "This could have been due to the high ash content and lower protein available in some microalgae."
Dr Gamboa-Delgado is confident that these results will prove useful for nutritionists interested in using or testing microbial sources of protein, and hopes to encourage further research in other commercial crustacean species.
Because yeast is still relatively more expensive than plant proteins, it's mostly used at low dietary levels as feed attractants or immunostimulants rather than as protein replacements, but Dr Gamboa-Delgado hopes that as the range of nutritional applications increases, biotechnology companies will develop new methods to mass-produce microbial protein from microalgae, bacteria and yeast.
Meanwhile, his research group hopes to determine the specific amino acids that are primarily transferred from microbial biomass and fishmeal, by analysing the stable isotope values of amino acids in feed ingredients and then in the muscle tissues of species fed those ingredients. The microbial biomass not only yields structural proteins but also facilitates or improves the metabolic use of nutrients delivered by other dietary ingredients. Determining the isotopic values in these nutrients may help to differentiate the source and fate of specific compounds.
To access the original research paper by Dr Gamboa-Delgado and his team, please click here: http://www.sciencedirect.com/science/article/pii/S0044848615302544