Effect of high-pressure homogenization on the rheology, microstructure and fractal dimension of citrus fiber-oil dispersions
Introduction
Citrus fruits (genus Citrus in family Rutaceae) are nutritionally and economically important fruit crops. They are widely distributed around the world and are most important component of horticulture industry of many countries (e.g., most Mediterranean countries, Brazil, Mexico, China, Spain, Greece and Italy) (Koppar and Pullammanappallil, 2013; Martín et al., 2010; Zema et al., 2018). Annual output of citrus fruits was about 17.22 million tons in 2017 (USDA, 2017), and about 1.94 million tons of citrus fruits were processed into orange juice (FAOSTAT, 2017). Citrus pomace is a byproduct of juice industries, and contains considerable amount of dietary fiber, polyphenols, flavonoids and other phytochemicals. The pomace of most citrus fruits is currently not adequately utilized despite being rich in above mentioned phytochemicals. If these pomaces are utilized well, one can not only reduce the cost incurred by their disposal as waste but also, become stable source of dietary fiber, polyphenols and other valuable phytonutrients. At present, the utilization of citrus fiber is still in its early stage; for instance, dietary fiber from external source is added to liquid products including fruit juice, yogurt, salad dressing and solid products such as cheese (Zhu et al., 2017). Many natural fibers possess remarkably high oil holding capacity (OHC) (Likon et al., 2013; Lundberg et al., 2014; Zhu et al., 2017), indicating that they can be excellent carriers of oil and lipid. Therefore, in order to make full use of fibers obtained from citrus pomace as carrier of oil and lipid, it is necessary to study the rheological and microscopic characteristics of citrus fiber-oil dispersion. This is because adsorption properties of fiber affected by microscopic characteristics (Ulbrich and Flöter, 2014), and there are some relations between the rheological properties and microscopic characteristics of citrus fiber-oil dispersion (Liu et al., 2015).
High-pressure homogenization (HPH) is now a common technology in food industry. Average particle size reduced and the microstructure is destroyed when the dispersion is forced through homogenizing valves at high pressure (Buggenhout et al., 2015). This high-pressure homogenization process is known to affect the oil adsorption, rheological and viscometric properties of dietary fibers and improve their OHC, and water holding capacity (WHC) (Buggenhout et al., 2015; Zamora and Guamis, 2015). In this context, Wang et al. (2018) investigated the effects of HPH on rheological properties and homogeneity of tomato fiber suspensions. These authors reported that the morphological, rheological and color properties of tomato fibers were significantly affected when subjected to HPH, especially at high pressures. Similarly, Leite et al. (2017) reported that the particle size distribution and the rheological properties of frozen concentrated orange juice were affected by the increase in number of passes and also in pressure of HPH. Therefore, the measurement and interpretation of physical properties (apparent viscosity, activation energy, surface morphology, storage modulus, and loss modulus) of citrus fiber-oil dispersion before and after HPH will provide important evidence-based support for broader application of citrus fibers.
Rheological property is an important characteristic of liquid materials, the understanding of which is important for operations such as mixing and pumping through pipelines (Rao, 1999). The citrus fiber is composed of soluble and insoluble parts, so it will form a complex dispersion when mixed with water and oil. Microstructure of the citrus fiber was found to change significantly after HPH treatment (Zhu et al., 2017), which is expected to effect the rheological properties. Despite this importance, there are dearth of studies on the rheological properties of the fiber-oil dispersions. Most of the literature dealing with the characteristics of fibers appears to focus on the adsorption of oil. A limited study on the rheological properties of the fiber dispersion also primarily confined to soluble polysaccharide-protein systems. For example, Mcdonald (2017) and Bi et al. (2014) studied the linear and non-linear rheological properties of the flaxseed gum-casein and flaxseed gum-soy protein isolate dispersions, respectively, and discussed the influence of the concentration of flaxseed gum on the apparent viscosity, activation energy and viscoelastic properties of these dispersions. Liu et al. (2017) studied the trend of rheological properties of flaxseed peptide-starch dispersion, and found that both the apparent viscosity and viscoelastic properties of the dispersion and gelatinization temperature of the starch component increased with the increase in flaxseed peptide. The approach used in the studies of Bi et al. (2014) and Liu et al. (2017) to establish link between macroscopic characteristics and microstructure of the dispersion will be useful in characterizing the citric fibers-based dispersions. The fractal analysis used by Bi et al. (2014) could also be useful in theoretically interpreting the microscopic characteristics of fiber dispersions and also in interpreting the effect of the complex network structure. In the fractal analysis of the citrus fiber-oil dispersion, the main components of citrus fiber (cellulose, hemicellulose, lignin, and pectin) and oil will not have effective and suitable fluorescent dye (Bui et al., 2008) to facilitate the characterization, and rhodamine B is used as the fluorescent dye, this is because macromolecule protein and polysaccharides in citrus fiber could be inlaid with rhodamine B (Hagiwara et al., 1997). The confocal microscopy together with fractal dimension has been successfully used to describe the uniformity and complexity of the dispersions (Bi et al., 2014). It has also been shown that the fractal dimension has a significant correlation with the rheological properties of the mixed dispersion system. Therefore, a combine of fractal and rheological analyses is expected to provide more detailed information and enable better interpretation of the characteristics of the dispersions.
Currently there are no publications establishing relationship among microstructure, rheological properties and fractal characteristics of the citrus fiber-oil dispersions. To address this gap in knowledge, this study acquired and analyzed the apparent viscosity, activation energy, viscoelastic properties and the microstructure of the citrus fiber-oil dispersions produced through HPH process. The fractal dimension of these dispersions was calculated and linked with homogeneity of the dispersions. The outcomes of this study will help broaden the application of citrus fiber-oil dispersions in food and pharmaceutical industry.
Section snippets
Materials and chemicals
Citrus fiber samples were provided by Fiberstar (River Falls, WI 54022, USA). Average particle size (D50) of citrus fiber samples, marked as Citri-FI 100M10, Citri-FI 100M40, and Citri-FI 100M100, was 10, 40, and 100 μm, respectively. The citrus was originally from Florida, USA, and the composition (provided by Fiberstar) is listed in Table 1. Corn oil (Q/XSP 0007 S) was used as the model oil and it contained 15% saturated fatty acids, 31% monounsaturated fatty acids and 54% polyunsaturated
Apparent viscosity of citrus fiber-oil dispersion
Apparent viscosity of citrus fiber-oil dispersion is shown in Fig. 1 (panel A to D). As can be observed from Fig. 1 (A&B), the apparent viscosity of citrus fiber-oil dispersions increased with the increase of fiber concentration and decreased with the increase in shear rate. The latter observation indicated that the fiber-oil dispersions were shear thinning in nature which is the characteristics of non-Newtonian fluids.
Fig. 1 (C & D) shows the apparent viscosity of citrus fiber dispersions
Conclusion
Rheological properties and fractal dimensions of the citrus fiber-oil dispersion produced through HPH using two different pressure levels were studied. The microstructure of dehydrated dispersion was observed by SEM to establish the effect of HPH on microstructure and rheological properties.
HPH significantly increased the apparent shear viscosity, storage and loss moduli. But, reduced the activation energy of the citrus fiber-oil dispersions. The Herschel-Bulky model represented the apparent
Acknowledgements
This research was supported by National Natural Science Foundation of China (31771896).
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