This week we harvested kiwifruit in one of my classes for a lab activity. I’m a big fan of kiwifruit. Last weekend I spent some time with the vines to make sure everything was ready.
In commercial production kiwi berries are picked long before they are palatable. The kiwi in the picture above felt like a golf ball and would taste very bad. The fruit is picked when it is physiologically mature. After harvesting the fruit is stored in coolers, where the starches will be converted to sugars. Eventually the fruit is sweet and softened. The fruit we picked this week will be sold in about 15 weeks.
To determine the appropriate time for harvest the firmness of the fruit is monitored, as well as the sugar content. Firmness is measured with a penetrometer. In California kiwifruit should be harvested before they reach 14 lb/sq. in. with an 8 mm tip, and somewhere in the high teens or low twenties is good. On Saturday I was getting readings around 20 lb/sq. in.
The refractometer tests sugar content. It looks like a small, 6-inch telescope. Some of the juice from the fruit is placed on a calibrated prism at one end and covered with a slip case. Looking through the scope will reveal a reading on a scale, which is the degrees Brix (a measurement of sucrose in solution). The minimum sugar content for kiwi harvest in California is 6.2 °Brix. On Saturday I was getting readings around 8 °Brix. After the starches in the fruit become sugars the number will be much higher (maybe 12-15 °Brix).
And so we harvested.
I picked up some kiwis at the grocery store recently. On a whim I decided to try juicing one of them. I used one kiwi and two lemons in two quarts of water with 1/3 cup of sugar. If you like the taste of kiwi (and I do) it adds a nice twist to lemonade.