Indice
A close look at the structure of food
In our daily life we consume food products with a large variety of structures. We will use common examples including homogenized milk, whipped cream, and cheese to explain how diverse structures are created from an identical starting material, in this case: milk. During processing the physico-chemical interactions among milk constituents such as proteins and fats, and their distribution within, give rise to what we call “the structure of food”.
Structure itself is the ultimate result of the most thermodynamically efficient way molecules can organize and coexist in an environment. In this example, milk proteins and fats can align in a way where the fat forms droplets and the proteins surround those droplets. This happens when milk fat and proteins are present in water that contains lactose and minerals as found in homogenized milk or from a food scientist perspective what is generally called an oil-in-water emulsion.
When air is blown in (i.e., whipping) to the mixture containing milk proteins, fat, water, lactose, and minerals, whipped cream is formed. In this case, the proportion of milk constituents is different than in homogenized milk and the air is dispersed as bubbles surrounded by proteins and fat. This structure is known as a foam.
The same milk proteins can form cheese upon enzymatic treatment by using i.e., chymosin which curdles the milk. Cheese is usually referred to as a gel and we can picture it as a sponge, where the scaffold is made of proteins filled with fat, water, lactose, minerals, and microorganisms [1] (Figure 1).
Now, let’s imagine, water is surrounded by molten fat, and the two liquids are repulsed by each other, so they try to stay as far apart as they can. However, when we start mixing them, water is forced inside the lipid matrix and to reduce its contact with lipids, it adopts a spherical shape (called a droplet). Proteins that are present in water, migrate to the droplets surface and release the tension that the molecules are experiencing: the interface between oil and water is now stabilized.
As the mixture cools, the lipids calm down because of thermal motion reduction, and when they are quiet enough, they align themselves and form crystal units that stick to each other, forming clusters known as aggregates. Different aggregates then interconnect and form a crystalline network locking the water droplets inside this network [2]. Guess what, we have just produced margarine. We bet you never thought that there was so much science spreading through your sandwich!
From these examples we can see that the same core molecules can give rise to very different complex structures. Structure is the ultimate expression of how ingredients interact. The difference between these structures relies on the varying interactions that molecules establish among each other [3]. Simply, we can think about these interactions, or lack of, as a consequence of how the molecules enjoy each other’s company. The structure of homogenized milk arises from the fact that proteins have an affinity with both lipids and water and therefore they prefer to stay at the oil/water interface, while lipid molecules like each other but hate water, so they prefer to segregate in droplets.
Food scientists like to visualize the food we eat every day as a structured entity and categorize them as emulsions, foams, gels, dispersions, suspensions, and more, as we have experienced with the case of milk, whipped cream, cheese, and margarine.
So far, we have described structures on a very small scale in a single category of foods, dairy-based and lipid-based food products. However, food is much more elaborate than a single microscopic structure. Indeed, the food we all enjoy is composed of several co-existing structured elements interconnected at multiple length scales, which form a multiphasic system. Examples include sausages, which are a dispersion of fat in a protein gel; chocolate, which is a dispersion of cocoa powder and sugar in a cocoa butter lipid network; ice cream, which is a partly frozen foam formed by air and water crystals where fat is dispersed as droplets; bread, which is a foam stabilized by proteins and carbohydrates; and pasta, which is an edible glass.
The interactions among ingredients are the key factor determining the structure of food and this is important because it influences both our sensorial perception (such as texture, palatability, melting, etc.) and the macro and micronutrient delivery and availability in our digestive tract [3]. In addition, the increasing consumer expectations for healthier, sustainable, and tasty food products (reduced in fat, sugar, salt, from vegetable sources, enriched in bioactive molecules, etc.), challenges food scientists to come up with new creations.
Can we modify the structure of food to make it taste better? Can we use alternative ingredients to structure new food products? Can we design structures to generate healthier food? These are typical queries, continuously stirring in the mind of a food scientist. This all derives from to one simple question: can we design new foods structures?
How do food scientists create new food products?
Food scientists have the ability to tailor existing food structures to develop new and innovative ingredients. The main reason to include new structures is to reformulate or create new foods for people with specific nutritional requirements (e.g. reduced sugar, fat, salt, gluten-free) or willing to integrate their diet with selected macro and micronutrients (e.g. minerals, omega-6 and omega-3 fatty acids, vitamins and pro-vitamins).
