1 23 World Journal of Microbiology and Biotechnology ISSN 0959-3993 Volume 32 Number 9 World J Microbiol Biotechnol (2016) 32:1-10 DOI 10.1007/s11274-016-2103-7 Recent advances on prebiotic lactulose production Azis Boing Sitanggang, Anja Drews & Matthias Kraume