More Information

2000 year old date seeds still viable

Flowering plant seeds can be broadly classified as (i) orthodox, which have desiccation tolerance and can be stored for varying periods without unduly impacting viability and the ability to germinate, and (ii) recalcitrant (many tropical species), which are best kept close to the moist state existing within ripe fruit and sown as quickly as possible without any significant post-harvest drying. There are some overlaps between these two categories and some researchers include an intermediate category, stressing that variations between all three really manifest as a continuum. But if we stay with the conventional grouping, many seeds of sub-tropical fruiting species are orthodox, and with appropriate preparation and care this allows growers to vary the time between the time that seed are collected and sowing, sometimes for several years while still producing good germination outcomes.

Seed suitable for storing is dried, often down to the 5% moisture level, and optimal storage life is achieved if the low hydration level is continuously maintained. Imbibition when water is available begins the rehydration process leading to activation of enzymes and other processes associated with germination (providing there is no endo-dormancy), and this will reduce subsequent storage lifetime. Low temperature is another beneficial storage factor as it slows down metabolism that would otherwise use up finite energy reserves and could generate damaging chemical species or reactions that may not be effectively inactivated or repaired. Other storage factors beyond moisture and temperature include differences in species genetics eg seed coat thickness and permeability, internal antioxidant levels and repair mechanisms, and management differences such as maturity of seed at harvest and the rate of subsequent desiccation, predation, pathogens or wounding. Low humidity and temperature conditions are the basis of international seed storage banks.

Under favourable conditions, how long can orthodox seed storage life be extended? There are many instances of seeds being kept for several years that are still in good condition, but most would find it hard to imagine it could go on for too much longer. Well, a 1960s excavation near the Dead Sea in Israel uncovered some ancient materials that were subsequently identified as date palm seeds (Phoenix dactylifera, Science (2008) 320, 1464). They had been kept in storage at room temperature for 40 years when 5 seeds were selected to determine their radiocarbon age and to see if any were still viable and would germinate. Two were used for the dating which was found to be 50 yrs BC, ie more than 2000 years old, and one of the other 3 germinated, with the seedling growing to 15, 31 and 121cm tall in 3, 7.5 and 26 mths resp. Genetic analysis suggested this plant had many commonalities with modern elite Egyptian, Iraqi and Moroccan varieties, and the researchers suggested that local high summer temperatures and the dry climate contributed to the extended seed longevity.

But dates are not the runaway longevity winner if we branch outside natural seed reproduction to what can be achieved in the laboratory from other ancient fruits. In a 2012 study (Proc Nat Acad Sci, 109, 4008-4013), Russian scientists found that immature placental tissue of the herbaceous perennial Silene linnaeana buried in Siberian permafrost squirrel caches, carbon dated at 31,800 years old, was able to be micro-propagated and then grown on to become fertile adult plants producing flowers and fruit. Seeds taken from the fruits of micro-propagated plants germinated normally, and these seedlings grew on to exhibit normal flowering and fruit set. This spectacular longevity had been achieved by cryopreservation 38m underground where the present day average annual temperature is -7°C at ground level.

Dates – are they just ‘candy that grows on a tree’?

The date palm originates from Middle Eastern countries and today most of the world production comes from that region and North Africa.  It’s one of the oldest domesticated fruit trees with a history going back millennia, and is ideally suited to hot and dry arid conditions where temperatures can soar to 50°C. In these regions dates form an important part of the economy, their culture and diet, with daily consumption in some countries sufficient for it be regarded as a staple food.  There is great diversity in the species and many thousands of varieties have been named, with multiple names given in different countries and even in different regions within countries.  Much of this confusing state of affairs regarding how many distinct varieties there really are should be clarified in coming years with the recent sequencing of the genome; this will also speed up further development of plants with even better properties.

The dried and pitted black dates we all know in the West contain 70-85% simple sugars.  These are mainly fructose and glucose in roughly equal proportions and sucrose usually decreases to low levels with ripening.  As a consequence, they’re a high-calorie food, 270-330kcal/100g, very sweet and have good storage properties.  But can they be good for you at such high sugar levels?  There’s so much of it that they can easily be used as sugar substitutes in prepared foods.  For the sweet tooths amongst us, the hope would be that at least they’re not 99+% pure sucrose, and surely the other 15-30% of the fruit must have something of nutritional value in there?  The answer is positive, although nutritional features can vary more than ten-fold across varieties, even amongst those of superior commercial-grade.  Generally they have 4-11% fibre, the normal low fruit levels of protein (1-3%) and fats ( 0.2-1.5%, almost no starch and some vitamins.  More than 80 volatile chemicals have been identified that contribute to their aroma.  Consuming 100g can supply 5-7.5, 5-13, 19-25, 33 and 3-4.5% of daily recommended intakes for Fe, Mn, Mg, K and Zn resp.

Dates are one of the few plant-based foods that are an excellent source of selenium, and if grown in non-deficient soils, 100g can provide several times the daily requirement.  Most importantly, they have very good levels of healthy antioxidant phytochemicals, again varying markedly between varieties.  A 2005 US study compared the total (free plus bound) polyphenol content of dates with 5 other common fruits (apricots, cranberries, figs, green grapes and plums, all dried to <20% moisture) and found that dates were the highest at 1959 mg/100g catechin equivalents, with the others being 402, 870, 320, 551 and 788 respectively.  For these 6 dried fruits, dates were 3rd highest in fibre and 5th highest in K, Ca and Fe.  The normal sun-drying approach for marketable product causes significant loss (up to 50% and more) of antioxidants, anthocyanins and carotenoids.

