Gregory John Oliver Martin

Autolysis of yeast cells can be induced by exposure to extreme stress, including high temperatures. This study investigated the cellular responses to heat stress during autolysis. Saccharomyces cerevisiae was heated to 60 °C either very rapidly (<4.5 min) or gradually (∼2.5 h) and then held at 60 °C for 24 h and analyzed using proteomics, electron microscopy, and biochemical analyses. Extensive proteomic remodeling occurred under both heating regimes, with 139 proteins enriched and 98 reduced by at least 10-fold. Protein markers linked to heat stress, autophagy, apoptosis, and necrosis suggested these processes occurred before cell death. While the amino acid profiles remained unchanged regardless of the heating rate, gradually heated yeast developed thicker cell walls and fragmented vacuoles, which contributed to greater protein release (90% versus 50% for rapid heating). Thus, gradual heat stress enhanced protein yield without altering the amino acid profile, providing valuable insights for improving industrial yeast extract production.

Dark anoxic incubation has been identified as a low-cost method to facilitate the mechanical rupture of microalgae such as Nannochloropsis via autolysis-induced cell wall thinning. During this process, concentrated slurries of cells are incubated in the dark at an elevated temperature, to deprive them of light and oxygen. This work analyzed the integrity of proteins and lipids during dark anoxic incubation and investigated the cellular responses of Nannochloropsis through an in-depth proteomic analysis. Proteomic analysis identified enzymes associated with cellulose hydrolysis and glycolytic and fermentative pathways that are presumably activated to produce energy in the absence of light and oxygen. Progressive biochemical degradation was observed during 48 h of incubation, including the proteolysis and leakage of proteins, and the lipolysis and subsequent peroxidation of lipids. This provides further evidence of autolytic processes occurring during prolonged incubation, which can be attributed to uncontrolled action of intracellular proteases and lipases. Importantly, the resultant formation of peptides and free fatty acids will affect their use in food and fuel applications. It is therefore important to optimise the incubation time and parameters to achieve cell weakening while minimising the unnecessary degradation of biomacromolecules. Graphical abstract

Single-cell protein from yeast can provide a sustainable means to meet increasing protein demands. For yeast protein to be digestible it must be released from within the cells using methods such as autolysis, enzyme proteolysis, and mechanical rupture. This study investigates the combination of autolysis or proteolysis and subsequent mechanical rupture on the protein release. The effects of autolysis and proteolysis using papain on the size, wall thickness, and internal structure of freshly harvested and 7 day stored Saccharomyces cerevisiae cells were investigated using particle sizing and electron microscopy. Samples were then subjected to high-pressure homogenization (single pass, 800 bar) and the extent of cell rupture and protein solubilization determined. During autolysis and proteolysis, intracellular contents were released, shrinking the cells, allowing the elastic walls to contract and thicken. Autolyzed cells were mostly deformed rather than ruptured by high-pressure homogenization, which expelled cell contents. Intact untreated cells with thin, stretched cell walls burst during high-pressure homogenization, with the proteins mostly remaining unsolubilized. Stored cells were smaller with thicker cell walls and generally more difficult to rupture. These findings can assist in developing efficient processes for recovering sustainably produced proteins from yeast and other microorganisms.

Filamentous algae (FA) can form readily harvestable floating mats or attached turfs that facilitate their application in wastewater treatment systems. However, large‐scale implementation is hindered by our inability to predict performance as a function of key operational parameters. A predictive mathematical model would be a valuable tool for designing efficient FA‐based systems. Developing accurate models is challenging due to dynamic environmental conditions and the spatial complexities of FA cultures. In this work, a model was developed to mathematically describe the biomass productivity of static FA cultures (mats and turfs) in relation to the incident light intensity and temperature. The model was validated against published data to investigate the influence of time‐dependent inhibition (inhibition from sustained light exposure) on productivity. When time‐dependent inhibition was included in the model, predictions were within ~10% of experimental values, however, without including time‐dependent inhibition there was a sixfold overestimation of biomass productivity. The model could also generate predictions of the effects of time‐dependent inhibition during diurnal light fluctuations using experimentally determined rate constants. The model represents a powerful tool for optimizing the design and operational parameters in FA cultures that could be further expanded to incorporate the influence of nutrient and CO ₂ availability.

Yeast biomass has untapped potential as a sustainable source of nutritional protein. To realise this, current production capacity and the demand for yeast-based products in food applications must be increased. This review explores the possibility of increasing yeast supply using low-cost and sustainable substrates such as lignocellulosic sugars, starch hydrolysates and waste lactose, and by utilising the waste yeast biomass that will be generated from future recombinant protein production. Candidate yeast strains and processes for producing biomass from these substrates are reviewed in relation to production efficiency and product functionality. The opportunity to lower production costs and control yeast cell properties using continuous cultivation is highlighted. Current knowledge of how yeast diversity, metabolism and physiology are influenced by growth conditions is brought together to understand how yeast biomass can be produced with desirable functional properties. In particular, this review provides insights into how the variety and adaptability of yeast can make it possible to adjust attributes such as protein and cell wall composition through strain selection and controlled production. Major gaps are identified as targets for future research, in particular understanding the functional properties of non-Saccharomyces yeast biomass that could be produced from lignocellulosic sugars, lactose and precision fermentation. Specific, controlled studies of yeast biomass functionality in relation to species and growth are now needed to help expand the scale of production and associated environmental benefits of nutritional yeast.