Increasing sweetness in open pollinated corn.
Summary of 2011 breeding work seeking to select for sweetness in the development of two open pollinated sweet corn varieties. -Jonathan Spero
The method I am using to select for sweetness is to first, select the plants that have sweeter ears, and second, to select the sweeter kernels from the ears found on those sweeter plants. Selection of sweeter kernels relies on osmotic pressure from higher sugar levels to cause sweeter corn kernels to commence wrinkling more slowly than kernels on the same cobs that have less sugars.
Top Hat and Tuxana corn are two f4 populations that were grown out in 2011. I now have 250 or more cobs each of "Top Hat", "Tuxana yellow" , and "Tuxana white" corn that have been selected for sweetness first by taste testing, and then by selection of those kernels slowest to commence wrinkling. The cobs, with selected kernels marked, are in the racks drying.
Step 1 - finding the plants that make sweeter ears.
This involves people out in the field tasting corn and choosing the sweetest ears when the corn is ripe to eat. For the most part, I selected plants with two ears so that the secondary ear could be removed and sampled while the primary ear was left undisturbed on the plant. I experimented with a procedure to harvest the corn ahead, so that it could be taste tested when a day or two old.
I experimented this year with a community based approach to taste testing and had several people came out to the farm to be corn tasters. I posted on a local email board and offered free corn to anyone who would come and participate in a 3 hour corn tasting session. I compared this approach to using a regular taste testing crew. I also worked to determine the role of a refractometer in a process that relied mostly on taste to find the sweetest corn. The methods used and the conclusions drawn are discussed below.
Step 2 - identifying and marking the kernels that commence wrinkling last
This process is critical to sweetness selection. I selected within the cobs for the kernels that were the last to commence to dry. This process should pick kernels that are homozygous for the sugary enhancer (se) gene as well as selecting for any other sweetness enhancing genetics that may be present in the population.
Top Hat (2011) is the f4 generation of Tuxedo. Seed prior to this year had not been selected for sweetness by kernel selection. Taste test selection for sweetness in 2010 in Top Hat yielded only very modest gains in sweetness in the 2011 crop. Slow drying Top Hat kernels have been selected in 2011 as described herein. The results will not be known until these seeds are grown out in the next generation.
Tuxana (2011) is the f4 generation from the cross of Anasazi x Tuxedo. The Anasazi parent corn is a landrace and this f4 generation exhibited variation in many characteristics, as might be expected. Achieving greater uniformity will take some time. A white, a yellow and a purple or multicolored corn are being or will/could be selected from this cross. In 2010 I selected and marked both the yellow and the white Tuxana kernels, and chose the kernels to plant in 2011 by looking for slow-to-dry kernels and then choosing separately the whitest white and deepest yellow kernels. In 2011 the white and the yellow Tuxana lines were grown separately and slow drying kernels selected from both.
The results with the white Tuxana line verified that the basic method works. The white corn was notably sweeter in 2011 than in 2010. Brix readings of 5 samples taken ranged from 18 - 23, compared with an average of around 15 the year before.
What I did not know is that yellow kernels with se tend to be lighter in yellow coloration than those without the se, and therefore, in selecting the deepest yellow kernels, I was working against myself. I did not gain in sweetness in the yellow Tuxana line in 2010, so the yellow line in 2011 was considerably less sweet than the white. By choosing last-to-dry lighter yellow kernels from the 2011 harvest (I now think it may be possible to see the right shade of color), I expect the Tuxana yellow corn to be sweeter in the next generation.
In the yellow Tuxana line, with about 1400 corn plants, I only selected plants with two good ears. Taste selection is easier when the secondary ear can be removed for tasting, and this way the primary ear remains undamaged. Testing was done in the yellow Tuxana with a 24 to 48 hour delay after harvest. I was able to find about 250 plants with two ears where the secondary ear was moderately or more sweet. Kernels that were slowest to commence drying on the primary ear from those sweeter plants have been selected and marked.
