Most people don’t think of the dead of winter as being the best time to harvest leafy greens. Most of them are probably buried under feet of snow. But researchers at the University of New Hampshire have discovered that spinach is sweetest when picked in the winter months.
Becky Sideman and Kaitlyn Orde have been studying the effects of temperature on spinach sugar levels at the New Hampshire Agriculture Experiment Station. Sideman is a researcher and extension professor of sustainable horticulture production at the University of New Hampshire, and Orde is a graduate student in agricultural sciences. They spoke with NHPR’s Peter Biello.
Why did you think to compare the taste of spinach harvested at different times of the year?
Becky Sideman: As with many of the projects we do, we get our inspiration from local farmers in the area. They’re really the innovators in this case. For many, many years, growers have been growing winter spinach and noticing that it seemed to be much sweeter in the winter months. And there’s some research indicating that fact. And so we just really put numbers with it.
And how did you conduct the experiment.
Kaitlyn Orde: Over two winter seasons, we chose primarily three spinach varieties and planted them at six different planting dates throughout the fall. At the third planting date, we also included five additional spinach varieties that we use to test for the sugar levels throughout the season.
So you tested across varieties and over a couple of seasons?
Yes.
Did the greenhouses mimic the temperatures you’d have in the summer? Was temperature the same in both seasons?
KO: No. Firs tof all, we had two very different winters.
Different in the sense that last winter there wasn’t a lot of snow, but the previous winter there was.
KO: Yes, and the temperatures were much colder than this past winter. We didn’t actually use a greenhouse. We used a high tunnel, which is not heated. So greenhouse would imply that there were some other supplemental heating. But high tunnels do not have additional heating throughout the winter, and that was one of the great things about this experiment in that there was no added cost for heat for winter greens.
So, Becky Sideman, maybe you could explain to us what temperature does to spinach. You mentioned speaking to farmers who say spinach was sweeter in winter months. What does temperature do to the plant?
BS: As it turns out, when spinach is grown under cold conditions—lots of plants do this, actually—there’s a couple different things that happen. One is that there’s actually just less water in the plant cell. So any things that make them sweet are at a higher concentration. So they do taste sweeter. But they also are making anti-freeze proteins that make them sweeter as well. So it’s kind of a multi-faceted process.
And when you say “sweeter,” you mean that you were actually testing the spinach in a lab, not just tasting it and saying, “Oh, that tastes sweeter”?
BS: That’s right. In order to put numbers to the sweetness, we use a hand-held refractometer. And that refractometer measures the soluble solids, which are mostly sugars, in the solution inside the spinach cells. We measure, essentially, the percentage of sugar.
So how did it taste?
KO: They all tasted great! We did observe a difference among varieties. We had one variety in particular that was the highest in both years for the sugar content. But they were all pretty close. In the first year, they got higher brix, but the second year, last year, was warmer, so they weren’t as high. About six or six and a half or so. But they all tasted great, even with the variation. I would recommend all the varieties.
Becky, how would you describe the taste?
BS: I would agree that it’s quite difficult, even though we did detect significant differences between the varieties at a given date. It’s very difficult to taste those differences in the heart of winter. It is, though, really noticeable when you eat fall-grown and spring-grown spinach versus the spinach harvested in the dead of winter. So it’s noticeably sweeter when it’s very cold conditions.
So what does this mean for people growing spinach in their gardens in New Hampshire? Should they just plant other stuff in the spring and then wait until maybe around now, August, to plant their spinach, and harvest in October, November when things get colder?
BS: That’s certainly a way to guarantee sweeter spinach through the fall, absolutely. One question that has nagged at us is: Is sweet spinach necessarily better spinach? I don’t think people necessarily associate “sweet” as being an important critical characteristic like they do with, say, fruits. So while it’s noticeable and delicious, I think not sweet spinach is also delicious.
KO: In addition to the sweetness, the study shows that we can produce greens all winter long in New Hampshire, so I think that’s the biggest benefit to consumers. There are a lot of growers in our state that have high tunnels that go unused in the winter months and they could fill them with spinach in the fall and they could choose a planting date that fits their target markets, whether it be the holidays or the spring, and they could supply locally-produced greens to the public.
