Food as soft matter

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A fantastic article about the soft matter aspects of food was recently written by Raffaele Mezzenga and his swiss colleagues in Nature Materials. The figure below concisely summarizes how many phenomena in the food sciences are the result of colloid interactions, with the example of casein proteins. These proteins can be treated as hard spheres or as the structural elements of a continuous network, depending on the interparticle interactions between them:

FoodDiagram.jpg

The top central figure shows the interaction energy (U/kT) as a function of volume fraction. At low volume fractions, but strong interactions between particles, fractal networks form. In the opposite case of high volume fractions with low interaction energies, dense suspensions are created. Three specific cases are illustrated around the central figure: A. Casein micelles in solution can be treated as hard spheres. The viscosity of the suspension increases until the system reaches a fully jammed state. The photo shows how the mixture can be turned upside down without separating. B. When a polysaccharide polymer is added to the system, various phases are possible through spinodal decomposition. Depending on the relative concentration, the system can form: (1) Xantham-rich droplets, (2) casein-rich droplets, or (3) a bicontinuous phase. C, D. The figures at the top show the similarities between ceramic materials (C) and casein networks in yogurt (D).

Phase Diagram of Milk

Milk.jpg

This diagram show the phase diagram of milk. Note that lactose sugar has a higher melting point than the milk fat and protein matrix. Tg defines the glass transition and Tf the freezing point of milk. The dotted lines near the glass transition indicate the delay time before nucleation occurs. The dotted line with circles at the ends and lables A & B describes the process for making powered milk. During process A, milk is heated and concentrated in an open system. In part b, a spray drying process occurs when hot gas introduced to the system to further evaporate the material.

Homogenization

You know how it says homogenized on every container of milk and as a kid you never knew what it was. It essentially changes the size of solids (colloidal particles) of fat and proteins in solution. By decreasing the size of the particles it creates a more stable dispersion. This is an emulsion rather than a colloidal suspension but there are ions within the micelles. Overall a very complex system.

"Milk is an oil-in-water emulsion, with the fat globules dispersed in a continuous skim milk phase. If raw milk were left to stand, however, the fat would rise and form a cream layer. Homogenization is a mechanical treatment of the fat globules in milk brought about by passing milk under high pressure through a tiny orifice, which results in a decrease in the average diameter and an increase in number and surface area, of the fat globules. The net result, from a practical view, is a much reduced tendency for creaming of fat globules. Three factors contribute to this enhanced stability of homogenized milk: a decrease in the mean diameter of the fat globules (a factor in Stokes Law), a decrease in the size distribution of the fat globules (causing the speed of rise to be similar for the majority of globules such that they don't tend to cluster during creaming), and an increase in density of the globules (bringing them closer to the continuous phase) owing to the adsorption of a protein membrane. In addition, heat pasteurization breaks down the cryo-globulin complex, which tends to cluster fat globules causing them to rise." [1]

Creaming.gif[2]

Cool movie of homogenization valve: [3]


Mayonnaise

Mayonnaise is an emulsion – mixture in which droplets of one liquid are suspended in another liquid which otherwise do not mix. These droplets are less than 1 micrometer in size, which is small enough to be able to pass through filter paper. Even though mayo is a mixture, it has thick texture and smooth appearance. Particles it is made of are always in constant motion and they don’t separate. In case of mayo, the main ingredients are oil and water. As we all know these two ingredients do not mix well because they separate from each other soon after mixing. Therefore, creating mayo emulsion requires more science and effort than one would think.


Mayonnaise.jpeg

http://whatscookingamerica.net/Sauces_Condiments/Mayonnaise.jpg

Mayonnaise consists of three main parts, as for any emulsion:

- Oil (dispersion phase containing particles suspended in liquid)

- Water (continuous phase in which the droplets (oil) will be dispersed into)

- Emulsifier (keeps oil and water from separating)

Other ingredients include one egg yolk and an eighth cup vinegar roughly for each cup of oil. The more oil you add the thicker the mayonnaise becomes. When the oil becomes separated into droplets, which are surrounded by a film of emulsifier, the oil then becomes immobilized and loses its fluidity. As more oil is added, more droplets are formed and the interfacial area between oil and vinegar increases.

