Understanding food structuring and breakdown: engineering approaches to obesity

Many of the processing operations of the food industry have been undertaken for millennia and many of the problems are equally old. For example, Garnsey points out that in classical Rome, most of the population ate cheap meat from roadside stalls or taverns, rather than cooking it themselves (1). Many food products are structurally complex. This structure, and its breakdown in the mouth, determines the taste, texture and eating pleasure of each product. Their manufacture is also complex. For example, bread involves the creation of microstructure, coupled with heat and mass transfer, and the flow and deformation of highly non-Newtonian materials of which our engineering understanding is limited (2). Food products need to be metastable to deliver taste and flavour 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 (3). To make these products efficiently, a combined understanding of chemistry and material science is needed, together with knowledge of how the processing the material receives affects its structure, chemistry and attractiveness. This type of understanding can be used to develop structured foods that are inherently more healthy. It is our hypothesis that one way forward will be for the food manufacturer to encourage a change in lifestyle and to develop the next generation of foods, designed to be convenient, cheap, nutritionally balanced and enjoyable to eat. These foods will be structured in such a way as to control the rate of release of macronutrients and slow the rate of the stomach emptying, thus limiting the amount of food that people consume. We have recently discussed the ways in which food science and engineering might be used to develop healthier foods (4). Here, we suggest how enhanced understanding of food structuring and breakdown might help in reducing 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; then we discuss the scope for reducing obesity by designing the next generation of food microstructures. The starting point for the design of structured foods is an understanding of the science underpinning the various performance functions. This comes principally from the product microstructure (5) 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. Consider the design and development of spreads. Margarine is a 20% water-in-oil/fat emulsion. Absorbed at the water/oil interface is a layer of emulsifier (for size reduction of the drops), protein, which assists in-mouth destabilization, and crystallized fat, which stabilizes the droplet. The continuous phase is oil with a crystallized fat network, which contributes to the rheological and mechanical properties of the material. Three main in-use functions are important to a consumer: (i) the appearance, controlled by water drop size, crystal size, colour; (ii) the spreading properties, controlled by fat-solids level, crystal size and networking, water-phase volume and drop size, emulsion stability and (iii) the organoleptic properties, controlled by fat-solids level, melting curve, crystal size, water drop size, emulsion stability, flavour 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. For in-mouth properties, however, it is necessary to understand structure breakdown. 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 flavour. 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 (6), fat crystal size, morphology and networking. From its invention until about 30 years ago, margarine changed little. However, increasing awareness of the effects of high dietary fat levels and saturated fats on coronary heart disease (7) led to the reduction of both saturated fat level and absolute fat level. Evidence also showed that high polyunsaturated fat levels in diet could aid the prevention of coronary heart disease (7). While the need to reduce saturated fat levels and increase polyunsaturated fat 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 interface (8) 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 to 40% and lower. Ultimately, this led to the development of phase-separating biopolymer systems, which could mimic the rheology (9) of water in oil/fat emulsions, allowing the development of 0% fat products (10). Most recently, spreads have been used as the delivery vehicle for fat-soluble actives that can reduce blood low-density lipoprotein cholesterol levels. The type of fat used in margarine is important and particular attention needs to be given to trans-fatty acids, a particular concern. However, it has been possible to remove trans-fatty acids from margarines by adopting the microstructure approach. All foods pass through a common unit operation, the gastrointestinal tract, yet it is the least studied and least understood of all food processes. To design the foods of the future, we need to understand what happens inside people in the same way as understanding any other process. The mouth, as mentioned above, is a complex environment in which the food is broken down by chewing action, mixed with saliva that contains enzymes and undergoes a temperature change as heat flow occurs (11). The foods are processed to the stage at which a bolus can be formed and swallowed. As these processes are taking place, molecules are released and delivered to the nasal cavity (12). A number of investigators are actively researching product microstructure design for in-mouth behaviour and delivery of molecular species, such as flavours, tastes (13), aftertastes (14,15) and physical sensations such as fatty feeling or mouth coating. The next stage is to consider how the food behaves in the stomach. Magnetic resonance imaging (MRI) studies (16,17) have shown that the stomach behaves as a poorly mixed system in which the walls oscillate a small amount, causing some surface mixing and erosion. At the same time, acids and enzymes are added to help break down the food into materials that can be absorbed into the blood stream. The time taken for complete digestion depends on the original food structure, with liquids being digested more rapidly than semi-solids or cellular structures. This area of research