True, Particle, and Bulk Density of Shrinkable Biomaterials: Evaluation from Drying Experiments A. Martynenko Department of Engineering, Faculty of Agriculture, Dalhousie University, Truro, Nova Scotia, Canada A method of evaluation of bulk, particle, and true density of shrinkable biomaterials from drying experiments was developed. Density was calculated from instantaneous shrinkage and mass measurements, using the correlation between dimensionless values of density q, shrinkage n, and moisture content X. Bulk density evol- ution during drying was found to be dependent on temperature and material shrinkability. For example, the bulk density of wild blue- berry increased from 1.03 to 1.26 g/cm 3 at 40  C and decreased from 1.03 to 0.38 g/cm 3 at 60  C. Particle density increased with moisture removal. True density was evaluated in the range 1.59–1.61 g/cm 3 for wild blueberry and 1.45–1.56 g/cm 3 for ginseng root. Keywords Imaging; Real-time; Shrinkability; Shrinkage; Solids density INTRODUCTION Density of porous biomaterials is important parameter, determining the heat and water diffusivity, microstructure, and texture of dried or semidried products. At least three different densities are widely used in scientific literature: (1) bulk density, accounting for overall volume; (2) particle density (excluding open pore volume); and (3) true density of solids (excluding open and closed pore volumes). The difference in volume, considered for densities calculation, is illustrated in Fig. 1. Bulk density is defined as the mass over the bulk volume, including air voids [1] : q b ¼ m s þ m w V s þ V w þ V a ; ð1Þ where air void volume V a includes both open pores V op a and closed pores V cp a : V a ¼ V op a þ V cp a : ð2Þ Bulk density is measured by using displacement, buoyancy, or pycnometry with sealed open pores. [1] It was established that the bulk density of porous biomater- ials ranges from 1.75 to 0.1 g=cm 3 , depending on the moist- ure content, air temperature, and drying technology. [2,3] Particle density is defined as the mass of the particle over the apparent volume, which includes solid material, liquid, and closed pore volumes [4] : q p ¼ m s þ m w V s þ V w þ V cp a : ð3Þ Particle density is defined similar to bulk density but without open pore sealing. [1] Using a gas pycnometer, Krokida and Maroulis proved that particle density depends only on the moisture content. [3] True (solid) density is defined as the mass of a solid particle divided by its volume, excluding open and closed pores [4] : q s ¼ m s V s : ð4Þ True density is a constant, intrinsic property of the material, calculated for a bone-dry sample. The significant similarity between particle and true density creates some confusion in interpretation. Despite the classical definition of true density as solid density, [2–5] some research publica- tions [6–9] interpret true density as particle density. However, from the definition [Eq. (3)], the particle density is deter- mined by the coexistence of three phases (solid, liquid, and air), whereas true density is determined only by the solid phase. True density is equal to particle density only at zero moisture content and negligible porosity. [1] From simple considerations, the true density of biomaterials is determined by the ratio of carbohydrates (1.53–2.0 g=cm 3 ), fat (0.89–0.91 g=cm 3 ), and protein (1.37–1.43 g=cm 3 ). Direct measurement of true density is complicated due to the closed pore formation at low moisture content. [10] To avoid the effect of closed pores, additional grinding of dried samples is required during moisture content determination. [2] Correspondence: Professor Alex Martynenko, Department of Engineering, Faculty of Agriculture, Dalhousie University, Truro, Nova Scotia, B2N 5E3, Canada; E-mail: alex.martynenko@dal.ca Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/ldrt. Drying Technology, 32: 1319–1325, 2014 Copyright # 2014 Taylor & Francis Group, LLC ISSN: 0737-3937 print=1532-2300 online DOI: 10.1080/07373937.2014.894522 1319