In the white, with about 900 corn plants, I dropped the 2 ear requirement, and did bite tests of the single ear of corn still on the stalk. Therefore, only immediate sweetness could be evaluated. The white Tuxana is the furthest along in sweetness selection, and could be the first ready to confirm and document that this process to enhance sweetness works. However, I need to select out all the colors to get to pure white. There was a lot of yellow and some red and blue remaining in this first generation of "all white" corn.
Methods of taste testing - trying a community based approach:
Our collaborative / community based model for corn tasting "paid" volunteers in corn. Taste testing must be done when the corn is ripe, and at that time there is plenty of corn from for the testers to take home from other than the selected plants. I posted a note on a local email list and about 8 people came out to the farm for a 3 hour session as corn tasters. Usually there was myself, an intern, and two community members as the tasting crew. I think all had a good time, learned something, and volunteers took home as much corn as they wanted. Later on, we switched to using a regular crew of three practiced taste testers.
We tried three methods of putting taste testers to work.
In the first method, the group of three tasters sat at a table alongside the corn patch. A runner, usually me, went and picked the corn and brought it back to the table for evaluation. I could bring in two at a time without confusing them. I would take the secondary ear of the first plant, place a red colored flag at the base of the plant, and put the ear of corn in my left pocket. I would similarly harvest the secondary ear of a different plant, mark the plant with a blue flag, and put it the cob in my right pocket. Then, after the tasters made their evaluation "in" or "out", I would hang a yellow flag from the top of the stalk of the chosen "in" plants and move my red and blue flags on to the next two corn plants. Tasters rated each ear on a 1 to 5 scale for sweetness, eating quality and appearance, and then made an "in" or "out" judgment. This method is convenient for the volunteers as they sit at a table instead of clambering through the corn. It leaves some record, as participants score the corn ear, and could allow for some statistical analysis. Using this first method, a group of 4 (3 tasters and a runner) could test 80 to 100 ears in a 3 hour session. With a couple thousand or more ears coming ripe, we needed to move faster.
In the second method, a group of three walked together down the rows. The lead person would harvest the secondary ear from the plant (or, if this was a delayed sweetness tasting, pick up the previously harvested, paper plate wrapped and rubber-banded ear), shuck the ear, and all 3 of us would taste it. We would collectively decide "in" or "out" using sweetness as first criteria but also considering eating quality and the appearance of both cob and plant. We rotated who would give an opinion first so that one person did not end up dominating the decisions. Participants should keep something of a "poker face" when tasting, as it seems natural to look to another taster's facial expression for an opinion. We kept no written records and simply flagged the keeper plants. Using this second method, a crew of 3 could test about 200 ears in a 3 hour session.
In the third method, I stopped using new community volunteers as this method requires experienced tasters. We first "tuned in" by all 3 people tasting and rating a few cobs, with refractometer readings take in addition. Then, the three tasters would separately work adjacent rows, keeping fairly close together. Most plants could be called "in" or "out" by only one person. If that person was uncertain, then one or both of the others would also taste the cob and give an opinion. A brix reading would be made if desired. Using this third method, a crew of three could test about 400 ears in a 3 hour session.
Brix: Although I relied primarily on taste tests, a refractometer has some use in sweetness selection. At the beginning of the tasting session, we, usually a crew of three, each took a bite of 6 to10 ears, compared opinions as to sweetness, and simultaneously took refractometer readings on corn from those cobs. This checked that we were able to agree on "sweet" and gave us a general brix range and a minimum acceptable reading.
