Lipases are active on a water/lipid interface and in this respect differ from esterase enzymes which cleave only water-soluble esters, such as triacetylglycerol.

Lipase activity is detected in, for example, milk, oilseeds (soybean, peanut), cereals (oats, wheat), in fruits and vegetables and in the diggestive tract of mammals. Many microorganisms release lipase-type enzymes into their culture media, and these may contribute to enhance the deterioration of foods.

As to specifity, fat-splitting enzymes, which preferentially cleave primary HO-group esters are distinguished from those which indiscriminately hydrolyse any ester bonds of acyl glycerols.

The 48 kcal lipase secreted by pig pancreas has probably been the best studied. It cleaves all acyl glycerols but preferred substrates are triacylglycerols, and worst of all monoacylglycerols, and in any case breaks the ester bonds at positions 1 or 3 only.

Oat and Aspergillus flavus lipases present no positional specificity whatsoever, whereas Geotrichum candidum lipase is specific for oleic and linoleic residues in any position, and Mucor miehei and Penicillium roqueforti lipases also show 1,3 specificity.

Acyl migration from position 2 to 1 is thermodinamically favoured and normally precedes enzymatic hydrolysis of that acyl residue; longer hydrolysis times are needed for completeness of this reaction unless an unspecific lipase is used.

When hydrolysis of emulsions is being studied, the size of oil droplets in as much as it affects the total oil/water interfacial area, the larger the oil/water interface and, therefore, the higher the apparent lipase activity. Interfacial area should be considered when substrate emulsions are used in the assay of enzyme activities.

Activity of lipases at the interface is explained by assuming that part of the lipase molecule is hydrophilic and part is hydrophobic, hence the enzyme sits ot the interface and the active site lies next to it. Hydrolysis of the ester bond occurs with the involvement of Serine, Histidine and Aspartic residues as in chymotrypsin peptide bond breaking. Lipase has a leucine residue within the active site in order to maintain a hydrophobic interaction with the hydrophobic lipid and bring it to the reaction centre, whereas serine proteinases do not have such a residue because their substrates differ.

Lipases with high specificity may be used to taylor food ingredients. An example may be found in the production of CBS, which are replacements for cocoa butter, and in the production of high polyunsaturate oils which may have nutritional advantages. Another example is the use of unspecific lipases when randomization of acyl groups is to be obtained as for instance in enzyme mediated interesterification for the production of margarines and spreads.