Specialist Crafts · Article 26

Sugar Syrup Physics

A reference article · 4500 words · 41 cited sources · last reviewed September 2026

Chocolate & Confectionery gives the candy stages as temperatures. This article supplies what sits underneath them: the concentration each temperature stands for, how a refractometer reads the same number, how elevation moves every target, and how glucose syrup, invert sugar, and acid decide whether cooled sugar sets as a crystal or a glass.

A candy thermometer is a hydrometer in disguise. Dissolved sugar lowers the vapor pressure of the water it sits in, so a syrup has to be hotter than plain water to boil, and the more sugar it carries, the hotter. As water boils off, the concentration rises and the boiling point rises with it. At one atmosphere, then, the temperature of a boiling syrup is a reading of how much sugar is in it, and the stages of candy making are concentration bands with texture names attached. Every rule in confectionery follows from that one relationship, including the ones about elevation, refractometers, and why a recipe carries the glucose it does.

Temperature is concentration

The ideal, dilute form of the relationship is the colligative boiling-point elevation, about 0.51 °C per mole of dissolved solute per kilogram of water (Ergun, Lietha, & Hartel, 2010). Real syrups leave ideality behind long before candy concentrations. The best available model, a Margules-type fit to more than a thousand measurements, still carries a mean deviation of about 2 percent and shows a minimum in the water-activity coefficient near 96 percent sucrose that simpler models cannot reproduce (Starzak & Peacock, 1998; Starzak & Mathlouthi, 2006). Hartel’s group put it plainly: literature data for the boiling point of sugar solutions, particularly at high concentration, “is highly inconsistent,” and one review cites 94 percent sucrose reported at 127.5 °C by one source and 135.6 °C by another (Ergun et al., 2010; Hartel, Ergun, & Vogel, 2011). The table that follows is a working reference, not a constant.

Two things the table does not capture. Impurities raise the boiling point further at the same solids; Hugot’s lower-purity columns run half a degree to four degrees Celsius above pure sucrose at 85 to 90 percent (Hugot, 1986). And a syrup carrying glucose or invert sugar boils higher than pure sucrose at the same solids, because the smaller molecules put more moles into the same weight: glucose solutions “boil at higher temperature than sucrose solutions of equivalent weight percentage,” and higher-DE glucose syrups raise the boiling point more than lower-DE ones (Ergun et al., 2010; Hartel et al., 2011). A doctored syrup reaches a given thermometer reading at slightly lower total solids than the table implies, which is one reason a recipe’s target temperature belongs to its formulation.

The stages, reconciled

The stage names are cold-water tests: a spoonful of syrup dropped into cold water forms a thread, a soft or firm or hard ball, or cracks. Published temperature ranges differ between reputable sources by five to fifteen degrees Fahrenheit, and where a source gives a concentration it derived it from a boiling-point table rather than measuring it. The table here keeps the temperature ranges that Chocolate & Confectionery already carries, which McGee, ThermoWorks, and King Arthur agree with to within a few degrees, and takes the concentrations from the boiling-point table above.

Where the sources disagree: the thread stage begins at 215 °F in McGee and King Arthur and 230 °F in ThermoWorks; the hard-crack floor is 295 °F in some tables and 300 in others; the top of “caramel” is anywhere from 335 to 360 °F depending on the author. McGee’s own text puts 90 percent at 270 °F where the popular tables say 95; Claassen’s 94 percent at 267 °F sits between them. None of this matters at the bench, where the cold-water test and the formulation’s own tested target settle it; it matters for anyone who wants to convert a thermometer reading into a solids figure, and that person should use the table, not the chart.

Formulation-level cooking targets from the confectionery literature: fondant 116 to 121 °C, fudge 116 °C, caramels and toffee 118 to 132 °C, butterscotch 146 to 154 °C, hard-boiled sweets 149 to 166 °C with under 2 percent final moisture (FAO, n.d.); Greweling’s hard candy at 156 °C (313 °F) without stirring (Greweling, 2013); pulled sugar 160 °C, spun sugar 150 to 155 °C, blown sugar about 135 °C (Domson Academy, 2026). The spread within a product is formulation: a syrup carrying more glucose reaches the same texture at a slightly different reading, and every professional target was set by test batch.

