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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
-80°C for chemical analysis, or into refrigerated storage
at 4°C 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
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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 57°C 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
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