Another reason to develop new structures is to facilitate the creation of new food products to support people with specific ethical beliefs and personal motivations (e.g. vegetarian and vegan). Increasingly the environmental impact of food is coming under the spotlight and new foods are being designed to reduce the impact of high energy, long distance transport, and resource demanding ingredients such as dairy proteins and palm oil [3].
The process begins with the simple question: what type of functionality do we need in this food product? This then leads to more specific queries: do we want it to taste like meat even if it is made from plants? Do we want it to be enriched with vitamins which maintain their nutritional properties during storage? Do we want to replace butter with oil without losing the texture of the product? After having identified and narrowed the challenge to work on, food scientists design several possible structures to accomplish the goal they set, allowing fantasies to run wild.
Then, they study the feasibility of each structure based on their understanding of the fundamental interactions among the molecules of different ingredients (hydrophobic, steric, electrostatic, etc.) and the factors governing them (temperature, time, shear, pressure, pH, ionic strength, etc.) [4]. Finally, food scientists assemble in laboratories the desired structure using the ingredients they have selected. This mind set is what allows food scientists to master the art of food product design (Figure 2).
By controlling the arrangement of molecules, we can develop new food products from sustainable sources with tailored properties or find innovative uses for known ingredients. For example, in meat alternatives, plant proteins are directed to form fibrous structures able to mimic the corresponding animal products through a thermomechanical process [5].
Oils are converted into semi-solid materials like palm oil or margarine by physically trapping the oil in a network made of proteins, polysaccharides, or other lipids, leading to novel and healthier fats [2]. Sugar has been engineered by spray drying after the addition of pressurized gas generating a porous crystalline structure. Consequently, sugar dissolves faster in the mouth and less is necessary to reach the desired sweetness [6].
Another example is the formation of powders where volatile gases are encapsulated in a hard lipid shell so that the aroma can be trapped in the capsule and released only during cooking [7]. Imagine the exciting flavours you could soon have in your cupboard waiting to enhance any meal. These examples show the endless possibilities for the development of new structures.
Designing protective and delivery systems for functional foods
Functional food can be defined as a category of food products that has proven health benefits when regularly consumed [8, 9]. Examples of new food structures for improving health related functionality are represented by beverage supplements, where lipid droplets encapsulate selected macro and micronutrients, such as vitamins, carotenoids, omega-6 and omega-3 [4, 10]. In order to incorporate and protect these nutrients in food products, specific and tailored structures need to be developed.
Among food products, beverages are consumed worldwide and represent the perfect target for functionalization through enrichment with bioactive compounds. Familiar examples include energy drinks, sport drinks, dairy-based and plant-based beverages (soy milk, oat milk, etc.) [11]. Together, let’s explore how we would design a beverage enriched with oils containing a high proportion of omega-6 and omega-3 fatty acids (e.g., fish oil and oat oil). These fatty acids are lipophilic molecules essential for maintaining human physiological functions and health, indeed they are also called essential fatty acids. In this scenario, we ask ourselves: what are the main challenges when working with these essential fatty acids?-
As you may recall, the first challenge is the immiscibility of oils with water, so we need to disperse the oil in the beverage as droplets and formulate an emulsion. The second challenge is that omega-6 and omega-3 fatty acids are prone to degradation (i.e., oxidation), which decreases their nutritional value and can be responsible for rancidity in food products. Therefore, we need to protect the fatty acids from oxidizing agents, but there is a paradox: when we produce an emulsion, we are also increasing the area where oxygen can enter and be in contact with oil (there are millions/billions of oil droplets in our emulsion!).
The third challenge is to consider where our emulsion will be used and in particular what is the acidity of the beverage? Will the beverage be thermally treated? Is there any other ingredient which breaks up the emulsion when added into the beverage? After considering all these aspects, we can design our emulsion based on the target beverage which can eventually fulfil the consumer’s requirements.
Here we report some examples of emulsions specifically designed to deliver and protect oils rich in omega-6 and omega-3 fatty acids in drinkable yoghurt. Considering these challenges, the solution proposed was to design an emulsion with an extract from spruce. Spruce extract is a novel, natural, value-added, emulsifying and stabilizing agent, which contains antioxidants able to protect the fatty acids from oxidation. In addition, the extract can form a stable emulsion and the latter can be added without any side effects to yoghurt’s physico-chemical properties [12].
