"The Green Desert"
DATES FRUIT < DATE PALM  <
OMAN A GRICULTURE
DATES: A Fruit of Promise for the Food Industry

Stefan Kasapis, Ph.D.

Associate Professor, Department of Food Science & Nutrition, College of Agriculture, Sultan Qaboos University. He is the author of over fifty publications in reputable journals in Food and Biological Sciences.

Date production in the world is only confined to a small number of countries, most of them being the Arab countries. However, the date industry in the Arab world is not yet fully developed and concerted efforts are still needed to fully utilize the tremendous potential of date substances as ingredients in processed foods for export and the local market. Date pectin, dietary fibre and syrup are some of the date substances which can find a plethora of applications as a thickener or gelling agent in processed foods, i.e., confectionery products, jams, table jellies, soft cheeses, yoghurts, etc. But date products such as these should be developed on a sound technological basis which requires adequate characterisation of the chemical composition and textural properties of dates. The College of Agriculture, SQU, has implemented a thorough study on the structure-function relationships of date ingredients that yielded, for the fist time, the so-called 'state diagram' of the fruit. The article elaborates on the study and the benefits it could bring to the dates industry and the market for date-product.

Date production in the world is only confined to a small number of countries, most of them being the Arab countries. However, the date industry in the Arab world is not yet fully developed and concerted efforts are still needed to fully utilize the tremendous potential of date substances as ingredients in processed foods for export and the local market. Date pectin, dietary fibre and syrup are some of the date substances which can find a plethora of applications as a thickener or gelling agent in processed foods, i.e., confectionery products, jams, table jellies, soft cheeses, yoghurts, etc. But date products such as these should be developed on a sound technological basis, which requires adequate characterisation of the chemical composition and textural properties of dates. The College of Agriculture, SQU, has implemented a thorough study on the structure-function relationships of date ingredients that yielded, for the fist time, the so-called 'state diagram' of the fruit. The article elaborates on the study and the benefits it could bring to the dates industry and the market for date-product.

Date Palm Cultivation in Oman

The importance of dates cultivation in Oman can not be underestimated. Across the country almost seventy five percent of the area allocated to tree crops is monopolized by the date palm, Phoenix dactylifera. 

Examples of red and yellow dates at various stages of ripeness.

Current estimates of the number of trees in the country approach seven million with the maximum fruit yield reaching 40 kg per palm. To date Khalas remains the cultivar of the highest quality but experimental introduction of new cultivars along with easier production practices are poised to increase the extent of large scale farming in the country. However, the size of cultivated areas is limited by the availability of water and traditional small-scale farms are irrigated by falaj water. Fertilization takes place manually by placing branches cut from the male tree among the sprays of the female tree. This can be an exhausting process and, alternatively, pollen can be mixed with wheat flour and frozen until the required time of fertilization, which is administered by a hand held spray. Ripe dates are collected by the farmer using a long rope along the tree, which allows him to climb to the top.Today Omani standards are in place in relation to handling following harvesting, pressing and packaging of the fruit. Collection centers act as intermediaries between the farmers in the production areas and processing plants. Both units are adequately equipped with cold and dry stores and transportation facilities. Furthermore, the government has intensified efforts to educate growers on the code of practice for improved quality control and distribution procedures which should reduce the cases of blemished dates encountered during post-harvest. 

Developmental Stages of Dates in Relation to Nutritional Properties

A good quality date drupe is a delicious fruit with a sweet taste and a fleshy mouthfeel. This is a high-energy food containing sugars and fibre thus being suitable for both people and livestock. To come to this state, the fruit pass through several separate stages of maturity, traditionally described by changes in colour, texture and taste/flavour. Green dates (Arabic kimri) contain maximum moisture and are firm in texture. At the second stage (Arabic khalal), dates begin to lose moisture and in parallel accumulate considerable quantities of sugar. In the third stage (Arabic rutab), loss of moisture is accelerated and the fruit becomes softer in texture. In the final mature stage (Arabic tamar), the fruit contain the least amount of moisture and maintain a soft texture and a sweet taste. In line with the dietary requirements of the modern consumer, dates are a good fiber provider (about 6.5%), contain brown sugar (70%) and they have a negligible fat content. Dietary fibre mainly consists of polysaccharides like cellulose and pectin, and insoluble proteins. The digestive process of humans is unable to metabolize fiber, which is excreted taking up malignant tumors. In mature dates, sucrose converts into invert sugar, which is a mixture of glucose and fructose. Sugars are in unrefined form and stock up healthy fibre, vitamins and minerals in the fruit. The high levels of sugar bind moisture effectively thus preserving the fruit by preventing bacterial growth. In addition, dates contain seven vitamins and eleven minerals whose importance as a dietary supplement was appreciated by the desert people who, for thousands of years, ate dates with goat or camel milk as a complete sustenance. Finally dates have tannins which are made mainly of polyphenols and in lesser amounts of flavone. These are responsible for the dark colour of dates in the post-harvest period. Relating Chemical Composition to Physical Properties of Dates Previous studies relating to physical properties of date palm composition have led to inconclusive results. They did not relate changes to the separate stages of maturity, nor did they investigate how these changes influence the textural properties and taste of dates at these separate stages. To fill this gap, the College of Agriculture, SQU, undertook a detailed study using sensory and texture profile analysis to evaluate the attributes of firmness and astringency in maturing dates, and relate them to chemical composition.

