Why Cold Keeps Aroma Down — Volatilization, Partition, Perception

Foundations Chapter 5 and Advanced Chapter 11 touched more than once on the fact that cold keeps aroma from rising. Approfondimento 11 designs its pairings on that assumption. This piece digs into the assumption itself. Aroma weakens for more than one reason. Volatilization, partition, perception — three stages, and every one of them turns against aroma at low temperature. This piece takes them in order.

Stage one: volatilization — less of it reaches the gas phase

A volatile molecule that carries aroma can be detected by the nose only once it has reached the air above the food, the headspace. So what has to be asked is how the aroma molecules divide between the liquid and the air. The quantity for that division is the partition coefficient.

The larger K_GL, the more of the aroma has gone to the air side — which is to say, the better it smells. So what settles this value? Whether a molecule stays in the liquid or leaves for the air comes down, in the end, to a balance of energy.

Two forces pull against each other here. The configurational entropy (ΔS_C) prefers the molecule scattered through the system to shut inside the liquid. It is on the side that always favors volatilization. Against it, the molecular interaction (ΔG_I) stands for what an aroma molecule gains by binding to the solvent molecules around it. The binding is by van der Waals forces and hydrogen bonds. In the gas phase there is nothing to bind to, so it takes the side that holds volatilization back.

And it is worth looking carefully at where in this equation the temperature acts. The T that appears in the entropy term is not acting.

It cancels against the RT in the denominator. Open the equation out and it reads:

The right-hand factor — the entropy contribution — no longer holds a T. The configurational entropy favors volatilization by the same amount whatever the temperature. It is a fixed step up, not a slope.

Temperature acts through the left-hand factor, which is to say through the interaction term. To leave the liquid, an aroma molecule has to break the hands it holds with the solvent molecules around it. The price is paid out of thermal kinetic energy. Lower the temperature and less can be paid, and fewer molecules can break their hands. Only the molecules that could pay the price reach the gas phase.

And T sits in the denominator of an exponent. So the effect is not gentle. Low temperature does not weaken the push; it makes the price unpayable. Cold gelato stands at a large disadvantage already at the first stage of raising an aroma.

Stage two: partition — the matrix will not let the aroma go

So far the talk has run on two things, the liquid and the air. Real gelato is more tangled than that. Within an emulsion there are four places an aroma molecule can sit. They are the fat globule's interior, the water phase around it, the fat–water interface, and the gas phase above. What comes out into the gas phase is settled as the outcome of a tug of war among the four.

What tells most among them is which of the two it prefers, fat or water. That preference is carried by the oil–water partition coefficient K_OW. Above 1 it prefers fat and is hydrophobic; below 1 it prefers water and is polar.

And an important consequence follows. A hydrophobic aroma (K_OW > 1) comes out into the gas phase less and less as the fat rises. It is drawn into the hiding place of the fat globule. The more hydrophobic the molecule, the sharper the effect. Where K_OW is larger by orders of magnitude, a slight rise in fat drops the aroma in the gas phase a great deal. Turn it around, and the less the fat, the more strongly a hydrophobic aroma comes through.

Here lies an asymmetry easily missed. A polar aroma (K_OW < 1) is barely touched when the fat rises. It stays in the water phase. So fat does not hold every aroma back evenly. It hides only the aromas that dissolve in it. This selectivity is why changing the base changes not only the strength of the aroma but its composition (Approfondimento 11, Advanced Chapter 11).

Fat is not the only hand that holds aroma back. Proteins and polysaccharides can bind aroma molecules. What is bound cannot reach the gas phase, so where the binding is strong the aroma in the headspace falls a great deal. Gelato, carrying milk protein and stabilizer, is slow to let aroma go in this sense too.

And in gelato much of the water has turned to ice. The aroma molecules are concentrated into the small unfrozen phase that remains. That phase is viscous at low temperature, and slow to diffuse through (Approfondimento 1 and 4).

And there is a fourth hand. The interface, counted at the start.

The boundary between fat and water is only a very thin layer against the volume of the whole system. It still governs where the aroma goes, to a great degree — and it does so precisely because there is so little aroma there. The seats where a molecule can stick at the interface are limited in number. But food flavorings are used at amounts far below that number to begin with. So the seats never fill, and a good part of the aroma put in is taken by the interface. What the interface takes comes out neither into the oil, nor the water, nor the gas phase.

Here a circumstance peculiar to gelato tells. Homogenization makes the fat globules fine. Smaller globules mean more total interfacial area for the same amount of fat. By the estimate in the literature, cut a globule's diameter to a tenth and the area grows tenfold. More area means more seats, and more aroma taken by the interface. So the finer the homogenization, the harder it is for aroma to rise.

Homogenization is a step needed for mouthfeel, and for partial coalescence (Approfondimento 5). The same operation works against aroma. Here too is a trade-off.