Reading concentration directly

A refractometer measures the refractive index of a solution, which rises with dissolved solids, and the degree Brix is the sucrose scale for that reading: one degree Brix equals one percent sucrose by weight at 20 °C (BAKERpedia, n.d.-a). The instrument reads the same quantity a thermometer infers, and for pâte de fruit, jams, sorbet bases, and any syrup that will be cooled and checked, it reads it directly and at any elevation.

Three cautions govern the reading. Temperature first: the scale is defined at 20 °C, and a reading at any other temperature must be corrected, by the instrument’s automatic temperature compensation (good over roughly 10 to 30 °C on handheld models) or by table; a 10 Brix sample read at 25 °C reads about 0.35 low, and a hot syrup has to be cooled to the instrument’s window before it can be read at all (Atago, n.d.; Bellingham + Stanley, n.d.). Purity second: the Brix scale is the ICUMSA sucrose table, and “Brix measurements are best applied to the analysis of solutions consisting of only one solute” (BAKERpedia, n.d.-a); acids, minerals, pectin, and dextrins all shift the refractive index. Third, and the one that matters most in confectionery: glucose syrup, invert sugar, and fructose have their own refractive-index curves, so a sucrose-scale reading on a mixed syrup is an equivalent-sucrose number rather than true solids. ICUMSA calls the corrected quantity refractometric dry substance and requires that results be labelled as refractometric (ICUMSA, n.d.); Hartel’s group notes that Brix values “do not represent the exact total solids (or water) content,” with the error growing as the sucrose fraction falls (Ergun et al., 2010; Kurtz & Eliason, 1979).

The older hydrometer scale, Baumé, survives on glucose-syrup drums and in pastry recipes. The working conversion is Baumé = 0.55 × Brix: an 80 Brix glucose syrup is about 43 Baumé, simple syrup at just over 50 Brix is about 28 Baumé, a sorbet base at 27 Brix about 15 (Figoni, 2011).

Elevation

Lower atmospheric pressure lowers the boiling point of water, and with it every stage temperature. The rule in Environmental Variables & Troubleshooting, about 1 °F per 500 ft, is the one the extension and test-kitchen literature use; McGee and ThermoWorks give it as 2 °F (1 °C) per 1,000 ft (McGee, 2004; ThermoWorks, n.d.), and the USDA’s Food Safety and Inspection Service states it more carefully as “just under 1 °F” per 500 ft (USDA FSIS, n.d.).

Neither the decay of pressure with altitude nor the vapor-pressure curve of water is linear, which is why the rule drifts. Weather matters as much as the last thousand feet: a swing of 3 kPa in barometric pressure, ordinary between a high and a low, moves the boiling point by about 0.9 °C, which is why ThermoWorks tells its users to look up local pressure rather than trust elevation (ThermoWorks, 2025).

The method that beats any table is the one every source agrees on. Boil plain water on the day, read the thermometer, and subtract the difference from 212 °F (100 °C) from every stage target (McGee, 2004; ThermoWorks, n.d.). It corrects for elevation, for the day’s weather, and for the thermometer’s own error in a single step. It assumes the syrup’s boiling-point elevation is itself independent of pressure, which Hugot says holds “between fairly narrow limits” and Holven measured (Holven, 1936; Hugot, 1986); at candy pressures the error is negligible. To calibrate the thermometer itself: a rolling boil that does not stop with stirring, in at least four inches of water, probe a few inches deep in the center and off the bottom, and compare to the local boiling point; an ice bath, a stirred slush of crushed ice and a little water, checks the other end of the scale and is the easier test (ThermoWorks, 2025).

Crystal or glass: the doctoring agents

A cooked syrup is supersaturated the moment it starts to cool, and what it does next is the whole of confectionery. The sucrose solubility curve, the glass-transition curve, and the metastable and labile zones between them make up the state diagram on which Hartel’s group places every confection (Hartel et al., 2011). Fondant and fudge are engineered to crystallize small and uniform. Hard candy, cotton candy, and toffee are engineered not to crystallize at all, cooled so fast through the labile zone that the sugar sets as an amorphous glass. Caramels and gummies sit in the rubbery state between. The craft is in the confectionery article; the figures are here.