is hampered by the available techniques that are either intrusive or too slow or of too low a resolution. In the near future, there will be measurement techniques (18), including rapid nuclear magnetic resonance imaging methods that will overcome these limitations. The small and large intestines are the most difficult part of the process to take engineering measurements. Therefore, little is known about the processes that occur from an engineering perspective. This is likely to change in the next few years as MRI is just starting to be capable of measuring the processes (19) and the first basic models (20,21) are starting to appear. The opportunities for food manufacturing to deliver health benefits and assist in solving the obesity problem can be divided into four areas. There are potentially three ways to design microstructures to control the delivery of fat and modulate people’s food consumption at lower fat levels: (i) Use of emulsions that are unstable in an environment similar to the stomach will result in coalescence and creaming in the stomach (22). This could lead to the fat forming a layer on the top of the stomach where it would interact with the receptors to send signals to the brain about the state of satiety. The nature of the fat is important here. It might be possible to design emulsions to break in the stomach to signal the feeling of fullness/over indulgence, and thus limit the amount of food consumed. The technical problem that needs to be overcome is to combine storage stability and in-use performance with instability in the stomach. (ii) Spiller has suggested that if fat passes from the stomach into the ileum without being digested, the body responds by sending signals to the brain, slowing the rate of stomach emptying and slowing digestion, the so-called ‘ileal break effect’ (23). Emulsions could be designed to give all the eating pleasures and performance of present products but with this new functionality. (iii) The physical state of fat (24) changes the rate of digestion, with solid fat crystals digested more slowly than liquid fats. However, the use of long-chain saturated fatty acids that are nutritionally unhealthy will need to be limited, and any solid particles will need to be hidden during consumption to prevent sandy/gritty sensations. A potential way around these problems is to develop Pickering emulsions with very small droplets having solid crystal shells at body temperatures and liquid oil cores (unsaturated/nutritionally good oils) (25). As convenient foods and the tendency to snack has become normal behaviour, people are eating softer, more liquid products. A consequence of this is that the proportion of digestible carbohydrate has increased at the expense of fibre. In addition, digestible carbohydrates are readily available and absorbed (26,27). This results in high levels of glucose in the blood shortly after a meal, which is then stored quickly inside the body, leading to a rapid onset of hunger and a desire to eat again. This pattern of blood glucose levels has been linked with an increased risk of insulin intolerance and type 2 diabetes (28). The food industry needs to develop soft liquid−1 foods that deliver glucose over an extended period of time. Potentially, this can be achieved, for instance, by encapsulating carbohydrates and starch inside non-digestible hydrocolloids, reducing the rate of amylase attack. Some types of starch are already known to change the rate of digestion. A challenge is how to achieve this ‘slow burn’ while retaining all the other product attributes. Liquid products can be made to self-structure inside the gastrointestinal tract (29) by choosing hydrocolloids that are acid-sensitive, such as alginate. As the liquid alginate solution reaches the stomach, the acidity causes spontaneous gelation. With careful selection of the alginate, it is possible to gel the whole stomach content, slowing both digestive processes and stomach emptying. Maintaining stability of the formulation in real foods is an ongoing challenge. Such problems will be overcome and self-structuring products will reach the market that deliver varying degrees of mechanical rigidity from a wide range of food products – liquids, semi-solids to solid foods. This could have a major impact on the obesity problem. Functional peptides can bind to receptors in the gastrointestinal tract and influence the feeling of satiety, impact on stress levels and can even affect our mood (30). However, most are unpleasant to eat and are often chemically unstable. One possibility is to encapsulate these materials in ways that allow release at the desired place and rate in the gastrointestinal tract. The structuring routes discussed here are available to encapsulate any type of active, including micronutrients and those that suppress appetite. In addition, by designing the product microstructure to positively interact with biosurfaces inside people, we will have biologically targeted products. People find it difficult to follow the current recommendation to eat between five and 10 portions of fruit and vegetables a day in order to get the right balance of micronutrients and the required daily intake of complex carbohydrates (31) (dietary fibre). In addition, the cellular nature of fruit and vegetables is important in order to fill the stomach and be slowly digested. In the future, microstructural design will allow convenient good-tasting products that deliver the required high levels of fibre, bulk and micronutrients to be developed. Clearly, the replacement of fat in products is critical in fighting obesity and there is a real need to reduce the amount of fat that is invisible to the consumer. Fat replacement was an area of interest a few years ago (32) and a number of products appeared in the market. Some of these are still on sale, for example, low-fat spreads, biscuits, cakes, etc. However, many of the products launched did not survive because of poor taste and texture. In some of these products, high levels of sugars were used to maintain succulent textures, thus resulting in only marginal calorific advantages. A system approach based