The glycemic index (GI) is used to classify foods nutritionally in terms of blood glucose response following consumption.  They can be broadly divided into those called low (GI <56), medium (56-69) and high (>69).  The goal for all of us should be to try and eat slow burn foods that are lower in these categories.  This avoids the rapid and more extensive elevations in glucose and insulin from high GI foods that very quickly lead to craving unhealthy snacks between meals, over-consumption, insulin resistance, metabolic syndrome, obesity etc.  But GI is not a perfect index of carbohydrate effects for all foods in the body.  For example, whole cucumbers have such low available carbohydrate content (1.4g/100g) that to measure the GI, 3.5kg would have to be consumed, an amount so far in excess of the average serving size that the result would be pointless. Plus, such low energy foods might have other positive features such as vitamins, minerals and fibre that may outweigh the carbohydrate considerations. Then  there’s the effect of other foods which may be eaten together with the test food and this can also have major impact.  Other foods like spices and herbs are eaten in such small quantities as flavouring agents (ie < 3-5g) that a high GI is not necessarily going to be the end of the day.  Introduction of the glycemic load that combines GI with serving size was an attempt to address this quality/quantity question.  However despite these confounding issues, low GI diets have been shown in many clinical studies to be associated with reduced risk of chronic disease.

With high levels of sugars in dates it might reasonably be expected that the GI would be high and especially unsuitable for diabetics.  This question is important globally but even more so for those countries where considerable daily intake is enshrined in their national cuisine and the incidence of diabetes is high.  For example, daily consumption in Oman of predominantly dried fruits is 55-164g/d, and in the neighbouring United Arab Emirates the prevalence of diabetes is the second highest in the world, with age-standardized rates (diagnosed and undiagnosed) and prediabetes among 30-64 year olds of 29% and 24% respectively.  The following study (Nutrition Journal 2011, 10:59) found that even diabetics who have to watch their blood glucose carefully could eat the studied date varieties as they were low GI and didn’t magnify the glucose response seen in healthy individuals.

This study was designed to determine the GIs of five commonly used varieties of dates in healthy subjects and their effects on postprandial glucose excursions in individuals with type 2 diabetes mellitus.  Composition analysis was carried out for five types of dates (Tamar stage, fully ripe and dried).  The weights of the flesh of the dates equivalent to 50g of available carbohydrates were calculated.  The study subjects were thirteen healthy volunteers with a mean age of 40 years and ten participants with type 2 diabetes mellitus (controlled on lifestyle measures and/or metformin) with a mean HbA1c (glycosylated hemoglobin) of 6.6% and a mean age of 41 years.  Each subject was tested on eight separate days with 50g of glucose (on 3 occasions) and 50g equivalent of available carbohydrates from the 5 varieties of date (each on one occasion).  Capillary glucose was measured in the healthy subjects at 0, 15, 30, 45, 60, 90 and 120min and for the diabetics at 0, 30, 60, 90, 120, 150 and 180min.  The GIs were determined as ratios of the incremental areas under the response curves for the dates compared to glucose.  Statistical analyses were performed using the Mann-Whitney U test and repeated measures analysis of variance.  Mean GIs of the dates for the healthy individuals were 54, 53, 46, 49 and 55 for Fara’d, Lulu, Bo ma’an, Dabbas and Khalas, respectively.  Corresponding values for those with type 2 diabetes were very similar (46, 44, 52, 50 and 53).  There were no statistically significant differences in the GIs between the control and the diabetic groups for the five types of dates, nor were there statistically significant differences among the GIs of the dates.   The results show low GIs for the five types of dates included in the study and that their consumption by diabetic individuals does not result in significant postprandial glucose excursions.  These findings point to the potential benefits of dates for diabetic subjects when used in a healthy balanced diet.

The GI of different dates can vary considerably depending on variety, harvest time, degree of ripening and dehydration, soils, cultivation conditions, seasons, storage conditions and so on.  Some studies have reported higher GIs than that above, so the findings are not an unequivocal recommendation that all dates can be consumed with gay abandon.  Desirable features that lead to a reduction in the magnitude and duration of blood glucose and insulin response from what might be expected with such high sugar concentrations are (i) about half of these are fructose and this has very much less effect on glucose response, and (ii) the presence of considerable fibre reduces the rate and extent of absorption through reduced gut transit time and increased viscosity of the food matrix.  Both of these are nutritionally positive with habitual consumption.  However, two points should be kept in mind with dried dates.  First as a high calorie food, eating 100g can represent 10-15% of the average adult daily energy requirement, so they should only ever be eaten in moderation.  Second, like all dried fruits their sticky/chewy nature means particulate matter can persist on teeth after eating and cause dental caries if oral hygiene is poor. The overview is that dates shouldn’t be summarily dismissed as lollies growing on trees, consisting almost entirely of sugar and not much else.  They can make a valuable contribution to a healthy diet when eaten in proper balance with other plant-based foods – fruits, vegetables, nuts, seeds, cereals and legumes.