I will always say that agricultural workers work harder than anyone. Not only is it physically demanding work, but there is a lot of attention to detail that is required. One primary example is during harvest. Not only are the workers harvesting as quickly as possible, but they are also checking for quality and fruit maturity indices as they harvest.
Maturity Indices
Since all crops are unique, they each have different parameters that determine when they are ready to harvest. These parameters are known as the maturity indices, which are based on:
- Whether the fruit are climacteric (continue ripening after harvest) or non-climacteric (do not continue ripening after harvest)
- The characteristics of the fruit that demonstrate the fruit maturity (e.g., if the skin color changes as the fruit ripens, as is seen with bananas)
- The customer and how much shipping time is required
If the fruit is climacteric, they can be harvested prior to ripening and still ripen properly. By doing this, it allows the crop to have a longer shelf-life. One example is kiwifruit. They are harvested prior to ripening and are gassed with ethylene to ripen the fruit each time they are sent out for orders. However, if the fruit is non-climacteric, it needs to be harvested to ensure that the minimum flavor characteristics are achieved prior to harvest.
The maturity index used is dependent on the crop. Some common examples include the color measurement, soluble solids concentration, and firmness of the fruit. When measuring these different maturity indices, a representative sample of the fruit is used.
Color
The color can either be measured using the skin color or the internal flesh color of the fruit. This maturity index is used when the skin or flesh color is a clear indicator of the fruit maturity. In this case, a colorimeter (the machine demonstrated below) can be used to measure the color or color swatches can be used out in the field to determine if the fruit has reached the desired maturity stage.
A common climacteric fruit harvested using color as a maturity index is peaches, by measuring their skin color. However, it is important to remember that the color will be different between different peach cultivars that are grown. (Peaches could also require a firmness measurement if there is a red color masking the color change that comes with ripeness.)
A common non-climacteric fruit that uses color as an indicator is strawberries. Since strawberries do not continue ripening after harvest, they have to be harvested at a maturity stage where the flavor meets at least the minimum customer expectations (horticultural maturity), and the fruit has as long of a shelf-life as possible. In California, strawberries need to obtain a minimum of 2/3 red/pink color on the strawberry prior to harvest (1/2 – 3/4 is the national standard). At this stage, they have also reached the minimum soluble solids concentration.
Soluble Solids Concentration (SSC)
The soluble solids concentration (SSC), also referred to as ‘Sugars’ or ‘Brix’, is a measure of the amount of sugars and titratable acidity (amino acids, organic acids, etc.) in the fruit. The SSC is measured by juicing a sample size of fruit and using a refractometer to measure the SSC content. There are also portable refractometers that can be easily used in the field for this purpose. Grapes are an example of a fruit that relies solely on the SSC to determine the harvest time.
Firmness
Fruit firmness is measured using a penetrometer. Just like with the other maturity index measurements, there are machines that can be used inside, as well as hand-held penetrometers that can be easily used in the field. Firmness is measured by maintaining the fruit in a stationary position, while penetrating to a specified distance and measuring the force required to reach that distance. The tip used to penetrate is based on the size of the fruit. Thus, you would not be using the same tip for pears as you would for cherries.
Is All Fruit Harvested Dependent on Just One Measure?
No. It is more common for there to be two maturity indices used to determine when fruit is ready for harvest. Some examples include:
- Kiwifruit – Firmness & SSC
- Figs – Skin Color & Firmness
- ‘Brooks’ Cherries – Skin Color & SSC (while harvest for ‘Bing’ cherries is based solely on skin color)
Destructible vs. Non-Destructible Methods
The methods described above are all potentially destructible methods. Skin color can go either way since the skin color measurement does not damage the fruit, while the flesh color measurement does. However, for SSC and firmness measurements, your fruit will be damaged in the process.
Engineers have been in the process of coming up with non-destructible methods to measure these different maturity indices. However, there is not yet a tool that can determine the maturity for all fruit types.
If you enjoyed this post, you might also like:
- What Banana Is Right For You: The Banana Ripening Process
- Postharvest Handling of Cut Flowers: Trimming & Packaging Fresh-Cut Roses
- Why Won’t My Avocados Ripen?
Further Reading:
Reid, M.S. “Maturation and Maturity Indices.” University of Florida. Web. 20 April 2017.