A similar approach has been also applied to other functional products. For example, in order to make functional cheese enriched in vitamin D and essential fatty acids, an emulsion with vitamin D solubilized in oil, and dairy proteins and lecithin as emulsifiers was designed and then added to milk to produce cheese. In this case, vitamin D and fatty acids were also protected in the resulting cheese, leading to a functional dairy product [13].
Moving to another food category, confectionary products can also be made functional. In this case cellulose and oil were used to create a structure able to replace less healthy saturated fats in chocolate spreads, enriching the product with essential fatty acids and fiber [14].
These few examples show how food structure design can be used to functionalize food products, increasing their nutritional value and health benefits.
Food structure design can be used to modulate the availability of macro and micronutrients during digestion. This allows us to control the caloric intake of food products or properly deliver nutrients in our digestive tract [3, 15]. For example, trapping oil in a network of indigestible carbohydrates leads to a reduction of oil digestibility [16].
However, this fact should be taken into account from a nutritional point of view and labels should report food composition and digestibility of each nutrient in order to avoid misleading information. A recent scientific article dealing with the World Health Organization’s drafted guidelines for reducing dietary saturated and trans fatty acids, pointed out the need to consider the structure of food products in terms of nutrients available in foods [17].
Let’s imagine that we develop a structure that makes any type of lipid inaccessible to digestive enzymes, meaning lipids (e.g., solid fat and liquid oil) will not be digested [18]. However, the label of the food product containing our new structure will still report the caloric content derived from the full digestion of these now indigestible solid fats, leading the consumer to the conclusion that the product is less healthy than similar products containing liquid oils.
This concept can be extended to other macro and micronutrients that should be reduced in our diet (e.g. salt and sugar). Instead of substituting them, we can develop a structure that makes them inaccessible during digestion while preserving sensorial attributes. On the other hand, food scientists need to consider this nutritional accessibility when developing new food structures: if the structure is not accurately designed and developed, it can lead to beneficial ingredients becoming unavailable causing nutritionally imbalanced food products.
Food structure design and nutrients availability
Food structure design can be used to modulate the availability of macro and micronutrients during digestion. This allows us to control the caloric intake of food products or properly deliver nutrients in our digestive tract [3, 15]. For example, trapping oil in a network of indigestible carbohydrates leads to a reduction of oil digestibility [16].
However, this fact should be taken into account from a nutritional point of view and labels should report food composition and digestibility of each nutrient in order to avoid misleading information. A recent scientific article dealing with the World Health Organization’s drafted guidelines for reducing dietary saturated and trans fatty acids, pointed out the need to consider the structure of food products in terms of nutrients available in foods [17].
Let’s imagine that we develop a structure that makes any type of lipid inaccessible to digestive enzymes, meaning lipids (e.g., solid fat and liquid oil) will not be digested [18]. However, the label of the food product containing our new structure will still report the caloric content derived from the full digestion of these now indigestible solid fats, leading the consumer to the conclusion that the product is less healthy than similar products containing liquid oils.
This concept can be extended to other macro and micronutrients that should be reduced in our diet (e.g. salt and sugar). Instead of substituting them, we can develop a structure that makes them inaccessible during digestion while preserving sensorial attributes. On the other hand, food scientists need to consider this nutritional accessibility when developing new food structures: if the structure is not accurately designed and developed, it can lead to beneficial ingredients becoming unavailable causing nutritionally imbalanced food products.
Acknowledgments
Authors would like to acknowledge Troy Faithfull and Joanne Fearon for their valuable feedback on the article.
Conclusion
Food is a complex material full of different structures that food scientists can tailor to make them taste better and be healthier. Next time you eat or drink any functional food or just simply a vegan burger, remember the critical thinking and creative process that food scientists applied to develop it.
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Fabio Valoppi
Marie Skłodowska-Curie Individual Fellow (ULTRA-OLEO project, grant agreement No 836071) at University of Helsinki (Finland). Fabio obtained a PhD in Food Science from University of Udine and the title of Docent (Adjunct Professor) in Food Materials Science from University of Helsinki. His research interests focus on the design and development of food materials, especially emulsions, fats, oleogels, and their physical and nutritional properties.
Francesca Bot
Senior R&D Specialist and researcher at University College Cork (UCC) (Ireland). Francesca obtained a PhD in Food Science from University of Udine.
Her research interests focus on design and development of infant formula and nutritional beverages based on dairy and lecithin ingredients and their techno-functional and nutritional properties.
