As part of our ongoing efforts to finding value-added uses for lactose, we carried out reactive extrusion on blends of lactose, glucose, and citric acid to polymerize lactose to indigestible carbohydrates. This treatment can also induce the formation of color and hydroxymethylfurfural (HMF). The objective of this study was to investigate the effect of formula and extrusion conditions on color and HMF formation. Data analysis was carried out with ANOVA followed by Tukey’s HSD test. Three citric acid (CA) concentrations (2, 4, and 6% w/w) were tested. Glucose comprised 20% w/w and lactose made up the remainder. All blends were extruded in duplicate at 2 feed rates: 15 and 30 kg/h. Color was measured as Hunter L, a, and b values and absorption at 420 nm, indicative for yellow-brown pigments. Absorption was higher for samples of all CA levels extruded at 15 kg/h and these extrudates also had lower L and higher b values (i.e., samples were darker and more yellow). Thus, the extent of caramelization increased when mixtures spent more time in the extruder at the low feed rate. Higher CA levels lead to more pigment formation at 15 kg/h, but not at 30 kg/h. Along with other caramelization products, HMF is formed in sugars heated with acids. HMF has shown toxicity in in vitro and animal studies. Thus, reducing its formation through optimum processing conditions and/or sample treatments is needed. HMF content was measured by RP-HPLC/ UV. Increasing CA in the raw mix produced extrudates with lower amounts of HMF. The lower feed rate resulted in significantly higher levels of HMF. We evaluated sample treatments to remove HMF using Amberlite (an ion exchanger) and activated charcoal. Filtration over Amberlite lead to 27–57% less HMF. Preliminary results indicate that charcoal treatment may further reduce HMF (>95%). However, because it also lead to losses in oligo- and polysaccharide yields, further optimization of this method is necessary. Table 1. Effect of formula (citric acid at 2, 4, and 6% wt/wt) and feed rate (15 and 30 kg/h) on color and hydroxymethylfurfural (HMF) formation

Item	15 kg/h				30 kg/h
2	4	6		2	4	6
A420	0.554	0.654	0.934		0.194	0.195	0.200
L	83.84	83.59	80.99		87.41	88.50	89.17
b	27.23	30.01	35.03		14.83	15.21	15.79
HMF (mg/g)	1.20	0.91	0.73		0.71	0.55	0.52
HMF after Amberlite (mg/g)	0.51	0.46	0.53		0.42	0.38	0.31