Fruits that are used to make these jams are picked from nearby farms by labour, in summer, lots of farms will be opened to everyone for picking, and they’ve planted various berries, and even beans. The fruits taste good and have a huge difference from those in supermarket, not always sweeter, but by all means very tasty in sour. I deliberately contrast strawberries from farm and surpermarket, for strawberries from farm, they are very inconsistent on maturity because I’m not sophisticated on picking after all, so that some of them taste very sweet and some of them very sour. Supermaket-bought strawberries taste the same and have no complicated circumstance as my sweet or sour strawberries, they have a relatively light taste, not particularly sweet nor particularly sour. Certainly it has a great relationship with varieites, the farms have planted many varieties of strawberries in order to allow them mature in batches, each kind has a mature period with size and taste varying greatly.
Generally farms will sell some of their fruits that are packed well besides allowing customers to pick their own. One type of fruit has two prices, PYO(pick your own) and Ready Picked. It raised my interest and I turned to the study of the price and found that fruits more difficult to pick had a greater difference between the two prices. Strawberry is the most effortless to pick, so it has a smallest difference on two prices, whereas the two prices of currant hidden in the depths of plants and laborious to pick differ twice.
Jam is seeminly simple to make but actually it is a food with many considerations in making. Thought is was only to boil fruit with sugar until thick? It is more than that simple, In fact making jam is a scientific experiment there. Take how much sugar to use speaking, even if you want your jam to be less sweet, you can not arbitrarily reduce the amount of sugar.
Because the original intention of making jam is to store fruits in sugar with high concentration which has a preservative effect, so that seasonal leftover fruits could be preserved for a long time. In most of the recipes, they would suggest that the amount of sugar shall maintain equal proportion or even higher with fruits, 100g fruits shall use 100g sugar. Personally I think that it is determined by the sweetness of fruits, raspberry or currant which is sour works with a ratio of 1:1 or higher, well strawberry which is sweet works with a ratio of reducing sugar to a half or a little bit more than a half, and that is the minimum.
In addition to sweetness, the pectin content is also an very important thing that needs to be considered. Whether jam is solid or not depends on the natural pectin in fruits. As we all know that pectin is mainly in peels, so fruits with thicker peels would have a higher pectin content such as apple and etc. At large, the lower the pectin content, the more the sugar, and thus relatively the less water dissloving out when bringing to a boil, the easier it is to reach the condensation point.
Speaking of condensation point, it is another scientific concept. When the density achieves the degree of condensing after cooling by boiling a pot of jam, it means that the solid, liquid and pectin in the pot have reached an appropriate ratio, at this time the moisture content in jam is extremly low, so the boiling point is higher than water which is 212F. That is why condensation point is determined by temperature when making jam professionally, it is considered to be boiled just right when the temperature in pot reaches 221F. It could be determined even without a thermometer, I’ll explain it later.
As for the fruits lower in pectin content, it is a little difficult in achieving the condensation point, but you can supplement extra pectin, many stores are selling purified pectins or sugar especially for jam added in pectin powder, it is the easy way out when making strawberry jam. Besides, it works only by reacting with acid,so lemon is a must-have. Those considerations would help you make a beautiful condensed jam but not a watery sugared fruit.
At last we come to the sterilization and preservation of jams. If you make jam a small a time for a week, you can omit this, but if you want to make jams in bulks like the jam products for sale, and guarantee a year’s quality under room temperature, then you have to read what’s next.
The bottles to pack jam shall be thoroughly sterilized by boiling with water, including the bottle caps. Pack the just cooked jam with high temperature in sterilized bottles as soon as possible. Packed jams need to be boiled with their whole bodies in water for a while, just for the sake of killing all the microbes under high temperature and forming an vacuum and negative pressure circumstance in bottles, this is similiar with how to make can. The principle of negative pressure shaped in bottled jam is that air in jam bottle will expand right after heating with water, and will push the bottle cap to overflow, but out of the reason that the whole bottle is underwater, water won’t flow into the bottle even a crack appears by the water pressure that pushes the botle cap with hot air. Finally the remaining air in the bottle shrinks after cooling and further forms negative pressure which is close to vacuum. Jams in bottles have all been sterilized under high temperature, plus vacuum circumstance in bottles, the expiration date has been prolonged greatly and even under room temperature they can be preserved long, as much as canned food.