The Science of Sugars

Sugar is a structural material

In gelato, sugar is a structural material before it is a sweetener. Dissolved in water, it lowers the freezing point and sets the amount of the unfrozen phaseThe liquid phase that stays unfrozen even in the freezing range. Sugar and other solutes remain dissolved in it. It is the reason gelato is cold and still soft.. How much of the water freezes at the temperature of the case, and how much stays liquid? The hardness of gelato, its scoopability, and its melt are decided almost entirely at that one point.

For the artisan, then, designing the sugars is not deciding how sweet to make the gelato. It is deciding what physical character the frozen dessert will have.

Two measures — POD and PAC

POD (Potere Dolcificante) is relative sweetening power — the strength of the sweetness the tongue receives. In English this property is called relative sweetness. PAC (Potere Anti-Congelante) is relative anti-freezing power — how strongly a sugar resists the freezing of water, and so how strongly it softens gelato. The effect it measures is called freezing-point depression in English. What matters is that the two move independently.

Dextrose, for example, has POD 74 and PAC 173 — about seven-tenths the sweetness of sucrose, and about 1.7 times its anti-freezing power. The smaller the molecule, the more particles the same weight brings, and freezing-point depression grows in proportion to the number of particles. This is why the PAC of the monosaccharides is high across the board.

The principle reaches beyond sugar. The molecular weight of ethanol is 46 — about one seventh that of sucrose. The same gram therefore brings more than seven times the molecules, and the PAC reaches 743. No sugar comes near that value. If handling alcohol is so difficult in Chapter 10, it is because of this number, before any question of flavor. Put the other way around — PAC is not a property of sugars. It is a property of the number of dissolved particles, and what the particle is comes second on this measure.

One material pushes that logic to the end. Salt. Its formula weight is 58.5 — larger than the 46 of ethanol. On molecular weight alone, it should work less strongly. But salt, dissolved in water, stops being a molecule. It splits into two ions, sodium and chloride. Freezing-point depression counts particles, so one unit of salt counts as two. Strictly, a little less than two — the separated ions still attract each other. The result is a PAC of about 1100 — past ethanol, and higher than any material in this book.

This is also a practical matter. Say the "pinch" of a salted caramel is 3 g per 1 kg — 0.3 percent. The total PAC rises by about 33. That is nearly the same effect as adding 2 percent dextrose. The taste was being decided, and the ice was being decided with it. It is not an amount that can be waved away as small.

Approfondimento — deep diveWhy do sugars lower the freezing point? — Colligative properties and molecular weight↓ Go deeper

What POD leaves out

So far, sweetness has been handled as a single number, POD. For practice, that is enough. But it is worth knowing what the number throws away. It throws away four things.

First. Sweetness has a shape in time. POD states the strength of sweetness at a single point. In the mouth, sweetness rises, peaks, and recedes, and the trajectory differs completely from sugar to sugar. Fructose rises sharp and high, and recedes fast. Sucrose reaches its peak late, and stays long. Dextrose sits between the two, with the lowest peak of the three. The same POD can be an early, strong sweetness or a late, lasting one — different experiences. Gelato is eaten over several seconds as it melts in the mouth, so the shape in time cannot be ignored.

Second. Mixed sugars taste sweeter than the sum. Mix several sugars, and the sweetness always comes out above what the individual sugars predict. The sugars reinforce one another in perception. The sum described later — amount of each sugar × POD factor — therefore underestimates the actual sweetness somewhat. The direction of the error is fixed, so the measure still works as a measure. Do not mistake the sum for the sweetness itself.

Third. The POD of fructose moves with temperature. Tab. 2-1 writes fructose as a range, 120–173, not because the measurements scatter. The value truly moves.

A molecule of powdered fructose holds one fixed shape. Dissolved in water, it begins to take several — a six-sided ring and a five-sided ring. Only the six-sided ring is sweet. The five-sided ring contributes nothing to sweetness. Just after dissolving, the six-sided form is the majority; with time, more of it crosses over than returns, and the sweetness falls. This interconversion is called mutarotation — the same mechanism that feeds the growth of lactose crystals.

The balance also answers to temperature. The higher the temperature, the further it tips toward the unsweet five-sided ring. Fructose dissolved in a hot drink is less sweet than the same drink gone lukewarm, for this reason.

