Dates - Tropical and Subtropical Fruits: Postharvest Physiology, Processing and Packaging - page 175

(Navarro, 2006). Since water activity ( a w ) reflects the avail-
ability of water for metabolic processes under the influ-
ence of different levels of sugar content, a w (orERH)isa
more useful parameter than moisture content, particularly
in dates. Therefore the most accurate criterion for deciding
on the storability of dates is a w (Navarro, 2006). Saleem et
al. (1997) reported that maintenance of a w in the range of
0.6-0.65 a w was needed for extending storage of 'Dhakki'
dates with better quality and maximum chemical stability.
In high-sugar foods below 0.7 a w , mold growth would be
inhibited. For most date varieties at a w of 0.7, the EMC
would be at 24% moisture (Barreveld, 1993).
Water sorption isotherms are very useful for optimiz-
ing storage, drying, and packaging conditions for different
foods, including dates (Myhara et al., 1998). Al-Kechaou
et al. (2007) developed a desorption isotherms of Tunisia
'Deglet Nour' dates between 30 ◦ and 60 ◦ C. The experi-
mental procedure was a gravimetric dynamic method with
discontinuous registration of weight changes. The results
were adjusted by the Guggenheim, Anderson, and de Boer
(GAB) model, with temperature found to be a dominant
factor with respect to moisture sorption phenomena. This
is demonstrated in Fig. 10.2, where an increase in temper-
ature induces a reduction in the EMC at a specific a w . This
behavior is consistent with the thermodynamics of sorption
and is indicative of the impaired hygroscopic nature of the
material (Rizvi, 1986). They display a concurrent increase
in EMC with increasing a w .
The desorption isotherms of 'Deglet Nour' date give
the S -shaped characteristic curve typical of many sorption
isotherms of food. The (GAB) equation is suitable for rep-
resenting the relationship between water activity and EMC
of date in the a w range of 0-0.9, and hence it may be applied
for process development (Al-Kechaou et al., 2007).

Color darkening in dates
Dates possess a high amount of sugars and a small amount
of amino acids and also contain a noticeable amount of
tannin polyphenolic compounds (Sawaya et al., 1982). Ac-
cording to Maier and Schiller (1960), enzymatic browning
of polyphenols proceeds at observable rates at room temper-
ature, whereas at temperatures above 38 ◦ C, sugar browning
predominates. However, the deteriorative changes became
minimal on storage under the nitrogen. Maier and Schiller
(1960) reported that the darkening at 49 ◦ C was caused pri-
marily by nonoxidative and nonenzymatic reactions. How-
ever, Maier and Schiller (1960) made both oxidative and
nonoxidative deteriorative reactions responsible for dark-
ening in 'Deglet Nour' at 38 ◦ C. Maier and Metzler (1965)
have reported the similar results thus making insoluble tan-
nins responsible for nonenzymatic oxidative browning in
dates. Mohsen et al. (2003) noted that vacuum packaging
was a useful technique for reducing darkening of the date
for long-term storage.

POSTHARVEST TREATMENTS

Date dehydration and rehydration
Dehydration is an important stage of date processing. Dry-
ing condition such as drying temperature, relative humidity
and drying time will affect color, flavor, shriveling, sepa-
ration of skin and flesh, and overall acceptability. Conven-
tional drying requires high temperatures and long times and
final products are characterized by low porosity and high
apparent density values. Vacuum-dried materials are char-
acterized by better quality retention of nutrients and volatile
aroma. However, the cost of the process is high (Tsami
et al., 1998). Microwave oven results in a rapid and uniform
drying process (Schiffmann, 1995). Drying temperature of
54 ◦ C at relative humidity of 50% is recommended for dry-
ing soft dates. Temperature of 55 ◦ -65 ◦ C for drying of soft
dates is practiced (Barreveld, 1993). Drying time depends
on initial moistures of the product. Temperature of 39 ◦ C
for 12 and 16 hours was recommended by Fasihian (1986)
for drying of 'Shahani' and 'Kabkab' date varieties, respec-
tively. Kulkarni et al. (2008) evaluated the processing and
dehydration conditions for the preparation of dehydrated
dates from immature date fruits ( Khalal stage). Process-
ing of dates was done by blanching in water at 96 ◦ Cand
subsequent dehydration at 60 ◦ C for 18-20 hours resulted
in good-quality dehydrated dates as compared to the dates
dried without heat treatment.
Artificial maturation due to the softening of texture and
the equilibration of sugars was achieved using a posthar-
vest hydrothermal treatment of date palm fruit to enhance
the quality and consumer acceptance of 'Deglet Nour' hard

T=30 ° C

0.4

T=40 ° C

0.3

T=50 ° C

0.2

T=60 ° C

0.1

0

0 0.2 0.4

Water activity

0.6 0.8 1

Figure 10.2. Desorption isotherms in the date-water
system (source: Al-Kechaou et al., 2007).