A sweet task: Omani study to determine right time to harvest dates depending on sugar content

Scientists are using near infrared imaging to help farmers determine when their dates are ready to be harvested.

Near infrared imaging is able to assess the sugar content in individual dates. Delores Johnson / The National

Every year, 250,000 tonnes of the fruit are harvested in the UAE, underscoring its regional significance. Now scientists in Oman are using technology to better appraise value by measuring the sugar content in each fruit.

There are few more appealing ways to round off a meal in the UAE than eating a handful of sweet and succulent dates.

Available in a host of varieties and at modest prices, these tasty fruits have been a key part of the diet in the Middle East for thousands of years, and their popularity shows no sign of waning. Each year, about 250,000 tonnes of the fruit are harvested in the UAE alone.

Given the significance of dates to the Middle East in cultural, nutritional and economic terms, it is no surprise that scientists in the region are working hard to produce better-yielding types of date palm and to streamline ways of cultivating them.

Important considerations for growers and processors also include deciding when to harvest dates, and determining their sugar content. Research by scientists in Oman, recently published in the Emirates Journal of Food and Agriculture, could help with both of these issues.

The work centres on assessing the total soluble solid (TSS) content of dates, a measure that approximately equates to the amount of sugar in the fruit, which increases as the dates ripen because water is lost and starch is converted to sugar. TSS content is one of the key parameters linked to the value of dates, regardless of whether they are eaten fresh or processed.

There are accurate ways to measure TSS content, but these methods are typically laborious and involve destroying flesh from the fruit to obtain a result. As an alternative, the scientists at Sultan Qaboos University are looking to use near infrared (NIR) imaging.

With an NIR camera, some halogen lamps and a computer, the scientists measured the dates’ reflectance of NIR light, which is light that has a wavelength of between 900 and 1,700 nanometres (by comparison, visible light has a wavelength of between 390 and 700nm).

In carrying out their study, the researchers are tapping into a body of research, dating back several decades, into the use of NIR techniques to assess foodstuffs.

Much of the focus has, however, been on grains and meat, according to Sundaram Gunasekaran, a professor of food engineering at the University of Wisconsin-Madison and editor of the book Nondestructive Food Evaluation: Techniques to Analyse Properties and Quality.

With grains, for example, moisture, protein and oil content are useful variables, and it takes commercially available NIR machines less than a minute to give readings for these. By contrast, albeit with some exceptions, fruits have tended not to be assessed so often with NIR.

“With fruits, they’re not as interested in composition; they’re more interested in defects, so it’s not as popular in the fruit industry,” said Prof Gunasekaran.

The significance of TSS in dates could make them another exception to the rule. For date growers, composition really is important.

The key for the scientists in Oman was to find out what relationship – if any – existed between NIR reflectance and TSS content.

That meant that once they had carried out their NIR measurement on a date, they then had to measure the TSS content of the same fruit the traditional way: tissue pulp was obtained, water was added, and this mixture was passed through a filter paper.

The researchers then used a piece of equipment called a refractometer to find out how much the resultant liquid refracted light, an accepted way of determining TSS content.

After these procedures were repeated for hundreds of dates of several varieties and at different stages of maturation, the scientists looked for a correlation between NIR reflectance and TSS content.

They found an inversely proportional relationship, with reflectance tending to be higher when the TSS content was lower.

There was wide variability in the relationship between the two variables, but the study’s lead author, Dr Manickavasagan Annamalai, of Sultan Qaboos University’s College of Agricultural and Marine Sciences, said the techniques could be improved so that commercially useful tests could be developed.

He suggested researchers could, for example, go on to create a “harvest index” that indicated when the fruit was ready to be harvested.

“Dates are harvested in bundles. Normally people now, if one or two fruits are showing ripening, [the bundle is picked],” said Dr Annamalai.

“[Using NIR] we can take an image of the whole bunch and see the percentage [sugar content]. It’s OK to pick if the bunch has more than 50 per cent of the fruits ready. We can say, ‘yes, this bunch can be harvested,’ but if it’s less than 50 per cent, we have to wait a couple of days. That kind of index can be developed with this system.”

NIR measures could also be used to grade dates according to sugar content, which could prove valuable to consumers who are keen to limit their intake of sugar while still enjoying the beneficial substances, such as antioxidants, found in dates.

“In the supermarkets, there’s no standard or expectation [of sugar content], but definitely it will be an indicator in the future, because people are demanding the ingredients and nutritional level [of foods]. At that point they will have to provide the details,” said Dr Annamalai.

He said using a technique similar to the one described in the study could lead to a system whereby dates on a conveyor belt are sorted by machine according to their sugar content.

The processing line could, using NIR reflectance as the key variable, “analyse each and every date”.

“If you need high-sugar dates, the system [could] automatically identify and remove [them] from the processing line. That would be the ultimate,” he said.

“This is very preliminary work. We still have to do the next level of investment. Then we [can] develop a system capable of measuring, non-destructively, the sugar content. It has huge applications right from harvest.”

So in the years to come, by selecting dates with low sugar content, it might be possible for all of us to enjoy the fruit regularly without even a twinge of health-related guilt. Or, instead, we could just decide to throw caution to the wind and buy the dates with the highest sugar content.

Even they will be much healthier than an average bar of chocolate – and probably tastier, too.