Original article Physical properties of supercritical fluid extrusion products composed of milk protein concentrate with carbohydrates Hongzhi Liu, 1,2 Richard Lebaron Hebb, 1 Novita Putri 1 & Syed S. H. Rizvi 1 * 1 Food Science Department, Cornell University, Ithaca, NY 14850, USA 2 Institute of Food Science and Technology, Chinese Academy of Agriculture Sciences, Beijing 100193, China (Received 27 June 2017; Accepted in revised form 31 August 2017) Summary Supercritical fluid extrusion (SCFX) is a novel process combining traditional extrusion and supercritical fluid technologies. The objective of this study was (i) to assess the effect of sugar, maltodextrin and corn- starch on the physical properties of milk protein concentrate (MPC) puffed extrudates and (ii) to compare the physical and textural properties of SCFX puffed MPC products with commercially puffed cereal prod- ucts. Results showed that the nutritious MPC puffed products with carbohydrates can be made with tex- tural properties (hardness, brittleness and crispness) resembling those of the commercial samples. The porosity of F2 (0.573) was assessed to the commercial product C1 (0.543) and C2 (0.595), the specific length of F3 (50.41) was assessed to C1 (45.51) and F4 (80.14) was assessed to C2 (78.12). The hardness of extrudate F3 (435.49 KPa) was assessed to C2 (482.7 KPa). The Young’s modulus of F5 (87.03) was assessed to C2 (80.57). The starches affected the crispness of the extrudates, and sugar affected the hard- ness and brittleness significantly. Keywords Carbohydrates, milk protein concentrate, supercritical fluid extrusion. Introduction Puffed food products have become a part of the pre- vailing lifestyle especially in western countries. Tradi- tional extrusion technology is now widely used in manufacturing a diverse range of food products including pasta, snacks, breakfast cereals, confec- tionary, texturised meat substitutes, infant food formu- lations, precooked beverage powders and extruded crispbread (Suhaila, 1990). Traditional extrusion uses steam expansion as the driver for expanding or ‘puff- ing’ the extruded product (Suhaila, 1990). Supercritical fluid extrusion (SCFX) is a novel process combining extrusion and supercritical fluid technologies to utilise supercritical CO 2 (SC–CO 2 ) as the blowing agent for puffed products instead of steam. With no need for steam, the SCFX can operate below steam tempera- tures allowing heat-sensitive food ingredients, such as protein, to be included in extrudate formulations (Rizvi & Mulvaney, 1992). SCFX researchers control the macro- and microstructure of SCFX extrudates (piece density, cell size, cell size distribution and expansion ratio) by manipulating process conditions (Cho & Rizvi, 2009). The proper application of supercritical fluid technology to traditional extrusion techniques creates a more versatile, robust and con- trollable process allowing for simultaneous expansion, flavouring and reduction in viscosity (Alam et al., 2014). The textural properties of extruded products depend strongly on the ingredient materials and the processing conditions (Sozer & Poutanen, 2013). Extruded prod- ucts piece and bulk densities have been widely used as a predictor of extrudate mechanical properties due to the simplicity of measurement and the reasonable cor- relation with the textural properties (Chanvrier et al., 2015). In general, more expanded products with lower product density exhibit low breaking strength and compressive modulus (Sokhey, 1996). Extrudate texture is an important quality parameter that is material-dependent and greatly affects con- sumer preference. However, most metrics for texture measurements show dependence not only on process- ing and handling techniques and/or ingredient combi- nations but also on irregularities in shape, size and curvature that is inherent to most food products (Canet & Sherman, 1988). The effects of size and shape on stress–strain behaviour of various visco-elas- tic food materials (Poisson ratios in the range of 0.3- 0.5) have been extensively studied with inconclusive *Correspondent: E-mail: srizvi@cornell.edu International Journal of Food Science and Technology 2017 doi:10.1111/ijfs.13624 © 2017 Institute of Food Science and Technology 1