Product/Process integration in food manufacture: Engineering sustained health

Food processing is a very old industry. Many of the processing operations have been practiced for millennia and many of the problems of the food industry are familiar; for example, Garnsey1 points out that in classical Rome, most of the population ate cheap meat from roadside stalls or taverns, rather than cooking it themselves. The modern food industry is about 200 years old; Thorne2 defines the beginnings of the industry to the production of the first-heat sterilization plant in France, developed by Appert in the early 1800s. The industry is now huge; for example, it is 13.6% of the total EU manufacturing sector, with a turnover of €799 billion in 2003. However, the sector is very fragmented with a few multinationals — Unilever, Nestle and Danone for example — competing on the global market with global brands and a large range of products, while smaller enterprises serve local markets and concentrate on regional preferences. In 2001 99% of the companies in the EU F&D sector had less than 250 employees. The business, thus, differs from (for example) the chemical and pharmaceutical sectors, in which a few very large companies dominate the market. Although the major multinationals and retailers can afford to use sophisticated technologies, much of the industry is low-tech. In the U.K., for example, retailers operate sophisticated supply chains that can deliver “cook-chill” products to the market place very rapidly. These products have been cooked and then cooled to a level where microbial growth is minimal;3 this type of food needs to be moved rapidly from manufacturer to consumer, or the shelf life is unacceptably small, and so this model is not possible in countries the size of the U.S. or in countries with less well developed infrastructures. The first concern of the food industry is product safety, and it has developed sophisticated design and quality assurance tools to do this. Thermal preservation is still the basis of a large industry. For example, canned food is given a very high thermal process to ensure that it is sterile, although the contents are, as a result, not of very high quality. The death of canning as a process has been predicted for the last 50 years, however, despite such predictions it is still used and studied.4 Alternative processes that give the same level of safety, but which produce higher quality food have been widely investigated by academics and industry. These include volumetric heating methods, such as microwave,5 radio frequency6 and electrical resistance (“ohmic”) heating,7 in which the key processing need is to have thermal uniformity throughout the material.8 In addition, nonthermal methods, such as high-pressure9 and pulsed electric fields10 have been studied extensively; here the heat applied is low, so quality degradation due to heating is minimized. These processes are academically fascinating, but as yet too expensive to be viable for normal food production. The food industry is highly innovative in terms of products, much less so in terms of processes. are structurally complex (see Figure 1), and this structure determines the taste, texture and thus eating pleasure, involve complex processes in their manufacture, e.g., bread — which involves the creation of microstructure, coupled heat and mass transfer, and the flow and deformation of highly non-Newtonian materials of which the engineering understanding is limited,11 need to be metastable in order to deliver taste and flavor on consumption, for example, the confectionary fats in chocolate are highly polyphasic, with six polymorphs melting within 20°C of one another, and the form that the consumer enjoys is not the thermodynamically stable one.12 Microstructures of some typical food products: (a) freeze fracture TEM of crystalline fat at the water/oil interface in a fat continuous low fat spread (image width 5 microns); (b) freeze fracture TEM of an air cell in a whipped cream, showing crystalline fat droplets stabilizing the foam structure (image width 20 microns); (c) confocal micrograph of an Ice cream showing phase separation of the biopolymers in the matrix phase (image width 160 microns; green is the milk protein and red is the polysaccharide); (d) Confocal micrograph of an air bubble in ice cream with de-emulsified fat particles (image width 125 microns); and (e) confocal micrograph (image width 80 microns)of low fat mayonnaise showing oil droplets (green) and swollen starch phase (red). Thus, to make these products efficiently, a combination of understanding of material chemistry and material science is needed, together with an understanding of how the processing which the material receives affects its structure, chemistry and attractiveness. In the 100 years or so of its existence, the chemical engineering profession has been highly successful in developing the scientific and engineering principles required for manufacturing bulk chemicals. Such products are specified by a chemical composition. However, engineers have been less central in the design of structured foods (Figure 1), where the product is specified by a wide range of functions, such as eating and cooking, as well as physical, chemical and microbiological stability throughout the product lifetime. This approach has worked so far, and the food industry has successfully produced a range of products that are attractive and enjoyable to eat. As a result of changes in lifestyle, however, diet is increasingly affecting health (Figure 2), and the incidence of obesity has more than doubled in the U.S. in the past 15 years. The consequences in health terms are very dangerous. Percentage of the US population (http://www.cdc.gov/nccdphp/dnpa/obesity/trend/maps/), state by state, with a body mass index above 30, regarded as obese, over the past 15 years In this article, we consider how engineering understanding of food processing can play a major role in the next stage of development of the food industry, and help reduce the incidence of obesity. We first consider a typical structured food process, that of margarine manufacture, and how understanding of product form and function has been used to reduce fat content. The discussion is then expanded to consider the scope for reducing obesity by designing the next generation of food microstructures. The starting point for the design of structured food products is an understanding of the science underpinning the various performance functions. This comes principally from the product microstructure,13 which in a typical structured food has dispersed phase-length scales of the order of 10 μm composed of liquid or crystallized particulates. These exist within a continuous phase which may be of complex rheology and