6362
Ind. Eng. Chem. Res. 2008, 47, 6362–6367
Designing Multiscale Structures for Desired Properties of Ice Cream James F. Crilly, Andrew B. Russell, Andrew R. Cox, and Deryck J. Cebula* UnileVer R&D Colworth Science Park Sharnbrook, Bedford, MK44 1LQ, United Kingdom
Ice cream is a complex multiphase structure consisting of ice, air, and fat as dispersed phases at a range of different length-scales, all embedded in a continuous phase consisting of unfrozen sugar solution known as the matrix or serum. The entire structure is the result of both the ingredients and all the processes used in ice cream manufacture including emulsification, freezing, and aeration. It is thermodynamically unstable and delivered quality can only be assured at low and stable temperatures. Physicochemical processes during storage can lead to loss of quality by coarsening of the ice particles, disproportionation of the air, and the loss of water from the matrix. Product design for specific sensory, stability, shape, and increasingly, nutritional properties, is a challenging task and must take account of all these aspects of the structure. Almost all properties are sensitive to the size, density, and morphology of the dispersed phases as droplets, cells, crystals, or even micelles. Finer structures, in general, result in more desirable organoleptic properties such as creaminess and smoothness but the interfacial dynamics are more rapid, leading to less stability. Even small changes in the relative densities of the dispersed phases such as in the case of low-fat or fat-free products can dramatically change key properties such as taste perception, mouth-feel, and rate of melt. Conventional formulation and processing techniques complemented by the use of specific additives such as emulsifiers and stabilizers enable some control, albeit limited, over the interfacial dynamics and stability. New ingredients and new technologies (such as low temperature extrusion) have been developed to enable higher levels of control on the interfacial behavior either through direct molecular intervention on an interface or new structuring processes wherein interfaces are created in a new or different way. Examples of new ways of influencing the ice, fat, and air interfaces will be discussed such as “ice structuring protein” and hydrophobins. Challenges that remain highlight the need for new types of molecular and microstructural interventions to achieve the next levels in design capability for the ice creams of tomorrow. 1. Introduction There is a widely held impression that the design and development of ice cream involves only issues relating to recipe and chefmanship borne out of kitchen skills. Although chefmanship is important, ice cream per se is a deeply technological product in all of its development, manufacture, distribution. and selling. Whereas product delivery to the consumer is aimed to achieve high appeal through various sensorial attributes, it is deep inside the structure where the science and technology is embedded. Designing ice cream to meet specific technical performance targets is a major challenge.1 This paper aims to show that, with deeper scientific understanding and some very exciting new technologies both in use and under development, some of the major barriers to better ice cream in terms of quality, stability, nutrition, and innovation can be overcome. 2. Design Challenge Ice cream is a product that truly operates on a range of spatial scales. On a macroscale, the sensory properties of the texture are perceived; these are determined by the microscopic details of the structure. In turn, microstructure is determined by complex molecular interactions. The main aim of ice cream manufacturers is to generate the correct microstructure in the ice cream to achieve the desired organoleptic characteristics such that the product can breakdown and melt away in the mouth, thus delivering the consumers’ preferences for taste. In addition, the structure needs to be sufficiently robust to withstand transportation and storage, so there is quite a balancing act to perform to * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: 44-1234-222748. Fax: 44-1234222007.
reconcile these simultaneous and often conflicting requirements. Therefore, in achieving the optimum microstructure, there are trade-offs between the formulation (levels and types of ingredients and actives such as process aids and stabilizers) and the processing regimes (heat transfer rate, temperature of freezing, etc). A general description of the science of ice cream is given by Clarke.2 Increasingly, as consumers demand healthier products, nutritional aspects of formulation become vastly more important. Whereas, for example, reduction of both saturated fat and sugar are desirable, they may not be immediately possible because these are crucial components for both the process conditions and the microstructure per se. A typical microstructure is one that consists of ice crystals and air bubbles in the size range 20 µm to about 100 µm, and fat droplets in the size range of 1 µm to 0.1 mm (Figure 1). These fine entities are embedded throughout a viscous solution of sugars, polysaccharides, and milk proteins known as the “matrix”. At another order of magnitude lower in scale, it is possible to identify the location of the fat. Fat droplets of size