There is a condition on the effect, though. If free emulsifier is drifting in the water phase, it catches the aroma instead and the interfacial effect is hard to see. That the seats at the interface stand empty is the premise of this account.

Weak in reaching the gas phase; then fat hides it, protein grips it, the interface holds it, a viscous phase will not move it. Partition deepens the disadvantage of volatilization.

Stage three: perception — the two roads of the nose

Last comes the physiology of the receiving side. Aroma is perceived by two routes. Orthonasal olfaction is smelling directly through the nose. Retronasal olfaction is where the aroma of food in the mouth passes from the throat up into the nose.

Gelato in the case or on the spoon is cold, and the aroma felt orthonasally is weak, held down by volatilization and by partition. The moment it enters the mouth, though, the case changes. Gelato is warmed by the mouth and begins to melt, and the aroma molecules held down until then are released all at once. This retronasal aroma is the main ground of gelato's flavor. Foundations Chapter 5 set "Why it melts in your mouth" and "Cold and aroma" side by side for exactly this reason. Warming in the mouth is what sets off the release. So the design of aroma has to aim not at the aroma smelled but at the aroma that opens as it is eaten.

And low temperature dulls taste itself as well. The way it dulls is not simple, though.

Cold weakens sweetness by two separate roads. Cooled to 5 to 10 °C, the strength of the sweetness itself falls. Separately from that, from somewhere below 21 °C adaptation to sweetness runs faster. The first mouthful is as strong as ever, but as the eating goes on it withdraws sooner. Strength and duration are struck separately. Why that should be is not yet explained by any single mechanism. There are hypotheses that account for it by the temperature dependence of the receptors, but these alone are known not to explain both.

And the fall differs wholly from sugar to sugar. Cool from 30 °C to 5 °C and sucrose loses more than six tenths of its sweetness. Fructose falls only about two tenths (Advanced Chapter 2). "Cold dulls sweetness" does not tell the same way for every sugar.

And here the first half of this section tells. What is warmed in the mouth is not the aroma alone. As the gelato begins to melt, the temperature at the tongue rises too. The sweetness that had been held down comes back there. So it is dangerous to think that sweetness is weak in the cold and sugar should be added to make up for it. What was added appears once the melting is done, as length. Foundations Chapter 5 says to tune the sweetness for the cold state. Advanced Chapter 11 insists that the final judgment be made on gelato frozen to the serving temperature. Both say it for this reason. Sweetness can only be measured at the temperature it is eaten.

Length comes after it has melted away

Follow the three stages, and the length of gelato turns out to be built differently from that of other foods. Length is the sensation that continues after swallowing.

In an ordinary food, length is a tail: the strength at the moment of entry decays over time. In gelato a further circumstance is added — the temperature rises in the mouth. What was held down while it was cold, in volatilization and partition and perception alike, is set free as it melts. So the length of gelato is not only a tail of decay but also something that rises up afterward.

This twist has two consequences.

First, the design of sweetness is checked against the length. As the first half of this section showed, sweetness is held down in the cold state. Add sugar meaning to make up for it, and what was added does not appear during the eating. It appears once the temperature in the mouth has come all the way up — which is to say, as length. A cup that is just right while it is eaten, and heavy once it is finished, is born by this mechanism. When Foundations Chapter 5 speaks of sweetness that is right in the cold state, that is a demand which includes the length.

Second, low temperature also works to shorten the length. From somewhere below 21 °C adaptation to sweetness runs faster, so as the eating goes on the sensation withdraws sooner. The asymmetry noted above — that strength and duration are struck separately — shows itself directly as a short length. So if a long length is what is wanted, the sounder course is not to add sugar. It is to stretch the release of aroma along the axis of time with fat and solids — to solve it on the partition side.

Length should be kept apart from aftertaste. Aftertaste is what remains in the mouth, and the chief of it is the thin film that fat leaves — what stays, as it were. Length is what continues: aroma and sweetness withdrawing over time. The first is a matter of amount, the second a matter of time, and they are dealt with separately.

The three stages settle the design

Volatilization is weak, partition leans toward the matrix, perception is dulled by the cold. This threefold disadvantage governs the design of flavor in gelato. The answers correspond to the stages, one by one.

• The answer to volatilization: do not push the serving temperature down needlessly. Serve within the range where aroma rises (Foundations Chapter 1, Advanced Chapter 7).
• The answer to partition: write the fat of the base into the design of flavor. Fat holds back only the aromas that dissolve in it. So between sorbetto and crema not only the strength of the aroma changes but its composition (Approfondimento 11).
• The answer to perception: aim not at the aroma smelled but at the retronasal aroma that opens in the mouth. Put it in strong, allowing for the suppression. Design the sweetness to feel right in the cold state as well (Foundations Chapter 5).

So the science is handed over to the craft. The three stages seen here — volatilization, partition, perception — are the physical ground on which Approfondimento 11, "Designing Pairings", builds its affinities. Science, in this piece, and craft, in Approfondimento 11, shake hands at this one point.