Production of CO2 clathrate hydrate frozen desserts by flash freezing

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Abstract

CO2 clathrate hydrate is a crystalline water–carbon dioxide structure that can enable novel, strongly carbonated frozen desserts. A CO2 flash-freezing process has been developed to form CO2 hydrates during freezing of a dessert mixture. In the process, liquid CO2 and liquid dessert mixture are emulsified and then sprayed into a chamber in which the CO2 flashes to a vapor causing the dessert mixture to freeze into a low-density powder. To ensure CO2 hydrate formation it is important to avoid ice formation because ice to CO2 hydrate conversion proceeds too slowly to occur during flash freezing. To avoid ice formation it is important to flash the emulsion to a pressure greater than 4.7 bars and disperse the mixture in fine droplets. In this work the key parameters of the process are described, which will enable development of commercial dispensers.

Introduction

Carbonated drinks are omnipresent in grocery stores, convenience stores, restaurants, bars and vending machines. Carbonation is often associated with refreshment (e.g. cold soda), excitement (e.g. pop rocks) and celebration (e.g. champagne) and can provide product differentiation. For example, bottled water, flavored beverages, fruit, and wine all come in sparkling and non-sparkling varieties. However, frozen carbonated desserts are not commonly available on the market. The closest product is an ICEE® or SLURPEE®, which is a lightly carbonated partially frozen beverage.

Carbonating a frozen food is more challenging than carbonating a liquid. Beverages and fruit have significant non-crystalline aqueous phases in which CO2 can be dissolved and retained. However, in frozen foods a significant portion of the water is ice in which the solubility of CO2 is extremely low, 10−8 g/g ice (Rhode and Price, 2007) compared to 10−3 g/g liquid water (Diamond and Akinfiev, 2003). To achieve CO2 concentrations similar to the CO2 concentration in soda and sparkling water, CO2 in frozen foods must be retained by a mechanism other than solution in the aqueous phase. One attractive method of retaining the gas is a solid CO2–H2O structure called CO2 clathrate hydrate.

CO2 clathrate hydrate (hereafter CO2 hydrate) looks like ice, but is actually a crystalline water–carbon dioxide phase. The water molecules form hydrogen bonds in a cage-like structure that is stabilized by the van der Waals forces of the enclosed CO2 molecules (Sloan and Koh, 2008). The minimum pressure for a stable CO2 hydrate phase increases with increasing temperature. Fig. 1 shows a pressure–temperature projection of the CO2–H2O phase diagram with a shaded region that indicates stable CO2 hydrate.

The concentration of CO2 in CO2 hydrates is typically 296 kg/m3, which is 50 times the density of CO2 in sparkling water (typically 6 kg/m3 (Descoins et al., 2006)). With this high CO2 concentration, if all of the ice in a standard ice cream were replaced by CO2 hydrate, the CO2 content of the ice cream would be much greater than the CO2 content of a soda. Using CO2 hydrates the gas is trapped in the crystalline phase of the dessert, rather than in bubbles or in the aqueous phase. Since frozen desserts always have a significant crystalline water fraction, CO2 hydrates are a very effective method of trapping CO2, even with large variations in the dessert recipe and gas volume.

There have been several efforts at making frozen desserts containing CO2 hydrates, as evidenced by the patent literature. For example, Gupta and Dimmel patented (2005) manufacturing CO2 hydrates by injecting pressurized CO2 into a water-filled reactor for 30 min, while maintaining the reactor at 273 K and agitating it. After 30 min, the reactor contents are cooled to 253 K, depressurized, ground to an acceptable particle size, and the resulting ice–CO2 hydrate granules are mixed with pre-cooled flavored syrup. Bee and Gerrard patented (2000) putting partially frozen water or dessert mixture into one part of a vessel and solid CO2 into another part of the vessel, sealing the vessel, warming it to approximately 274 K and then agitating the vessel until sufficient CO2 hydrate formation has occurred. Then the CO2 hydrate dessert is fully frozen by placing the vessel in a blast freezer. Hinman et al. (1983) patented contacting granulated ice with CO2 gas at 253–273 K and pressures sufficient for CO2 hydrate formation and maintaining the contact for a period of time to allow CO2 hydrate to form in the granules. The patent suggests that the resulting carbonated granules could be mixed into an aqueous liquid for simple home preparation of a carbonated beverage. Despite these and other patents for making CO2 hydrates, CO2 hydrate desserts are not found on the market, suggesting that these processes are not commercially practical.

Here a novel flash-freezing process is described that forms CO2 hydrate directly as a dessert mixture is frozen. The process involves combining high-pressure liquid CO2 with a liquid dessert mixture and then spraying these combined fluids into a chamber at a lower pressure. In the spraying process the CO2 evaporates, absorbing its heat of vaporization from the dessert mixture, which in turn freezes the mixture. CO2 hydrate is formed throughout the dessert mixture. The dessert is completely hardened in a rapid single-step process with a minimum of moving parts. In this study, both an apparatus for CO2 flash freezing and an apparatus for characterizing the resulting powder are developed. Using data taken during a typical flash freezing run and measurements of the CO2 content of the powder, the physical processes that occur during flash freezing and their impacts on the resulting powder are identified.

Section snippets

Materials and methods

Fig. 2 shows a schematic of a batch-implementation of the CO2 flash-freezing process. Before production, the mixture tank is filled with the ingredients mixture (mixture), which includes all water, sugars, fats, stabilizers, emulsifiers and flavors in the dessert recipe. Pressurized nitrogen gas drives the mixture through a heat exchanger (ice box 1) and then a nozzle (mixture nozzle) into a small chamber filled with liquid CO2 (emulsion chamber). Liquid CO2 is supplied through the dip tube of

Results

The pressure and temperature profiles for a typical trial with the CO2 flash-freezing batch apparatus are shown in Fig. 5. The labels T1 through T5 and P1 through P3 correspond to the sensors shown in Fig. 3. Pre-cooling the ICT takes about 25 min. At the start of the trial the ICT temperature (T3 and T4) is stabilized at approximately 230 K (in this particular trial the ICT was slightly overcooled). When the trial begins the vent temperature (T5) drops rapidly to 238 K. The mixture (T1) and

Discussion

The measured initial CO2 content of the flash-freezing powder, 0.12–0.25 kg CO2/kg powder, approaches the expected content of pure CO2 hydrate powder, which is 0.27 kg CO2/kg powder; and hence the process can cause nearly complete CO2 hydrate formation in the dessert mixture. The measured initial CO2 content of the powder gives a lower limit on the actual CO2 content at the time the ICT is opened because CO2 is lost from the powder during the transfer process from the ICT to the PTM. The

Conclusions

CO2 hydrates provide a viable solution for carbonating frozen foods because CO2 hydrates trap CO2 in the crystalline phase and enable high CO2 concentrations. CO2 hydrates can easily be formed as a dessert mixture is frozen using CO2 flash freezing. The process is rapid and can cause nearly complete crystallization of water into CO2 hydrates in a single step.

It has been argued that to ensure CO2 hydrate formation during flash freezing it is important that the mixture is dispersed in fine

Acknowledgement

The support of Nestle S.A. for this research is gratefully acknowledged.

References (18)

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