## 1 Introduction  
One of the foremost concerns for economically efficient ethanol production from biomass through the biochemical pathway is the high price and usage rates of cellulolytic enzymes. Several factors have substantial negative impacts on the enzymatic hydrolysis of biomass, such as lignin, acetyl groups and crystallinity of cellulose. The main negative impacts of lignin are inhibition of fiber-swelling (reducing cellulose accessibility) and enzyme deactivation through irreversible adsorption [1]. These factors reduce enzyme performance, such that higher quantities of enzyme are required to achieve acceptable hydrolysis rates and efficiencies. A typical cellulase loading of 15 FPU/g cellulose would equate to \(\sim\)30 g of enzyme per liter of ethanol produced. Considering a $10/kg cost for cellulase, this level of enzyme accounts for 27%-40% of biofuel production costs [2,3], assuming a liter of ethanol is sold for $0.94. Thus, it is essential for the enzyme cost to be reduced below $2/kg protein, or strategies must be developed to substantially reduce enzyme dosage rates [4, 5, 6] so that cellulolytic enzyme costs would be comparable to starch enzymes used in the corn-to-ethanol industry ($0.02-0.03/gal ethanol). At present, a range of cost estimates for cellulolytic enzymes have been made, including $0.10/gal [7], $0.30/gal [8], $0.32/gal [9], $0.35/gal [10], $0.40/gal [11] and $0.68/gal [10] of ethanol produced. A sensitivity analysis of the effects of feedstock price and fermentation residence time on the costs of enzyme production showed that at a baseline cost of $60/MT for poplar, the enzyme costs would still be $1.30/gal ethanol [10]. However, a reduction in microbial growth and fermentation period would reduce the capital costs, and thereby drop the enzyme costs. Because enzyme production costs can only be reduced so far, developing techniques to lower enzyme utilization in biomass hydrolysis is a critical aspect of reducing the production costs of ethanol from biomass [11, 12, 13]. Several approaches have been evaluated to improve enzyme performance, and hence reduce the net costs of hydrolytic enzymes. A wide range of pretreatment technologies has been developed to enable high saccharification yields at lower enzyme loadings [14,15]. Some of these pretreatments target removal of monomeric sugars [16,17], hemicellulose oligomers [18], lignin and lignin degradation products [19, 20, 21], while others may also target increased biomass porosity, or a combination of these effects. A more recent approach has been the development of techniques that maintain enzyme secondary structure and solubility, and hence activity for an extended period of time. Amphiphiles such as emulsifiers and proteins have been found to improve the activity of a variety of enzymes by alleviating one or more of the potential enzyme inhibitors mentioned above. Examples of this beneficial effect have been reported for enzymes such as horseradish peroxidase (HRP) [22], lipoxygenase (LOX1) [23], lipase [24,25], xylanase [22], \(\beta\)-glucosidase and cellulase [1,26, 27, 28, 29]. Various mechanisms of action have been described to explain these effects that will be described further in this manuscript. These emulsifiers and proteins contain simultaneously polar and non-polar regions, which impart them with surface-active properties [30] that can stabilize the two-phase system. Improving enzyme activity through the application of amphiphiles not only reduces the operating costs of ethanol production from biomass by reducing the amount of enzyme required, but also reduces fermentation time, thereby lowering capital costs. Therefore, due to the significant need for improved enzyme activity, and the demonstrated effective action of protein and surfactant-based enzyme stabilizers, we review the different mechanisms of action of these additives on cellulase stabilization.