Glucose syrup inhibits sucrose crystallization two ways. Its longer saccharides adsorb onto crystal faces and block growth, and because it is a solute that is not sucrose, it lowers the sucrose supersaturation at any given water content: in Hartel’s example, replacing sucrose with corn syrup from 100:0 down to 30:70 drops the sucrose supersaturation at 12 percent water from 523 to 185 g per 100 g water (Hartel et al., 2011). In thin-film experiments 14 percent corn syrup cut sucrose crystal growth rates several-fold and 30 percent cut them further (Chen, Nowakowski, Green, & Hartel, 2015); 10 to 20 percent corn-syrup solids retard crystallization in the amorphous state and 50 percent or more prevents it (Gabarra & Hartel, 1998). Invert sugar, whether added or formed in the pot by acid, does the same job as a second solute; Greweling lists it as “a tenderizer, doctoring agent, and humectant” and warns that excess makes products “soft and overly sweet” (Greweling, 2013). The confectionery rule is that about 10 to 15 percent invert sugar in the finished sugar is what “is required to give a non-crystalline product” (FAO, n.d.; Wolf, 2016).

The sharpest published thresholds are Hartel’s for caramel and jellies. A sucrose-to-corn-syrup ratio above 1 in caramel, or above 2 in jellies, “always led to crystallization,” while ratios below 0.4 in caramel and 0.8 in jellies “always indicated inhibition” (Hartel et al., 2011; Miller & Hartel, 2015). Nougat follows the same line: below 1 it stays ungrained, above 1 it grains. Cotton candy carries no corn syrup at all and relies on speed alone, which is why it is the least stable confection there is.

Published formulations, for scale rather than as recipes: hard candy at 48 percent sucrose, 32 percent glucose syrup, 20 percent water, cooked to 150 to 166 °C, which puts glucose at about two-thirds of the sucrose weight (Wolf, 2016), or 70:30 sucrose to 42 DE corn-syrup solids in Hartel’s state-diagram example; citric acid in hard candy at 0.1 to 1.0 percent, pH 3 to 4, target moisture 2 to 3 percent and never above 5 (Ozel et al., 2024). Fondant at 62 percent sucrose, 16 percent glucose, 22 percent water, boiled to 88 percent solids at 117 °C and cooled to 37 °C before beating so the crystals stay under 20 to 30 µm (Wolf, 2016), or sucrose 100, water 50, glucose 25, cooked to 120 °C and beaten at 38 to 43 °C (Talbot, 2009). Pulled sugar at 1 kg sugar, 350 to 400 g water, about 200 g glucose, a dozen drops of tartaric acid at 155 °C, cooked to 160 °C (Domson Academy, 2026). Why fondant is cooled before it is beaten: a supersaturated syrup agitated cold throws many small nuclei and gives a smooth cream, agitated hot it throws few large ones and grains, and the induction time follows classical nucleation theory (Hartge, Flöter, & Vilgis, 2023).

Dextrose equivalent. DE is the degree of starch hydrolysis expressed as reducing sugars, dry basis, with pure dextrose at 100; anything under 20 DE is a maltodextrin, and Codex defines glucose syrup as 20 DE or above at 70 percent solids or more (BAKERpedia, n.d.-b). The confectionery grades are 42 and 63 DE. The higher the DE, the smaller the average molecule: more sweetness, more hygroscopicity, a lower glass transition (42 DE solids about 79 °C, 20 DE about 139 °C), and more boiling-point elevation at the same solids (Hartel et al., 2011). High-maltose syrups are the confectioner’s answer to stickiness, because maltose is less humectant than glucose. “Corn syrup” is glucose syrup made from maize; retail light corn syrup is a 42 DE-class syrup with salt and vanilla.

Inversion. Sucrose plus water under acid and heat gives glucose and fructose, and cream of tartar or lemon juice in the pot is the traditional doctor that predates glucose syrup. No peer-reviewed figure exists for the fraction inverted in a typical boil; the literature gives the target, 10 to 15 percent invert for a non-crystalline product, rather than the yield. Invert is markedly more hygroscopic than sucrose because the glass transitions of its parts are so low, fructose at 5 to 10 °C and glucose at 31 °C against sucrose at 62 to 70 °C (Hartel et al., 2011; Ergun et al., 2010), which is why an over-inverted hard candy weeps in humid air and why acid in hard candy is held to a fraction of a percent. Even salt added during a fudge boil contributes to inversion and makes the batch softer (Greweling, 2013).