further structured by polymer or particulate networks. As a spread, to provide lubrication and flavor during eating, in frying, as a heat-transfer medium and flavor carrier; in baking, as a source of fat for texture control. a layer of emulsifier, for size reduction of the drops protein which assists in-mouth destabilization and crystallized fat (Figure 1a) which stabilizes the droplet. The microstructure of margarine. Appearance — controlled by: water drop-size, crystal size, color Spreading properties — controlled by: fat-solids level, crystal size and networking, water-phase volume and drop-size, emulsion stability, Organoleptic properties — controlled by: fat-solids level, melting curve, crystal size, water drop-size, emulsion stability, and flavor level. In designing margarine these functions are characterized and related back to the controlling structural parameters. Appearance, for example, can be related to spectral reflection and absorption properties; spreading to rheological and mechanical properties. When it comes to in-mouth properties, however, it is necessary to understand the structure breakdown mechanisms involved. This is shown in Figure 4. In the early stages of mastication, the margarine is mixed with saliva, and the fat phase starts to melt. This causes the water drops to coalesce and form an increasingly coarse emulsion leading to inversion and sudden release of water-phase components (e.g., salt), and an increased perception of flavor. To characterize this behavior, simple tests have been developed: for instance, conductivity is measured as a function of temperature; perception in mouth viscosity and melting rates are determined using trained panels. Underlying all these properties is the product formulation in terms of: phase volumes, fat blend formulation, emulsifier/protein levels and microstructure, such as water droplet size,14 fat crystal size, morphology and networking. The “in mouth” destabilization of margarine Pasteurization to increase product shelf life, production of a controlled mean-water droplet size, production of a minimum fat crystal size, control of the fat crystal network. To minimize fat crystal-size rapid cooling is applied in high-surface to volume scraped surface heat exchangers, so that nucleation dominates over growth. Fast cooling, however, forces the fat into a metastable α polymorphic form that transforms over a period of a few minutes into the more stable β' polymorphic form.15 In the absence of shear this would lead to extensive networking and very hard products. The material is, therefore, “worked” in a pin stirrer, which provides residence time for the polymorphic transformation, and applies mechanical energy to the system to break up the crystal network. The sequence of scrapped surface heat exchanger followed by pin stirrer is repeated one or more times in order to develop an appropriate degree of crystal network in the desired polymorphic form, before packing and distribution at 5°C, to minimize kinetic effects on structure, such as Ostwald ripening and microbiological growth. The first scraped surface unit also subdivides the emulsion droplets to produce a fine emulsion with typically 5 μm water droplets. Reduction of saturated fat level (SAFA), reduction of absolute fat level. The first of these was further amplified as evidence showed that high polyunsaturated fat levels (PUFA) in diet could aid the prevention of CVD.16 From the product and process design perspective, while the need to reduce SAFA and increase PUFA levels provided the opportunity to design new “soft spreads” it also set challenges in controlling crystal networks with minimum solids levels and control of crystallization at the water oil interface17 to stabilize the water phase with fewer solid particles. The need to reduce fat level set even greater challenges in controlling emulsion structure and stability as levels were reduced to 60% then 40% and lower. New processing and structuring technologies were introduced.18 It was found for fat continuous low fat spreads, for example, that inversion processing of a water continuous premixes led to smaller water droplet sizes and better products. Structuring of the water phase with biopolymers was required to control stability and improve organoleptic performance. Ultimately, this led to the development of phase separating biopolymer systems which could mimic the rheological behavior19 of water in oil/fat emulsions allowing the development of 0% fat products.20 Most recently, spreads have been used as the delivery vehicle for fat soluble actives which can reduce blood LDL cholesterol levels. Sauces and dressings where the structure of the oil in water emulsion (Figure 1e) controls the performance of the product and low fat versions are designed to have hydrated starch grains to mimic oil droplets. Whipped cream and ice cream, which depend on crystalline fat droplets at air bubble surfaces to stabilize the foam (Figure 1b and d) and to give the required oral properties. In both these products the water phase contains biopolymers that additional have their own microstructure (Figure 1c). Currently, the development of new structured foods is still separated into product development and process scale-up phases. Product development is carried out at the laboratory/mini-process unit scale using a combination of qualitative models of structure/performance behavior and experience. These allow the development of first prototypes that are characterized, analyzed, and refined using the types of method described earlier for margarine. Some of these will relate quantitative measures of structure to performance; many infer structure performance relationships from empirical measures of a bulk property. Final evaluation and acceptance is by consumer testing. Frequently in the product development phase standard equipment that is poorly characterized or controlled from a processing perspective is used. Furthermore, preservation regimes very different from those required in final manufacturing are often employed. As noted earlier, process innovation in the food industry has also been comparatively slow compared to others. For a particular product type, a limited range of unit operations have been employed for some considerable time, and the same process line is used to make a range of different product structures; as is the case in spreads processing. A particular design objective and its processing solution, such as long shelf life via UHT processing, can come to dominate the product design, so that it is applied broadly across product types and leads to large compromises in product structure and performance, and to problems in the scale-up and commercialzation of new products. 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