Sugar. Water. Micro-organisms. And why sweetened condensed milk does not spoil easily.
How can a can of unopened sweetened condensed milk be kept for a year at room temperature without spoiling? Well it can because it is so sweet, makes it difficult micro-organisms to survive, eventually killing time!
Sweetened condensed milk is pasteurized milk where much of its water has been removed and additional sugar mixed in. Naturally, it would be very concentrated in sugar, giving the food we add it to – such as coffee, bread, and desserts – a flavor and sugar boost.
This high sugar concentration is also the sweet secret to why sweetened condensed milk does not spoil easily. A lot of sugar can dissolve in water because they love and cannot help being attracted to each other i.e. hygroscopic (read the science why this is so in the 1-Minute NomNom “Sucrose yet so far“).
With so much sugar and water bonded to each other, the water
“… in unavailable to the micro-organisms [that might spoil the milk]. Just as sailors at sea cannot quench their thirst with seawater, micro-organisms cannot easily quench their thirst with sugar syrup. Their cells shrivel and they cannot function.” (Source: How Baking Works)
With the loss of function, the micro-organisms cannot survive and die, effectively killed by the high sugar concentration in sweetened condensed milk!Why do the cells shrivel? Without water, the “water activity is lowered“. At the same time, water cannot flow into the cells via osmosis because the milk is so concentrated; in fact, water might flow out which is what causes the cells to shrivel.
With water activity lowered, and the osmotic pressure at such a disadvantage to the micro-organisms’ cells, it is no wonder the micro-organisms can no longer function. It is also no wonder then that we can keep cans of sweetened condensed milk at room temperature for months, even up to a year, and when we do use it, it’s still “fresh”!
Like this? We condense science to make it sweet for learning so like me to discover more. All you need is a minute a day to explore the world’s marvels through the phenomenon of food!
photos: in order – depositphotos/tempakul; tempakul; belchonock; akulamatiau; edesignua; kung_mangkorn