The present invention relates to a method for preparing oligosaccharides which can be used among others as food additives to reduce calorie content, to sweeten food products, to increase the fiber content of food products, to improve the texture of food products and to stimulate the gut microbiome bacteria. Furthermore they can be applied in the fields of animal feed, or other applications. More particularly, this invention is directed to a high temperature hydrolysis of xyloglucan polysaccharide to defined xyloglucan oligosaccharides. The invention further relates to oligosaccharide hydrolysates produced with the method of the invention and to the use of said oligosaccharide hydrolysates in human and/or animal nutrition, as prebiotic or other uses. Further provided are novel endoglucanases for use in the method of the invention as well as in other applications.