Microstructures formed by spray freezing of food fats

Abstract

The spray-freezing of two food fats, tripalmitin (PPP) and cocoa butter (CB) and mixtures thereof, has been modeled experimentally using a novel single droplet freezing apparatus configured so that temperature profiles or samples for microstructure analysis can be obtained. For 2 mm diameter droplets suspended in a cold air flow at temperatures around 2-15°C, initial cooling rates were on the order of 10 K s -1 and the temperature profiles could be correlated directly to DSC data collected at 20 K min -1 , indicating that minimal supercooling of the materials occurred in the droplet form. Microstructure analysis confirmed that PPP crystallized preferentially in mixtures, and that the surface structure was very sensitive to storage conditions. The bulk structure was much less sensitive, and the internal microstructure of the PPP droplets revealed distinct nucleation sites, which were absent from the CB: These persisted in the mixtures up to 50 wt%. X-ray analysis indicated that the fats crystallized in their more stable forms, namely, β for PPP and Form V/VI in CB.Spray freezing of liquids and emulsions by exposing fine drops to cold, dry gas (usually air) has been advocated as a manufacturing route for new products and product forms by several workers (e.g., 1-3). This route allows thermally labile products to be converted to the form of a free-flowing powder, avoiding structure and activity modification associated with the temperatures required for spray-drying. It offers very high cooling rates, both via large surface area-to-volume ratios and high rates of convective heat and mass transfer (i.e., where volatiles give rise to evaporative cooling) and, when operated correctly, yields a fine particulate product with well-defined size range. For aqueous systems, a subsequent freeze-drying step is required to give a stable product. Commercial processes based on this technology are available (see Ref.2), particularly for spray-chilling for microencapsulation. A number of patents have been filed recently on a variant where the feedstock is contacted with a cryogen spray, typically liquid nitrogen, to give extremely high cooling rates (4-6). Spray-freezing also offers the opportunity to generate new microstructural forms, or direct routes to known forms, driven by the high rate of cooling achievable in drops compared with bulk liquid.Spray-freezing into liquid has received considerable attention in the pharmaceutical and other controlled release sectors as a way of generating novel microstructures and microencapsulates and for stabilizing enzymes. Interest in the food sector is relatively recent, partly because of the cost associated with the cooling media: spray-freezing into cold air avoids the need to separate the product solids from liquid, which in addition would have to be food grade and noncontaminating. Hindmarsh and co-workers have shown that spray-freezing in air or cold nitrogen gas can yield novel structures from sucrose solutions (7) and emulsions (8), and MacLeod et al. (9) have demonstrated that spray-f...

Controlled freezing and freeze drying: a versatile route for porous and micro-/nano-structured materials

“…107 -109 The morphologies of the particles produced by spray freezing could be engineered in different ways by varying the spray geometry and cooling rate, 110 the use of a four-fluid nozzle, 111 or using solid-in-oil-in-oil suspensions as the feed. 112 Spray freezing for the preparation of particles has also been used for food, 113 fat, 114 and inorganic particles such as catalysts for oxidation of methane and CO. 115 Hollow biodegradable PLA particles with porous shell walls have been prepared by both emulsification/freeze drying and spray/freeze drying. The porous particles were then suspended in an aqueous solution containing a water soluble drug and the resulting suspensions were exposed to plasticizing solvents (dichloromethane or compressed CO 2 ).…”

Development of a single droplet freezing apparatus for studying crystallisation in cocoa butter droplets

“…Tanner (2011) reported simulations of this nature. An inward freezing shell model was used to describe crystallisation and gave a good description of the T d -t profiles reported by Gwie et al (2006). The video microscopy results here, however, suggest that an inward freezing shell model is not an accurate description of the process.…”

“…SDFA devices have also been constructed for use in MRI machines for mapping emulsion creaming and solidification within droplets (Hindmarsh et al, 2004). Gwie et al (2006) used an SDFA to study spray-freezing of tripalmitin and cocoa butter. Their SDFA was later modified to fit into a laboratory X-ray system by Pore et al (2009) which were consistent with the temperature-phase transitions reported for static bulk samples crystallised under isothermal conditions by Van Malssen et al (1999).…”

Modeling and Validation of a Three-Stage Solidification Model for Sprays

“…The model validation for the single drop has been performed with Mathematica and are based on the experiments of Gwie et al [3]. The geometry reflects a cylindrical container with a diameter of 2 cm and a height of 5 cm with a conical diffuser at the bottom.…”

“…The model has been validated for a single CB droplet by simulating the experimental results of Gwie et al [3]. Subsequently, the model has been implemented into a modified version of the computational fluid dynamics (CFD) code, KIVA-3 [4], and then has been validated for a freezing CB spray.…”