Here lies an interesting implication for a cold dessert. Cold lowers the tongue's sensitivity to sweetness, so a cold gelato starts at a disadvantage in sweetness. Fructose, however, becomes sweeter as a molecule the colder it gets. The direction in which the tongue dulls and the direction in which the sugar sweetens point opposite ways. Among the sugars, fructose alone pushes back against the cold — which is why the high side of the range in Tab. 2-1 can be used.

And the prediction has been measured on the tongue. A study compared, sugar by sugar, how much sweetness is lost on cooling. From 30 °C to 5 °C, sucrose loses more than 60 percent of its sweetness. Dextrose (glucose) loses about 40 percent. Fructose loses only about 20 percent. Of the three, the drop for fructose is strikingly small.

The study does not say why fructose alone resists. But mutarotation predicts exactly this difference. A sugar that tips toward its sweeter form in the cold would cover part of what the tongue loses. The reasoning of the molecule and the measurement on the tongue, coming by separate roads, arrive at the same place.

Approfondimento — deep diveWhy cold keeps aroma down — volatilization, partition, perception↓ Go deeper

Fourth. This table is not the one correct answer. Tables of sweetness differ from source to source. Several tables give the same sugar different numbers. The source this book relies on offers its own table as one among many. It adds a caution — a table of sweetness valid for every occasion is not easy to build.

Take the caution at face value. The same source carries two tables of sweetness, and they do not agree. For the dried glucose syrup, one table — a table of composition — gives DE 40 a sweetness of 50. The other lists sweetness alone, as ranges by DE; read DE 40 there, and the value comes out somewhat below 50. At low DE the gap opens wider. Tab. 2-1 takes the former reading — one product, average molecular weight 730, with sweetness and anti-freezing power taken from the same line. The anti-freezing power of 47 comes from that molecular weight, so the sweetness belongs to the same line.

Even within a single book, this happens. Tab. 2-1 is a starting point, not a terminus.

What, then, does practice do? The answer is almost disappointingly simple — request the technical data sheet from the maker of the ingredient you use. Makers that publish sweetness and anti-freezing power are not rare. The published value of the product actually on your bench beats a table of general values, and no argument is needed. The source this book relies on recommends exactly this.

This is not an armchair point. The trehalose of Tab. 2-1 is the example. The trehalose on the Japanese market is the dihydrate, two molecules of water inside the crystal. About one tenth of the product is water. The maker publishes its sweetness as about 40 percent of sugar. The literature abroad gives 50, without saying which crystal form. Under the single name "trehalose", one tenth of the weight and ten points of sweetness move. This is why Tab. 2-1 writes it as a range.

The anti-freezing power moves for the same reason. PAC is decided by molecular weight. With the water of crystallization aboard, the same 1 g holds less of the sugar itself. The PAC of trehalose is therefore 90, not the 100 calculated from the anhydrous form. The same thing happened with dextrose, treated as its monohydrate. The name on the bag does not tell you the molecular weight inside.

Glucose syrup and DE

Glucose syrup (sciroppo di glucosio) is a mixture of sugars obtained by hydrolyzing starch. It comes in two forms. The dried form holds nearly 100 % solids; the liquid form, about 80 %. As sugars they are the same — the difference is only the water. In a formulation — the amounts and proportions of the mix — the same weight differs by a fifth in solids. Always know which form is on the scale. Below, the general name glucose syrup covers both.

It is not a single sugar, so its character is set by how far the hydrolysis has gone — the DE (dextrose equivalent)A measure of how far a starch has been hydrolyzed. Dextrose is 100 and starch is 0..

The higher the DE, the closer the composition comes to dextrose; the average molecular weight falls, and both POD and PAC rise. This is not theory alone. The literature lists sweetness and average molecular weight by DE; the correspondence stands in the table itself — DE up, molecular weight down, sweetness up. The lower the DE, the more of the long chains remain, with little sweetness and little freezing-point depression. That is a structural material — it neither sweetens nor softens, and only adds solids. The principle at work is the one already stated. Freezing-point depression is decided by molecular weight; at the same weight, smaller molecules mean more particles, and PAC rises. DE stands in for that average molecular weight. The PAC of 47 for the dried syrup (DE 40) in Tab. 2-1 is calculated from this molecular weight.

Because of this, low-DE glucose syrup earns its place as a corrector for missing total solids. It corrects wateriness without upsetting the balance of sweetness and hardness. Use it heavily, though, and the long chains bring a viscous, heavy feel in the mouth.