To take a reading, we would slice off at least 15 or 20 kernels, squeeze them through a hand operated garlic press, and drip the liquid onto the refractometer plate. Refractometer readings are fuzzy and readable only give or take probably one or two numbers. Furthermore, those brix readings seem to increase as the corn juice settles. I found 2 people could agree (sweeter vs. less sweet) with the refractometer maybe 4 times out of 5. We found that the refractometer missed corn with an off taste, and could not judge other traits of eating quality. It required field-cleaning the refractometer each time. We used the brix reading to help tune in our sense of taste, and then as one more vote on the crew if tasters were unsure or in disagreement. In the first ears of the tasting session, we used it often, maybe 5 to 10 times. Once we got going, we used the refractometer only occasionally.
Delayed sampling:
Sampling the corn 24 to 48 hours after harvest (instead of at the moment the ear was harvested from the plant) let us taste the corn more like when a customer would buy it in a store. Some sugars in corn turn to starch more quickly than others and I wished to better choose corn that would stay sweet. I needed to be able to track the tasted cob back to the right plant. To do this, the secondary ear was harvested and wrapped in a paper plate with a number written on it. The plate was secured around the harvested ear with a rubber band. A corresponding number label was attached to the plant with the primary ear undisturbed. The wrapped plates were each left at the base of the plant. We had some animal pressure (fox, skunk, opossum) but the paper plate and rubber bands slowed them down, so only perhaps 5% of the cobs were lost. Looking back, if cob and plant are to be labeled anyway, it may have made more sense to bring the corn inside for tasting, and to return after tasting to flag the keeper plants by number.
Kernel Selection
This is where increase in sweetness should occur quickly. The primary cobs from the selected plants were harvested about 2 weeks past eating stage when the first kernels on the more mature plants were starting to wrinkle. Kernels should be far enough along in development to make viable seed, but the kernels on most cobs should still be smooth and not yet showing signs of wrinkling. At this point the juices in the corn are just beginning to gel. If you poke a kernel with your finger nail, it will no longer squirt.
I harvested the ears of the selected plants when 10% - 20% of those plants had cobs with kernels that were showing any sign of wrinkling. In the Top Hat, fairly uniform for maturity, harvest was done all at once. With variation in maturity in the Tuxana, I harvested twice, about a week apart. The selection goal is to find and mark the last kernels to begin to show signs of drying on each separate ear. I went through the un-husked cobs daily in order to catch the cobs when the kernels were just starting to wrinkle.
Each morning, I peeled back a bit of husk on each bob and looked for wrinkles. If there were none, I smoothed the husk back over the corn and left the cob to check again in a day or two. If I saw any wrinkles, I peeled back and removed the husk. I set the cobs up with air flow all around or rotated them frequently, as a difference in exposure to drying air also affects commencement of drying of the individual kernels. The kernels on the cob, now open to the air, commence to wrinkle over the course of the day. It is necessary to mark the smooth kernels before all kernels commence wrinkling, at which point it is too late to make selection. After the initial flush of cobs that are ready to mark on the day of harvest, the number of cobs ready for kernel selection peaked on the 3rd or 4th day after the corn was harvested, and selection was complete by about the 9th day after harvest.
There is a point where it is easy to discern already wrinkling from not yet wrinkling kernels, and it is clear which kernels to choose for an hour or more. During that time, we used a marker (black on the yellow kernels, blue on the white kernels) or a paint brush (white on the purple kernels) to mark those kernels that still looked full when other kernels around them were starting to wrinkle. I set the cobs with marked kernels in racks to dry and will pick out the marked kernels from the cobs during the winter.
How much time did it take to do this ?
2011 labor hours total for both Top Hat and Tuxana corns:
Prepare soil, grow corn crops 94 hours
Sample at milk stage 163*
kernel selection 149
pick out kernels 30
* this included our experiments with community corn tasting and delayed sampling, each of which added roughly 30 hours of work time.
In 2012, I hope to repeat this process. At the end of two cycles I hope to have corn that is uniformly sweet and comparable in sweetness to hybrid se corn varieties on the market today.
October 2011
Jonathan Spero
Lupine Knoll Farm
spero.jonathan@gmail.com
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