Mayo owes its appearance to the fact that light is constantly reflected off of the suspended particles because they are smaller than wavelengths of light. This property makes the substance seem uniform to the naked eye, even though it is a mixture. It is interesting to see how mayo looks under microscope (image below).

Mayo microscope.jpeg

http://farm1.static.flickr.com/124/346778951_c910881021.jpg

In the upper image you can see course emulsions formed after the addition of only one tablespoon of oil. In the lower one, you can see the sample after adding one-fourth cup of oil. Both images have magnification of 200.

Here is an interesting article on mayonnaise modeling: http://www.tudelft.nl/live/binaries/5ba8080d-6331-49cb-9d68-658e450299f9/doc/DO05-4-2mayonnaise.pdf

Food science and food technology

All in all, food is the most essential part of human's life. By all means, it decides people's quality of life. As a result, two interrelated academic disciplines, food science and food technology, have emerged to bring better understanding and develop advanced improvements. Food Science is a discipline concerned with all technical aspects of food, beginning with harvesting or slaughtering, and ending with its cooking and consumption. It is considered one of the agricultural sciences, and is usually considered distinct from the field of nutrition. Food science is a highly interdisciplinary applied science. It incorporates concepts from many different fields including microbiology, chemical engineering, biochemistry, and many others. Food technology is the application of food science to the selection, preservation, processing, packaging, distribution, and use of safe, nutritious, and wholesome food.


Some of the sub-disciplines of food science and technology include:

  • Food safety - the causes, prevention and communication dealing with foodborne illness
  • Food microbiology - the positive and negative interactions between micro-organisms and foods
  • Food preservation - the causes and prevention of quality degradation
  • Food engineering - the industrial processes used to manufacture food
  • Product development - the invention of new food products
  • Sensory analysis - the study of how food is perceived by the consumer's senses
  • Food chemistry - the molecular composition of food and the involvement of these molecules in chemical reactions
  • Food packaging - the study of how packaging is used to preserve food after it has been processed and contain it through distribution.
  • Molecular gastronomy - the scientific investigation of processes in cooking, social & artistic gastronomical phenomena
  • Food technology - the technological aspects
  • Food physics - the physical aspects of foods (such as viscosity, creaminess, and texture)


Some recent examples of significant developments that have contributed greatly to the food supply are: Instantized Milk Powder, Freeze Drying, and Decaffeination of Coffee and Tea.

Starch

As described in the interesting properties of cake batter (see Emulsions and foams), starch is an essential component of the human diet in world civilizations. Pasta, bread, corn and potatoes are only a few examples. Starch granules are typically on the order of microns to tens of microns in diameter, with a blob-like shape. However, the sizes and shapes can vary depending on the source (potato, corn, wheat,...). Potato starch is seen in the image below.

Starch1.jpg

[4]

Though again exact composition may vary, starch is made of two polysaccharides: amylose and amylopectin. Amylose chains wind up into helices, while amylopectin tends to form branched, tree-like structures. Amylopectin typically makes for 80% by weight of starch, while amylose takes the remaining 20%.

Starch2.jpg

[5]

When starch is put into cold water, nothing happens really. But when the mixture of water and starch is heated past the gelatinization temperature, typically around 60-70C, then the granules swell, the amylose is pushed out of the granules and loses its helices. One then has a dispersion of amylopectin granules surrounded by a solution of amylose in water. When the water is cooled back down and no stirring occurs, the amylose polymers link the amylopectin granules together, and we get a gel.

Starch3.jpg

[6]

This is why, for example, if you leave badly-rinsed pasta or rice to sit and cool down in a pot, then it will become quite sticky fairly quickly; to prevent stickiness - synonymous with gel formation -, one must either add butter or oil, or stir the pot as it cools down.

Note that starches can be processed so that they are more heat- or shear-resistant. Moreover, the food industry is by far not the only one that has great uses for starch. For more common applications and information for starch, you can look up the wikipedia page [7] and its links.