Substitution in practice

This independence allows an adjustment that keeps the sweetness and changes only the hardness. Replace part of the sucrose with a high-PAC sugar, and the gelato softens while the total sweetness stays almost unchanged. The reverse also works. A mix too soft can take a sugar like trehalose — low POD, PAC on par with sucrose. It holds sweetness down while adding solids.

Substitution has a second motive, apart from the design of the freezing point. Sucrose used alone at high concentration crystallizes readily, above all at the surface. Replacing part of it with other sugars is also that prevention (Chapter 13).

"Almost unchanged", the last paragraph said. The "almost" can be erased.

Swap in a single sugar, and the sweetness always moves. Dextrose is less sweet than sucrose, so replacing gram for gram loses that much sweetness. The price of softness is being paid in sweetness.

So pair two sugars. Take one less sweet than sucrose and one sweeter, and bring the average back to 100. The standard pair is dextrose and invert sugar. Counted per unit of solids, dextrose is about 81 — the 74 of Tab. 2-1 with its water of crystallization removed. Invert sugar is 127–130. One sits below sucrose, the other above. Pair them in equal amounts and the average is about 105, close to sucrose. Their PAC, meanwhile, is 190 for both. The pair carries in 1.9 times the anti-freezing power while barely moving the sweetness. To harden, run it backward — shrink the pair back to sucrose, and hardness rises with the sweetness unmoved.

Two cautions.

The first is the basis. The calculation above is per unit of solids — not the basis of Tab. 2-1. Read from the table, dextrose monohydrate is 74 and the 70 % invert syrup is 90. Both sit below sucrose, and no mixture of the two reaches 100. One tenth of the dextrose is water of crystallization; three tenths of the syrup is water. Equal on the scale is not equal as sugar. Build the pair on solids, and let the water the syrup brings be caught on the total-solids side (Chapter 5).

The second concerns the number 100 itself. A Spanish manual of practice goes as far as naming this pair. Dextrose 70, invert sugar 130; the sum is 200, half is exactly 100 — no different from sucrose, it says. But it comes out exact only on that book's own table. The source this book relies on holds dextrose somewhat sweeter, and the same calculation gives 105. As the fourth point showed, there is no single table of sweetness. Whether the pair works as intended can only be confirmed on the table you actually use — best of all, the maker's data sheet.

This PAC value maps directly onto how hard the gelato is at serving temperature.

One sugar stays out of the sum — lactose. In Tab. 2-1 its PAC is 100, the same value as sucrose. But that is a nominal factor. Lactose dissolves worst among the sugars (Chapter 3), and at serving temperature most of the water is already ice, leaving a narrow unfrozen phase. Every component shares that narrow water, so the least soluble sugar — lactose — stays partly undissolved. An undissolved sugar joins no count of particles. Freezing-point depression counts dissolved particles, so undissolved lactose does not register on this measure. Manage lactose with the milk solids-not-fat, apart from the other sugars. It stands in Tab. 2-1 to show its position among the sugars — one sixth the sweetness of sucrose, the same nominal PAC. It is not there to be added into this formula.

Undissolved lactose does not simply sit there. Under poor temperature control it crystallizes, and is felt as sandiness on the tongue (Chapter 3).

Approfondimento — deep diveSandiness from lactose — supersaturation and crystallization↓ Go deeper

One warning, before the measure is put to work. "So many PAC points means so many degrees" — such a conversion table, in fact, exists. The Spanish manual lays one out, from −10 °C to −18 °C. The rule: every 20 points of PAC lowers the serving temperature by 1 °C. But its author states plainly — this is not the conclusion of scientific research; no such research exists; it is the result of daily work.

The conversion table, then, is a rule of thumb born in one workshop, not a theory that derives temperature from formulation. Nothing guarantees the same numbers in another workshop. So this book does not borrow the table. Total PAC serves as a measure — it reads which way hardness moves when the sugars inside a formulation are exchanged. How to use the measure in practice is the business of Chapter 5.

On top of that, the numbers in that table do not sit on the same scale as this book's total PAC. The lactose that was just removed from the sum — that book adds it. A formulation with one tenth milk solids-not-fat carries about 50 g of lactose (Chapter 3). Onto the example's PAC of 229, a full 50 would ride. The same mix, counted differently, yields numbers more than 20 percent apart. Before importing another book's target values or ranges, confirm what its numbers include. Not borrowing the table had a second reason after all.