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Food dispersions includes emulsions such as milk, cream, sauces, etc. The main characteristic of these foods is the presence of small particles, and the consequent high interfacial area between the particles and the continuous phase. The properties of food colloids are defined by the interactions among the particles.

Food emulsions consist of an oil phase containing hydrophobic compounds and an aqueous phase containing water-soluble compounds. One phase is dispersed into the other, defined as oil-in-water emulsions or water-in-oil emulsions, dependently if water or oil are the continuous phase, respectively. Emulsions are thermodynamically unstable, and phase separation can be deaccelerated or even prevented through kinetic factors. The origin of destabilization is based on gravitational force, attractive and repulsive forces among the particles, etc.

The destabilization can then be seen by creaming, flocculation, and coalescence. In addition to these, emulsion phase inversion and Ostwald ripening are phenomena that can happen in emulsions. Creaming is a phase separation caused by the upward migration of droplets due to density difference between phases. Flocculation is the aggregation of droplets due attractive forces. Coalescence is the merging of droplets.

The dispersion of water in oil for the production of mayonnaise is one of the most known examples of food emulsions.

Stokes Law and phase stability

Even in apparently stable systems, with a shelf life of several years, the number and size of droplets change with time. Stokes’ Law gives the creaming / sedimentation rate for an isolated, rigid, uncharged droplet: U=2/9 R2dρg/η. R stands for the radius, dρ for the density difference, g for gravity and η for viscosity. Creaming may be considered as negligible compared with Brownian motion when U is less than 1 mm/day. Stokes’ Law shows how to prevent or minimize creaming: i) Reduction of droplet size, for instance by the addition of considerable amounts of amphiphiles such as surfactants, or by the use of homogenizers at high operating pressure. ii) Reduction of density differences between the phases. Density difference between the oil phase and water phase is, depending on other factors, about 50 kgm-3. While the density of large droplets is similar to the oil phase, very small droplets have a density closer to that of the aqueous phase. iii) Tuning the viscosity of the continuous phase, by adding polymeric thickeners, for instance gums. iv) in the moon.

 

A conversation where bacteria and fungi are mentioned usually triggers a red alert in our head since they are associated with some mean diseases. However, when we look back in history, the activity of yeast and bacteria were essential for our lifestyle, being the major responsible for many tasty foods and beverages that were and still are part of our culture.

Imagine a world without bread, beer, wine, cider, coffee, mushrooms, pickles…it would be for sure less interesting! It is estimated that there are one trillion different species of microorganisms on Earth, which shows the tremendous variety of bacterial and yeast species.

Beer fermentation is the process where the sugars coming from the malt are converted into alcohol and carbon dioxide by the activity of yeast and in the absence of oxygen. Traditionally, beer fermenting yeasts can be divided in two types: ale and lager.

 

The pre-activation of yeast, where multiplication and yeast mass increase takes place, is a fundamental step for an healthy alcoholic fermentation of wort

Ale yeast

In the old times they were defined as top-fermenting yeasts since their cells would be collected from the top of the fermentation vessel. The most relevant yeast is Saccharomyces cerevisiae (also called brewer’s and baker’s yeast) and it requires fermentation temperatures around 18ºC – 22ºC (64ºF – 72ºF). In comparison to a traditional pale lager, ale beers usually display a fuller body and more intense fermentation-derived flavors. In some cases, there will be a more dry and crispy character, which can give an unique combination to that beer.

In my opinion, the versatility of ale yeast is a strong advantage when comparing to the lager, which makes it possible to use for a large variety of beer styles: amber ale, brown ale, stout, porter and, one of my favorites, Indian pale ale. The traditional wheat beers from Germany (Weißbier) and Belgium (witbier) fall also in the ale category, where specific ale yeast types that give that nice banana and herbal aromas are chosen!

Lager yeast

The most common lager yeast is Saccharomyces pastorianus, which is a hybrid of two Saccharomyces strains. This means that its general characteristics are like those of the ale yeast but the optimal conditions for fermentation and the resulting beer will be different. Lager yeasts were defined as bottom-fermenting organisms because the cells were collected from the bottom of the tanks after fermentation. However, that distinction does not make sense in the current processes where conical vessels are used and both yeast types are collected from the bottom. Thus, lager yeast is currently associated with “cold fermentation” since it is done at temperatures between 7ºC – 15ºC (45ºF – 59ºF). This temperature slows down the metabolism of yeast which results in longer fermentation times.

Due to their lengthy fermentation and lagering period, the fermentation-derived flavors will not be as evident as in the ale types. The combination of malt and hops is the greatest contributor to the aroma complexity we can find in some lager beers such as the Dunkel Bock or the Saaz-seasoned Czech Lagers. Lager beer is the world’s most sold type of beer, being a fresh golden tone drink ideal to refresh the beer lovers like us.

Sour beers

In the recent years there has been a trend of intense flavors and aromas in beer, with high levels of bitterness but also acidity and sourness. The sour beers, where the lambic type is included, are made by spontaneous fermentation. This means that there is no controlled addition of yeast under sterile conditions, but you make usage of the natural yeast and bacteria present in the surroundings instead. In the old times, Belgium beers were all made in this spontaneous manner and it would take a few years to have a relatively stable beer production.

Among several types of bacteria and yeast, Lactobacillus, Pediococcus and Brettanomyces are the most relevant organisms for this kind of beer, producing acidity and giving that sour, dry and tart profile like sometimes you find in wine. Currently, it is possible to make this kind of sour beers in a more controlled way and you can even buy blends of these bacteria and yeast to produce a sour beer at home.

The variety of yeast and bacterial strains will increase more and more in the next years and many craft brewers are isolating their own blends of yeast and bacterial strains, which can give unique flavors and expand the range of beer styles. If you are already brewing, what are your favorite yeast strains and how did you choose them? Tell us your yeastperiences in the comments below.

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