Water, the glass, and why hard candy goes sticky

The cook temperature sets the water content; the water content sets the water activity and the glass-transition temperature; those two numbers decide texture and shelf life (Ergun et al., 2010). Hard candy finishes at 2 to 5 percent water and a water activity of 0.25 to 0.40; caramel, toffee, and fudge at 6 to 18 percent and 0.45 to 0.60; gums and jellies at 8 to 22 percent and 0.50 to 0.75; marshmallow at 12 to 20 percent and 0.60 to 0.75; fondant and creams at 10 to 18 percent and 0.65 to 0.80 (Ergun et al., 2010). Measured commercial pieces run the same way: a hard mint at 0.30, chewy caramels 0.45, a translucent gummy 0.70 (AQUALAB, n.d.).

Amorphous sucrose has a glass transition of about 57 °C dry in the classic measurement (Roos & Karel, 1991a), 62 to 70 °C in later work (Hartel et al., 2011), and water plasticizes it hard: 69 °C at zero water falls to about 29 °C at 5 percent, and sucrose does not form a glass at room temperature at all above about 5.5 percent water (Ergun et al., 2010). A commercial hard candy sits near 65 °C at under 1 percent water and 35 to 40 °C at 6 percent (Hartel et al., 2011). Higher-DP glucose-syrup solids raise the glass transition of the blend and monosaccharides lower it (Roos & Karel, 1991b; Gabarra & Hartel, 1998).

A sugar glass is hygroscopic. Its surface takes up water from the air until the local glass transition falls below room temperature; the surface goes rubbery and tacky, then flows, then grains as the mobile sugar crystallizes (Nowakowski & Hartel, 2002; Ozel et al., 2024). The rate of sticking is a function of the gap between temperature and glass transition rather than of temperature or humidity alone (Foster, Bronlund, & Paterson, 2006), and tack peaks at about 11 to 13 percent water before falling again as the syrup becomes a liquid (Burke & Hartel, 2021). Because water activity is the equilibrium relative humidity divided by 100, a hard candy at 0.25 to 0.40 gains moisture in any air above about 25 to 40 percent relative humidity, which is nearly all kitchen air; cotton candy stayed stable for two years at 11 percent relative humidity and crystallized within three days at 33 percent (Hartel et al., 2011). Hence the practice: sugar work goes into an airtight box with silica gel the moment it is cool, spun sugar is assembled and served within about two hours, and a humid day is a reason to postpone pulled sugar.

Isomalt is the showpiece sugar because it “absorbs virtually no water up to a relative humidity of 85 percent at 25 °C”; it is about half as sweet as sucrose and dissolves to only 24 g per 100 g at 20 °C (McNutt & Sentko, 2003; Beneo, n.d.). Sources put its cooking temperature at 160 °C (Ozel et al., 2024) to 170 °C or so (Domson Academy, 2026), with the piece worked at about 135 °C.

Caramelization

Above the stages the syrup stops boiling and starts to break down. Sucrose loses water and rearranges into hundreds of compounds: hydroxymethylfurfural and the difructose dianhydrides from dehydration, then oligomers up to hexasaccharides from condensation, and their dehydrated derivatives; chemically, caramel is dominated by that oligomeric material rather than by the aroma volatiles people notice first (Golon & Kuhnert, 2012). The glass transition of the product falls at first, as small degradation products plasticize it, then rises as polymerized caramel forms, which is why a pale caramel is stickier than a dark one and why overcooked sugar sets harder (Jiang, Liu, Bhandari, & Zhou, 2008).

A worked correction

A recipe cooks caramels to 248 °F at sea level and the kitchen is at 5,280 ft. Water boils there at 202.4 °F on a standard day, where the linear rule would say 201.4. The offset is 212 minus 202.4, or 9.6 °F, so the target is 238 °F; on a day the barometer is 3 kPa low, water boils nearer 200.8 and the target is 237. Boiling water first and subtracting the measured difference takes both into account and corrects the thermometer at the same time. The same recipe read by refractometer needs no correction at all: the firm-ball syrup is about 87 to 88 percent solids at any elevation, and a cooled sample read at 20 °C says so directly, with the caveat that a syrup carrying glucose reads as equivalent sucrose rather than true solids.