Why chocolate turns white in a warm car, and what to do about it

A bar left in a parked car often comes out with pale streaks or a dusty gray film, and most people assume it has gone bad. That film is called bloom. Azelis, a food ingredient company, describes fat bloom as the most common kind. It says warmth softens the cocoa butter, which separates from the other ingredients, rises to the surface and sets again. Hill Country Chocolate, a Texas maker, says bloomed chocolate is safe to eat. It adds that mold on chocolate is extremely rare and would look fuzzy and off-colored, unlike a smooth white film.

What heat does to cocoa butter

Tempering explains why a little warmth causes so much visible change. Callebaut says tempering gives the cocoa butter a stable crystal form, and the satin gloss and hard snap come from that. Cocoa butter can set in several forms. The crystal table in the Wikipedia article on tempered chocolate lists six. Form V, the one tempering aims for, melts at about 34 °C (93 °F). Form VI melts at about 36 °C (97 °F), is hard, and takes weeks to form.

A bar does not have to go liquid to change. Hill Country says heat above 75 °F starts to soften cocoa butter. Lia Andrae's University of Illinois dissertation on chocolate storage found that high temperatures with fluctuations sped up the move to form VI. Chocolate held at high temperature came out harder and more fracturable. It took longer to melt, tasted less sweet with less cream flavor, and had a rougher surface. So heat can change how a bar eats, even though the look is what you notice first.

Full melting does more damage to the bar's structure. Callebaut says chocolate that is simply melted and left to cool will not be glossy. It also says badly crystallized chocolate turns gray. A bar that slumped on the dashboard and set again has lost its temper, and that is why it looks so different from the one you bought.

How hot a parked car gets

Those melting points are easy to pass in a car on a hot day. A study from Arizona State University and UC San Diego measured six cars over three days in Tempe, Arizona, with outdoor temperatures in the 100s °F. After an hour parked in the sun, cabin air averaged 116 °F, dashboards 157 °F and seats 123 °F. In the shade, cabin air was closer to 100 °F, with dashboards at 118 °F and seats at 105 °F.

Most days run cooler than that test, and milder weather will give lower numbers. Its results still show two useful things. The dashboard is the worst place for a bar, running 34 degrees hotter than the seat in the sun. Shade helps a lot, but on a hot day it will not save a bar, since those shaded readings all sat above the 93 °F melting point of form V. Callebaut puts the ideal for storing chocolate at 12 to 18 °C (54 to 64 °F), so a car can leave that range quickly.

Fat bloom or sugar bloom

Touching the surface will usually tell you which kind you have. Azelis says fat bloom shows as grayish or whitish streaks that feel slick. Sugar bloom is a dry, hard white film, and Hill Country describes it as dry, gritty specks. A warm car mostly points to fat bloom, but moisture causes the other kind. Whitakers Chocolates explains that moisture dissolves sugar on the surface and leaves a grainy coating once it evaporates. Temperature changes that bring condensation can set this off.

Rushing a warm bar to the refrigerator has a catch for that reason. The storage guidance from Callebaut says refrigeration is generally not recommended for confections, because of the moisture inside the fridge. It warns that condensation on chocolate returning to room temperature can cause sugar bloom. If you do use cold storage, let packaged chocolate come back to room temperature before you open it.

What to do with the bar

Your simplest option is to eat it, once it has firmed up somewhere cool and dry in its wrapper. Expect the texture to be different. With sugar bloom, Whitakers notes the taste may be slightly off as well, though the bar is still safe to eat. A bar you dislike eating can still be useful. Hill Country says fat-bloomed chocolate melts and re-tempers well for baking.

Getting the shine back takes re-tempering, which you can try at home with a thermometer. Callebaut's seeding method is one to follow, and its numbers are written for a chocolate melter.

• Melt the chocolate with the thermostat at 45 °C.
• Lower the thermostat to about 31 °C for dark chocolate, or about 29 °C for milk and white.
• Add about 5 percent unmelted chocolate pieces at room temperature, which also helps cool the melt. Callebaut gives that as its rule of thumb when the melted chocolate is at about 40 °C.
• Stir well so the stable crystals in the pieces spread evenly.
• If the pieces melt too quickly, the chocolate is still too hot. Add more and keep stirring.
• Use it when it has thickened slightly, then pour it into a mold and let it set.

The seed pieces do the work in that method, so take them from a fresh bar. Hill Country's rescue advice covers fat bloom specifically. For sugar bloom, its advice and the Whitakers page stop at saying the bar is safe to eat.

How to keep it from happening again

Steady, cool storage prevents most of this, and Azelis says keeping chocolate at a constant, cool temperature helps avoid fat bloom. Hill Country suggests 60 to 68 °F in a dry, dark place, which is a little warmer than Callebaut's ideal range. Callebaut adds a maximum relative humidity of 70 percent and airtight packaging that blocks light where possible. It also says to keep chocolate away from drafts and strong odors. On a car trip, keep the bar off the dashboard and out of direct sun. Bring it inside when you park, because the Arizona readings came after a single hour.