It's all about the water.
. Over the next few weeks, she'll break down the science of and principles behind food preservation, with accompanying recipes so you can put your newfound knowledge to work.For those of us in northern climates, strawberries are the first tangible evidence that summer has truly arrived. These fragile berries, filled with water and sugar, are beloved both by people and by the kajillion-strong microbe population.
Pressure canning, meanwhile, is about raising temperatures to a point so high that nothing will survive. Sure, we could expose our strawberry harvest to the extreme heat of a pressure canner, but to what end? The 240°F temperature would kill any and all microbes, but it'd also turn the strawberries into a jar of overcooked mush. It'd be like launching a nuclear bomb when a BB gun would have done the job.
Think of it this way: Imagine that all the water molecules in a piece of food are just people in a room. And then think of a microbial pathogen as a really annoying guy who's always trying to get into the room and make trouble. One solution would be to get as many people out of the room as possible before the annoying guy arrives; that, in essence, is dehydration. With fewer people in the room, he has fewer people to bother.
From a chemical perspective, it's helpful to think of this riddle: When is water not really water? When it's salt water.Sugar molecules bond with the water in the strawberries to form a syrup and lower the fruit's water activity.the water activity of the strawberries enough to make a microbe's survival damned hard. But how much is enough? Measuring water activity in a food requires complex and expensive laboratory equipment that most of us don't have.
Add heat to the sugar-covered strawberries and the water activity is reduced even further through evaporation , buying you even more time. Most preserved foods rely on such combinations of chemical and physical barriers to ensure no pathogens can grow. Even the simple act of sealing the sugared and cooked strawberries in a clean glass jar, then placing them in the fridge, is a line of defense—one more wall that makes it difficult for ambient microbes to move in.
In our above example, we tossed fresh strawberries with sugar, then cooked the two together. One might assume that we made jam during that process, but we didn't. While definitions can vary, most would agree that for jam to be jam, it can't just be cooked fruit in a sugar syrup. It requires a thickener called pectin, a natural fiber in fruits and vegetables that keeps the outer peel or skin firm and intact.
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