on understanding the dependence of performance on the physical chemistry and structuring of fat is required. To be successful, products must be developed that deliver flavour at the desired rate. If flavours are delivered in the wrong way, an imbalance is perceived, resulting in hollow or unpleasant flavours. Products need to coat the surfaces of the oral cavity and the throat to generate the physical feeling of fat and delivering pleasant aftertastes (14,15). Low-fat emulsions with as little as 5% fat can give the feeling of fat as they break in the thin film between the hard palate and the tongue (15). The sensation of melting is potentially the most difficult to recreate. It is clear that, to reduce fat significantly, food manufacturers will need to extend the functionality of any fat used, for instance, by building smart structures, such as stable duplex emulsions with a thin coat of fat around a structured water phase and an inner core of fat acting as a flavour sink. The replacement of sugar with artificial sweeteners has had a significant effect on the intake of calories. However, sweeteners taste artificial and are frequently reported as having health issues (33). Potentially, an alternative route will be to increase the efficiency of sugars by designing microstructures to interact with the receptors for longer and to deliver a larger proportion of the sugar present to the receptor. This should give the perception of sweetness from a low-sugar product. What is needed is to make binding specific to the soft surfaces of the mouth to avoid binding to teeth for long periods of time, resulting in increased tooth decay. Sugar often has a structural use, for instance, to aid retention of water (in cakes and biscuits) in order to impart succulence on chewing, to control ice formation and ice content in ice cream and frozen desserts, and to control the material properties of composite products such as chocolate and sorbets. There is potential to replace at least part of the sugar in these applications. We have suggested ways in which understanding of food structuring can deliver healthy foods to the consumer in the next decade. Historically, this is not an area in which engineers have been active, but it is one in which the skills of the engineer can be applied (34). Several areas need to be developed. At present, new food product development uses largely qualitative models of structure–function relationships, necessitating cycles of prototyping and testing to develop the final product. A key requirement to help us move forward will be the development of better understanding of the mechanisms underlying the functional behaviour of a given food structure and the ability to quantify this using mathematical modelling. Some progress has already been made (35). If, however, we are to exploit the opportunities of foods with health advantages from their behaviour in the human digestive system, it will be necessary to better understand and predict the digestion and subsequent metabolism of structured functional foods. The structuring process is integral to the design of a structured product. Hill rightly argues that product and process development for structured consumer products needs to be carried out concurrently (36). This is not only from the perspective of facilitating scale-up, but also to use the opportunity of developing novel structures through innovative processing. An example would be the adoption of extrusion processing for ice cream. This technology allows ice cream to be extruded at a low temperature, −12°C, compared with −5°C for the traditional process using scraped surface heat exchangers. Extruder processing reduces the time required to take the product to the normal storage temperature of −25°C and therefore the impact of ripening processes on ice crystal size distribution. Additionally, however, the process has significant effects on the structuring of the air phase, resulting in smoother creamier products for a given composition. Here, the process gives a better product. To achieve integrated product and process design, prototyping equipment in which the structuring conditions are well defined and controlled must be used. Similarly, our ability to predict structure–process relationships must be developed from its current level. The manufacture of structured foods is still mainly based around a limited number of traditional unit operations in centralized factories taking advantage of economies of scale. These unit operations allow limited control over product structure. To produce some of the structures that will be needed for functional behaviour in the human digestive system, new unit operations will be required. It will be necessary to rethink production of structured particulates from the bulk processing of material to operations that structure at the individual particle level. An extreme example would be the application of micro-fluidics to structured material design (37). In addition to achieving greater control over product microstructure, such technologies move scale-up from the duplication of process conditions in larger volumes to scale-up by parallelization. While this might at first seem to simplify scale-up, it will bring new challenges in distributing fluids uniformly to large assemblies of small channels. The ‘one size fits all’ solution will not necessarily be applicable for foods providing health will need to some level of product for particular in the extreme make products to deliver to the consumer. to this problem will from on production in centralized factories to new to the may be used to make a material that is at points based on products may be at point of such a system for the manufacture of foods has been The of this has been to how better understanding of the structuring and breakdown of foods during processing and eating could be used to make products that are inherently healthy. Clearly, this is not the whole to the obesity problem. However, the current industry approach is often To achieve the increase in healthy consumption required for the health of the population will new science of the type of interest was