From straw to salmon: a technical design and energy balance for production of yeast oil for fish feed from wheat straw.

BACKGROUND: Aquaculture is a major user of plant-derived feed ingredients, such as vegetable oil. Production of vegetable oil and protein is generally more energy-intensive than production of the marine ingredients they replace, so increasing inclusion of vegetable ingredients increases the energy demand of the feed. Microbial oils, such as yeast oil made by fermentation of lignocellulosic hydrolysate, have been proposed as a complement to plant oils, but energy assessments of microbial oil production are needed. This study presents a mass and energy balance for a biorefinery producing yeast oil through conversion of wheat straw hydrolysate, with co-production of biomethane and power. RESULTS: The results showed that 1 tonne of yeast oil (37 GJ) would require 9.2 tonnes of straw, 14.7 GJ in fossil primary energy demand, 14.6 GJ of process electricity and 13.3 GJ of process heat, while 21.5 GJ of biomethane (430 kg) and 6 GJ of excess power would be generated simultaneously. By applying economic allocation, the fossil primary energy demand was estimated to 11.9 GJ per tonne oil. CONCLUSIONS: Fossil primary energy demand for yeast oil in the four scenarios studied was estimated to be 10-38% lower than for the commonly used rapeseed oil and process energy demand could be met by parallel combustion of lignin residues. Therefore, feed oil can be produced from existing non-food biomass without causing agricultural expansion.

Oleaginous yeast as a component in fish feed.

This study investigates the replacement of vegetable oil (VO) in aquaculture feed for Arctic char (Salvelinus alpinus) with oil produced by the oleaginous yeast Lipomyces starkeyi grown in lignocellulose (wheat straw) hydrolysate. VO is extensively used to partially replace fish oil in aquaculture feed, which can be seen as non-sustainable. VO itself is becoming a limited resource. Plant oils are used in many different applications, including food, feed and biodiesel. Its replacement in non-food applications is desirable. For this purpose, yeast cells containing 43% lipids per g dry weight were mechanically disrupted and incorporated into the fish feed. There were no significant differences in this pilot study, regarding weight and length gain, feed conversion ratio, specific growth rate, condition factor and hepatosomatic index between the control and the yeast oil fed group. Fatty and amino acid composition of diet from both groups was comparable. Our results in fish demonstrate that it is possible to replace VO by yeast oil produced from lignocellulose, which may broaden the range of raw materials for food production and add value to residual products of agriculture and forestry.

Enhanced ethanol production from wheat straw by integrated storage and pre-treatment (ISP).

Integrated storage and pre-treatment (ISP) combines biopreservation of moist material under airtight conditions and pre-treatment. Moist wheat straw was inoculated with the biocontrol yeast Wickerhamomyces anomalus, the xylan degrading yeast Scheffersomyces stipitis or a co-culture of both. The samples and non-inoculated controls were stored at 4 or 15 °C. The non-inoculated controls were heavily contaminated with moulds, in contrast to the samples inoculated with W. anomalus or S. stipitis. These two yeasts were able to grow on wheat straw as sole source of nutrients. When ethanol was produced from moist wheat straw stored for four weeks at 4 °C with S. stipitis, an up to 40% enhanced yield (final yield 0.15 g ethanol per g straw dry weight) was obtained compared to a dry sample (0.107 g/g). In all other moist samples, stored for four weeks at 4 °C or 15 °C, 6-35% higher yields were obtained. Thus, energy efficient bio-preservation can improve the pre-treatment efficiency for lignocellulose biomass, which is a critical bottleneck in its conversion to biofuels.

Nutrient effects on biocontrol of Penicillium roqueforti by Pichia anomala J121 during airtight storage of wheat.

The biocontrol yeast Pichia anomala inhibits the growth of a variety of mold species. We examined the mechanism underlying the inhibition of the grain spoilage mold Penicillium roqueforti by the biocontrol yeast P. anomala J121 during airtight storage. The biocontrol effect in a model grain silo with moist wheat (water activity of 0.96) was enhanced when complex medium, maltose, or glucose was added. Supplementation with additional nitrogen or vitamin sources did not affect the biocontrol activity of the yeast. The addition of complex medium or glucose did not significantly influence the yeast cell numbers in the silos, whether in the presence or absence of P. roqueforti. Mold growth was not influenced by the addition of nutrients, if cultivated without yeast. The products of glucose metabolism, mainly ethanol and ethyl acetate, increased after glucose addition to P. anomala-inoculated treatments. Our results suggest that neither competition for nutrients nor production of a glucose-repressible cell wall lytic enzyme is the main mode of action of biocontrol by P. anomala in this grain system. Instead, the mold-inhibiting effect probably is due to the antifungal action of metabolites, most likely a combination of ethyl acetate and ethanol, derived from glycolysis. The discovery that sugar amendments enhance the biocontrol effect of P. anomala suggests novel ways of formulating biocontrol yeasts.

Amino acid supplementation, controlled oxygen limitation and sequential double induction improves heterologous xylanase production by Pichia stipitis.

Heterologous endo-beta-1,4-xylanase was produced by Pichia stipitis under control of the hypoxia-inducible PsADH2-promoter in a high-cell-density culture. After promoter induction by a shift to oxygen limitation, different aeration rates (oxygen transfer rates) were applied while maintaining oxygen-limitation. Initially, enzyme production was higher in oxygen-limited cultures with high rates of oxygen transfer, although the maximum xylanase activity was not significantly influenced. Amino acid supplementation increased the production of the heterologous endo-beta-1,4-xylanase significantly in highly aerated oxygen-limited cultures, until glucose was depleted. A slight second induction of the promoter was observed in all cultures after the glucose had been consumed. The second induction was most obvious in amino acid-supplemented cultures with higher oxygen transfer rates during oxygen limitation. When such oxygen-limited cultures were shifted back to fully aerobic conditions, a significant re-induction of heterologous endo-beta-1,4-xylanase production was observed. Re-induction was accompanied by ethanol consumption. A similar protein production pattern was observed when cultures were first grown on ethanol as sole carbon source and subsequently glucose and oxygen limitation were applied. Thus, we present the first expression system in yeast with a sequential double-inducible promoter.