Heterogeneous enzymatic reactions are used in many industrial processes including pulp and paper, food, and biofuel production. Industrially-relevant optimization of the enzymes used in these processes requires assaying them with insoluble substrates. However, platforms for high throughput heterogeneous assays do not exist thereby severely increasing the cost and time of enzyme optimization, or leading to the use of assays with soluble substrates for convenient, but non-ideal, optimization. We present an innovative approach to perform heterogeneous reactions in a high throughput fashion using droplet microfluidics. Droplets provide a facile platform for heterogeneous reactions as internal recirculation allows rapid mixing of insoluble substrates with soluble enzymes. Moreover, it is easy to generate hundreds or thousands of picoliter droplets in a small footprint chip allowing many parallel reactions. We validate our approach by screening combinations of cellulases with real-world insoluble substrates, and demonstrate that the chip-based screening is in excellent agreement with the conventional screening methods, while offering advantages of throughput, speed and lower reagent consumption. We believe that our approach, while demonstrated for a biofuel application, provides a generic platform for high throughput monitoring of heterogeneous reactions.