The question of whether making jam conserves mass illustrates fundamental scientific principles applied to a common kitchen process. When fruit, sugar, and acid are combined and cooked, the total mass measured at the end is noticeably less than the initial ingredients. This apparent loss of mass tests the boundaries of the Law of Conservation of Mass. Understanding the process requires examining the difference between a theoretical, perfectly contained environment and the reality of a bubbling pot on a stove.
The Principle of Mass Conservation in a Closed System
The Law of Conservation of Mass (LOM) states that mass can neither be created nor destroyed in a chemical reaction or physical transformation. Instead, the atoms of the starting materials are simply rearranged to form new substances. If one were to make jam in a hypothetical “closed system,” where no matter could enter or escape, the total mass would remain exactly the same from start to finish.
A sealed, non-reactive pressure cooker, for example, would function as a closed system for this experiment. Any water that boiled and turned into steam would remain contained within the vessel, contributing its mass to the total measurement. Even if the sugar caramelized or the fruit compounds broke down, the sum of all components—solids, liquids, and gases—would equal the initial mass of the mixture.
Physical Mass Loss Through Evaporation
The reason a cook observes a decrease in mass is that jam is made in an “open system,” specifically a pot on a stovetop, which allows matter to escape. The primary driver of this mass reduction is the physical process of evaporation, where the heat energy causes water molecules to undergo a phase change from liquid to gas (steam). This steam carries a significant amount of mass away from the pot and into the surrounding air.
Jam boiling is deliberately performed to achieve this loss of water, concentrating the fruit solids and sugar into the thick, spreadable gel texture that characterizes the finished product. To achieve the correct texture and preservation properties, the mixture must be boiled until the total soluble solids (TSS) reach a concentration of approximately 60 to 65% by weight. This necessary concentration requires the physical removal of a substantial portion of the water initially present in the fruit and any added liquid.
Minor Mass Shifts from Chemical Reactions
While evaporation accounts for the bulk of the observed mass decrease, minor mass shifts also occur at the molecular level due to chemical transformations within the jam. The high heat and acidic environment drive several chemical reactions that rearrange the components of the mixture. One example is the subtle caramelization of sugars, which can release trace amounts of volatile compounds or gases, such as carbon dioxide, contributing minimally to the overall mass loss.
Additionally, the long-chain pectin molecules, which are carbohydrates derived from the fruit, can undergo hydrolysis during prolonged heating. This process breaks the large pectin chains into smaller molecules. This rearrangement can be accompanied by the formation of tiny amounts of other volatile substances that escape into the air. The destruction of heat-sensitive compounds, such as Vitamin C, also produces degradation products, some of which are volatile and escape the system.
Quantifying Mass Loss and the Open System
The practical goal of jam making is to reduce the mass of water until the desired concentration of solids is achieved, which is typically when the total soluble solids (TSS) content reaches 65% or more. This high concentration is necessary for proper gel formation and to lower the water activity to a point where microbial growth is inhibited, ensuring the jam’s preservation. Cooks do not typically measure the mass of escaped steam directly, but rather the result of that mass loss.
The most precise way to quantify this concentration is by using a refractometer, which measures the Brix value, a direct reading of the TSS in the final product. A less expensive, but equally effective, method is using a thermometer to determine the setting point of the jam. As water evaporates and the sugar concentration increases, the boiling point of the mixture rises above that of pure water, reaching approximately 104°C (219°F) when the ideal sugar concentration is reached.