Soluble dietary fiber is a growing food ingredient market. Twin-screw extrusion of lactose with an acid catalyst has been reported as a way to polymerize lactose to oligomers which are indigestible and analyze as dietary fiber. Lactose was dry blended with citric acid at two different concentrations, 1 and 2%. Glucose was added to raw mixes at 0, 10, or 20% (w/w). Samples with 2% citric acid yielded a higher concentration of indigestible oligosaccharides (52.3-59.8%, w/w) than 1% citric acid samples (37.1-49.9%). Glucose did not affect the yield. Glucose addition was beneficial and reduced the motor torque and specific mechanical energy of the extruder, and extruded products were lighter in color. The generated oligomers had a degree of polymerization that ranged from 3 to 5, as determined by mass spectrometry. Testing if the oligomers confer a beneficial effect (to be classified as dietary fiber), is still required.