A latte is a simple combination of espresso and steamed milk, but its delicate structure makes reheating a challenge. While it is technically possible to warm a cooled latte, the process compromises the beverage’s original taste and texture. This quality loss results from chemical changes that occur when the milk and coffee components are exposed to heat a second time.
The Science of Degradation: Why Reheating Changes Taste and Texture
The primary reason a reheated latte tastes different lies in the denaturation of milk proteins. Milk contains whey proteins, such as $\beta$-lactoglobulin, which create the velvety microfoam when steamed. When these proteins are heated again, especially above $65^\circ\text{C}$ ($149^\circ\text{F}$), they unfold and aggregate.
This irreversible change destroys the foam structure, causing the milk to separate and become watery or grainy. The unfolding proteins expose sulfur-containing thiol groups, which react to form volatile compounds. These compounds impart a distinct “scalded” or “cooked” flavor, replacing the delicate, sweet flavor of the original steamed milk.
The coffee component also suffers degradation upon reheating. The complex aroma of espresso is due to volatile aromatic compounds, which are susceptible to heat and oxidation. Reheating accelerates the breakdown of these compounds, leading to a flatter, less fragrant flavor profile.
Specifically, chlorogenic acids present in coffee break down into quinic and caffeic acids when exposed to heat. These resulting acids are intensely bitter, causing reheated coffee to taste stale and acrid. Desirable roasty odorants like 2-furfurylthiol decrease, while less pleasant compounds like methanethiol can increase, contributing to the decline in quality.
Best Methods for Reheating a Latte
The goal of reheating is to use the lowest heat for the shortest duration to minimize damage to the milk proteins and volatile coffee compounds. The stovetop method offers the most control and is the preferred technique. Pour the latte into a small saucepan and place it over the lowest heat setting.
Stir the latte constantly and gently with a whisk or spoon to distribute the heat evenly and prevent scorching. The ideal temperature for reheating is just warm, not steaming hot. Keep the temperature well below the $65^\circ\text{C}$ threshold where milk proteins begin to denature. Remove the latte from the heat immediately once it reaches the desired warmth.
The microwave is a faster option, which reduces the time the coffee is exposed to heat and potentially preserving more flavor. However, microwaves heat unevenly, increasing the risk of scalding the milk. To mitigate this, use a low power setting, such as 50% power.
Heat the latte in very short bursts, typically 15 to 20 seconds at a time, stirring thoroughly between each interval. This technique helps redistribute the heat and prevents localized overheating that causes the milk to separate. Regardless of the method used, the original microfoam will be lost.
Safety and Storage Considerations
When dealing with a milk-based beverage, safety is a concern due to the risk of bacterial growth. Perishable foods, including lattes, should not remain in the “Danger Zone,” the temperature range between $40^\circ\text{F}$ and $140^\circ\text{F}$ ($4^\circ\text{C}$ to $60^\circ\text{C}$). Bacteria multiply rapidly within this range.
A latte should never be left at room temperature for more than two hours before being discarded or refrigerated. If the ambient temperature is above $90^\circ\text{F}$, this window shrinks to just one hour. If you plan to reheat a latte, it must be promptly refrigerated at $40^\circ\text{F}$ or below.
For safety, any refrigerated leftovers should be reheated thoroughly to an internal temperature of $165^\circ\text{F}$ to ensure harmful bacteria are killed. A latte should only be reheated once, as repeated heating and cooling cycles further compromise both the safety and quality of the drink.