Using Dry Matter as a Reliable Harvest Indicator for Vegetables
October 8, 2019 at 11:24 pm | Updated October 8, 2019 at 11:24 pm | 7 min read
Dry Matter in Vegetables
The use of dry matter to fix harvest time is well-established for fruits. This practice is, however, not wide-spread for vegetables, except for tomatoes. Many vegetables are consumed before ripening and eaten at various stages of fruit development. So, is dry matter estimation useful in the case of vegetables? Recent research studies show that DM is indeed a good indicator of harvest time and quality and can be used commercially.
Dry Matter Changes Are Not Simple
Dry matter (DM) is the solid contents of a fruit or a plant minus its water content. It is made of structural and non-structural carbohydrates, starch, sugars, oil, pigments, and minerals.
DM content has been established as a reliable indicator for harvest time, post-harvest taste, and ripeness in the case of most fruits, as increases in DM are associated with fruit development. It is this relationship which is used to judge ideal harvest conditions for fruits.
However, DM does not show a simple linear increase for many vegetables. The simple rule of thumb of “more time on the plant gives more DM” doesn’t apply to all vegetables.
Moreover, some vegetables are not even fruits, such as members of the cabbage family or Brassicaceae (also called Cruciferae). Cauliflowers and broccolis are flowers, cabbage and kale are leaves, and Brussels sprouts are stem buds!
The variation in DM can be complex and vary among different categories of vegetables. Similarly, the reason why DM is a good indicator of harvest time and post-harvest quality does not remain the same. Therefore, the relationship between DM and quality needs to be examined individually.
Three very different categories of vegetables will be considered to see DM’s effects; these are the climacteric tomatoes; bell peppers, which are partially climacteric; and leafy cabbages.
Dry Matter in Tomatoes
Though tomatoes are used as vegetables, they are botanically a fruit. Since they are used when ripe, they resemble other fruits the most where dry matter accumulation is concerned. Therefore, it is easy to see why DM estimation is popular in tomatoes to fix harvest time.
Tomato is a climacteric fruit, which can ripen after harvest and needs to be plucked when it is fully mature. DM will increase in tomatoes after harvest as it ripens, presumably due to a loss in water content through transpiration. The highest DM and SSC values are found in fully ripe tomatoes.
DM, soluble solids content (SSC), firmness, and external color are the criteria used to judge the quality of tomatoes. SSC makes up nearly 50% of DM near maturity.
Industries which make processed tomato products, such as ketchup and juices, are interested in maximizing both DM and SSC and pay more for crops with higher DM. Therefore, it is important to leave the crop on the plant as long as possible but harvest them in good time to get them to the market and processing factories.
Non-destructive measurement of DM is important, as its levels have to be checked not only on the field but also at other stages of the supply chain during processing.
Near infrared (NIR) spectroscopy in hand held devices has been tested successfully to establish fruit quality for tomatoes meant for use as a vegetable and to make puree and juices.
The Felix F-750 Produce Quality Meter, which is based on NIR spectroscopy, can measure DM, SSC through BRIX, and external color, and is an example of one such device available on the market.
The best DM at which tomato should be harvested will vary depending on the variety. Total
DM in different cultivars can vary from 5% to 8% of fresh fruit weight (See Figure 1).
Figure 1: Dry matter (DM) and fruit size (FD) at harvest for different varieties and their hybrids, Pascaul et al. 2013. (Image credits: DOI: 10.1093/jxb/ert349)
Dry Matter in Bell Peppers
Bell peppers or capsicums come in green, yellow, orange or red colors. These are the same pepper variety, at different stages of maturity. The different colors arise due to varying concentrations of pigments in the skin and pulp. Green comes from chlorophylls, flavonoids make peppers yellow, and carotenoids turn them orange to red. Ethylene is necessary for the development of color in the peppers, and without it, the peppers will remain green.
Bell peppers are usually harvested when they are green, and it is easy to mix immature with mature ones. Plucking them before full maturity can result in poor ripening color development. Since bell peppers will continue to grow in size until harvested, plucking them too soon also lowers harvest quantity.
Figure 2: Cubiu fruits arranged according to their different stages. The green stage, the turning stage (greenish-yellow fruits), ripe and yellow, and lastly the fully ripe or red stage.
Junior et.al. 2016 (Image credits: DOI: 10.4236/fns.2016.712111)
Studies show that bell peppers harvested when their dry matter is maximum are the best. At this stage, the amount of ascorbic acid, which keeps the vegetable hard, is also less. Ascorbic acid content continues to decrease post-harvest.
Bell peppers harvested with DM of 7.35%, the maximum, also have the longest storage time of twenty one days. Immature peppers with 5.19% DM can be stored for only nine days under the same temperature and relative humidity.
Keeping the fruits on the plants or delaying harvesting after the maximum DM is reached provides no advantages in terms of fruit weight and size, ripening, or shelf life. Sensory attributes like crispness, color, and flavor are also the best when peppers are harvested at maximum DM.
For example, in the case of the variety ‘BARI Misti Morich-1’ in Bangladesh, the ideal/ maximum DM is reached forty five days after anthesis.
Thus, dry matter is not only a good indicator of harvest time but also of the quality and quantity of yield in bell peppers.
NIR spectroscopy has proven to be useful in detecting differences in DM and SSC in bell peppers. The technology has proven that there are differences in DM and SSC, depending on whether the vegetables are grown indoors or outdoors.
Dry Matter in Cabbage
Harvest indices are being calculated based on DM and appearance for cabbages, where DM doesn’t rise linearly.
Dry matter peaks to 8.6% and is accompanied by a moisture content drop. After the first peak, the DM drops for three weeks and then starts to increase again. Research has shown that the first peak of DM is the best time to harvest, so the window for the ideal harvest-time is very small here.
The cabbage head harvested at the first DM peak has the minimum loss in weight later, post-harvest. If harvested later, cabbages suffer weight loss due to a decrease in water content through transpiration and cracking of leaves. There is scope for more water loss if cabbages are harvested in the earlier stages, as well. Early in the development, the cabbage head is not compact but is loose as the head is unfilled, so more water content is lost.
As storage time increases, the visual attributes of freshness, color, and crispness decrease. Crops harvested around the first peak of DM showed the least loss in quality on storage. It is likely that cracks in older cabbages lead to increased moisture loss and cause wilting and yellowing of outer leaves.
Moreover, the cracks and blemishes that form in outer leaves also lead to diseases. Once again, cabbages harvested at the first DM peak are more resistant to diseases, as younger cabbages are not tough enough to resist pathogens.
So the ideal harvest time for the cabbages is during the first peak of DM, as yield quantity can be retained and the keeping quality of the vegetable is the best for direct consumption. High DM and low moisture content guaranteed during this time are also the desired qualities for processing cabbage.
Cabbage variety Green Coronet, in Sri Lanka, reaches the first peak of DM between 75-80 days, which is the best time to harvest it.
More Applications of Dry Matter
For many years, the F-750 Quality Meter, produced by Felix Instruments, has been successfully used in the field and supply chain for fruits to make rapid, non-destructive measurements of DM, SSC, and internal and external color within a few seconds. With this small device, decision-making has become easier for farmers, suppliers, and retailers. With the crop in hand, they can optimise earnings by using produce at the correct stage. It is time now to extend this well-established technique to vegetables and reduce food loss.
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture
Featured image credit: https://www.flickr.com/photos/honey-bee/
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