What is the Origin of Texture Variation in Dates?

From late May to late August 1997, dates of the khalas variety, were harvested at weekly intervals from the Experimental Station of the University. The fruit were immediately packaged in strong polyethylene bags and placed into frozen storage at -80C for chemical analysis, or into refrigerated storage at 4C for textural and sensory analysis. To look at texture, samples of fresh dates were cut in a horizontal direction, destoned and the firmness was measured using the Instron Rheometer. Samples were punctured in the horizontal direction using a cylindrical plunger until they were fractured. As shown in Figure 1, there is a dramatic drop in the values of firmness as the dates matured from about 190 to 30 x 104 Pa at days 107 and 170 respectively, following pollination. 

Figure 1

Degree of esterification (،) and firmness (D) of maturing Khalas dates

To rationalise the softening of the fruit, we examined the chemical nature of the pectin polysaccharide, which constitutes the main gelling agent of date materials. Pectin is a galacturonic acid whose esterified form with methyl groups can form a gel network assisted by the presence of high levels of sugar. These conditions, of course, are met in dates, which contain up to 70% unrefined sugars. The methyl ester content of pectin was determined by standard chemical analysis and results are reproduced in Figure1. A definite decrease in the degree of methyl esterification (DME) was observed as the dates matured. 

Sensory Evaluation of Dates at Progressive Levels of Maturity 

There is a strong positive relationship between the sweetness of dates and the amount of sugar, which increases gradually following pollination. However, this is not the only sensory attribute that changes during maturation of the fruit. There is a sensory experience in the oral cavity that includes drying sensations, and roughing or tightening of the oral tissues. To quantify this phenomenon, representative samples of dates at each weekly interval were removed from the polyethylene bags and brought to room temperature. The date pits were removed by slicing each date into two halves and the fruit flesh retained. Taste panelists were trained by informally evaluating samples of green dates for high astringency and of mature dates for low astringency. After training, astringency was rated on a six-point scale from 'not astringent' to 'extremely astringent'. The mean scores of the astringency of dates are presented in Figure 2. The green dates (about 124 days after pollination - kimri) were significantly more astringent (3.8) than the scores observed at the mature stage (1.8 at 167 days after pollination - rutab). Dates contain a layer of tannin a little below the skin of the drupe which is associated with the sensation of astringency. Gratifyingly, the total tannin content follows closely the astringency scores in Figure 2 thus identifying the chemical origin responsible for the reduction in astringency of dates.

 

Total tannin content on a dry weight basis (،) and astringency (D) of maturing Khalas dates.

Using the State Diagram of Dates for Product Development

In the preceding discussion, the chemical composition of dates was related to textural properties and mouthfeel at various stages of maturity. The knowledge generated should be used to develop food recipes, since appearance, taste, texture and, generally, all sensory related properties of food products are what determine their appeal. This is common practice within the framework of modern food processing, with manufacturers trying novel and versatile ingredients to help them meet the ever-increasing consumer expectations. We feel that date ingredients are a unique group, which can improve a wide range of food characteristics both in terms of the desired sensory and shelf-life related properties. This can be demonstrated with the construction of a state diagram. In its simplest form, the state diagram of dates represents the pattern of change in the physical properties of the material as a function of increasing levels of solids. This type of data gathering is depicted in Figure 3. Curve AP demarcates the freezing temperatures below which crystallisation is observed in the cooling processes of date products. Curve DEH defines the glass transition temperatures which can occur at high temperatures in concentrated formulations; for example, Tg' is about 57C for a solids content of 100%.Understanding the state diagram provides vital clues to the liquid-like, rubbery, crystalline and glassy consistency of date-foods. 

  State diagram of temperature versus percentage solids obtained from mature dates incorporating the curve of the glass transition temperature () and the freezing curve (s).

Thus at concentrations and processing temperatures above those demarcated by the curve AP, clarified syrups at various levels of thickness can be made for use in relishes, pan cakes and gourmet dishes. Unclarified syrup contains date pectin whose structuring ability allows use of the mixture in biscuits, cakes, table jellies, jams and low-fat processed cheeses. At subzero temperatures, partial crystallisation of glucose and fructose will take place thus making the date pulp suitable for use in sugar-coated breakfast cereals and hard candies. At a solids content above 70% (curve DE), the glassy consistency should be used in the making of ice creams, gummy bears and non-crystalline confectionery products like fudges. Finally, the front-page illustration is a date and orange cake with icing which is worth making and, indeed, it is made with considerable success by our undergraduate students as part of their Food Product Development Course at the College of Agriculture. 

Epilogue

Date palm cultivation is very important in Oman with the commercial product coming into two forms, fresh and dry. Although the local market is the most important buying outlet, exports have been rising and the food industry appreciates the enormous potential of the export sector. To fulfill this expectation, it is vital that date compounds are found as functional ingredients in processed foodstuffs. A start has been made by relating chemical composition to textural and sensory attributes of maturing dates, and by constructing a state diagram. The latter has important applications in determining areas of use as well as product stability during storage. The results, once distributed to the food industry, will forge a strong link between the College and the industry in an effort to develop appealing food products that can contribute to the Omani wealth.

Acknowledgements

The author is grateful to Ahmed Al-Alawi, Mansoura Al-Amri and Insaaf Al-Marhoobi for technical support, and to Drs Nejib Guizani and Shafiur Rahman for stimulating discussions

©Adapted from SQU Miraat AlJamea.