Whenever two water solutions containing different amounts of sugar (for example) are on opposite sides of a plant’s cell wall, water molecules will move spontaneously through the cell wall in the direction of the more concentrated (stronger) solution, making it less concentrated, diluting it. That’s osmosis.
When you cook the berries in plain water without any sugar, water molecules moved into the cells, where some dissolved sugars already existed, until the cells could hold no more water and burst.
Ruptured cells, having lost their crisp cellular structure, are mushy cells.
On the other hand, when you cook a fruit in water with lots of sugar, more sugar than exists inside the cells, water molecules move out of the cells into the external sugar solution. The cells will shrink like deflated balloons, but they won’t burst and their cell walls will still be more or less intact, retaining their toothsome texture. The berries therefore won’t be softened as much by cooking in sugared water as they would be in plain water.
Sugar also has a strengthening effect on the fruit’s cells even when they’re deflated, because it reacts with the proteins in the cell walls.
In Nature, osmosis moves water from a solution of low concentration (of sugar, salt, etc.) through a cell wall or other kind of membrane, into a solution of higher concentration on the other side, thereby diluting or watering down the more concentrated solution. But food producers often want to make a solution more concentrated; that is, to remove water from it, the exact opposite of what osmosis would do.
To accomplish this, they reverse the osmosis process by forcing water out of the dilute solution, through a membrane, and into a more concentrated solution. The process, called reverse osmosis, can require a substantial amount of pressure, as high as 1,000 pounds per square inch, to counteract the natural osmotic pressure and reverse the natural direction of water flow.
For example, the watery whey from cheese making was once considered a waste product and, when discarded, an environmental pollutant. But today, through reverse osmosis, the water is removed and the protein is sold to food manufacturers as the “whey powder” or “milk protein concentrate” that you see in the lists of ingredients in processed foods.
Reverse osmosis is also used to purify water. In this case, the pure water “squeezed out” of the impure water is, of course, the desired product.
In a solution of sugar in water, there are both sugar molecules and water molecules. If there aren’t many sugar molecules (that is, if the solution is dilute), the water molecules can freely bombard the walls of their container without much interference from the sugar.
If those walls happen to be the walls of a plant cell, which are somewhat permeable to water, many of those water molecules will succeed in passing through to the other side. On the other hand, if a sugar solution is strong (concentrated), the sugar molecules will interfere severely, and not as many water molecules will succeed in penetrating the cell wall.
In much of the country, locally grown strawberries can be found in farmers’ markets for only a few weeks in late spring. Make the most of this window. Select small, firm but ripe berries in perfect condition. In this method, standing periods alternating with short cooking times yield a preserve with deep red color and fresh flavor.
In making preserves, jams, and jellies, the proportions of the three primary ingredients, fruit pectin, sugar, and acid (lemon juice), are crucial. The gel is formed by the action of the acid on the pectin, so too little pectin or acid will prevent gel formation and you’ll have syrup instead. Too little sugar will make a tough jelly, while too much sugar will make a weak one. Simply put, the ingredients must be measured carefully.