Methyl isobutyl ketone is also used as a denaturing agent for denatured alcohol. Ethanol and denatured alcohol are altogether different. Ethanol is sometimes denatured (adulterated), and made poisonous, by the addition of methanol. The double-stranded PCR products are denatured using heat and formaldehyde to produce ssDNA. The disulfide bond enhances BiP's binding to denatured proteins. When cooked, avidin is partially denatured and binding to biotin is reduced. IGF-1 in milk is not denatured (inactivated) by pasteurization. Soils difficult to remove are pigments and dyes, fats, resins, tar, waxes, and denatured protein. For example, protein drugs such as insulin may be denatured by stomach acids. They help prevent protein unfolding and help re-fold denatured proteins. In capillary electrophoresis, it is used for stabilizing (single) strands of denatured DNA. The skin that forms on curdled milk is another common example of denatured protein. One designation in particular, SD-40B, indicates that ethanol has been denatured using denatonium benzoate. In practice, the proteins are denatured in a solution containing a detergent (SDS). They contain harmful protease inhibitors that are denatured by cooking.