Assessments of Drying Characteristics and Physio-Organoleptic Properties of Dried Pineapple Slices under Different Pre-Treatments
The aim of this study was to assess the drying characteristics and physio-organoleptic properties of dried pineapple slices under different pretreatments. Four sliced pineapple treatments (ascorbic acid, salt solution, lemon juice, honey dip) and one control, each replicated four times were dried in an oven at a temperature of 65°C for 16 h. Statistical analysis using SPSS for drying characteristics of the treated pineapples revealed that treatment is significant at 5%. Ascorbic acid treated sample had the best degree of drying with mean moisture content of 20.01% followed by salt treated sample (20.10%), lemon juice (20.55%), control (21.12%) and then honey dip treated samples (25.03%). Also, the results revealed that the best time for drying the sugarloaf pineapple is 14 h at a temperature of 65°C. The sensory analysis for the most preferred dried pineapples in terms of taste, color and flavor revealed that honey treated sample was most preferred, followed by ascorbic acid, lemon juice/control and salt treated samples.
Pineapple production is widespread in Ghana and is grown largely in the Central
and Western regions of Ghana. In Ghana, tropical fruits produced during bumper
harvest are either consumed fresh, sold at relatively cheap prices, or are allowed
to go waste due to inadequate processing facilities. It is estimated that out
of the achievable yield of 32000 to 40000 kg acre-1 of pineapple
produced, 25% goes to waste (Yeboah and Kunze, 2004).
Post-harvest losses are a major challenge for pineapple producers especially
in Ghana. Usually, a fully matured harvested pineapple takes about seven days
to deteriorate. Much of the pineapple fresh fruits go to the industry for slices
and juice production, while about five million tonnes are exported for the fresh
fruit market (Smart and Simmonds, 1995). Little attention
is given to drying of tropical fruits for consumption and storage, even though
enormous market exists for dried fruits in Ghana. Drying remains the oldest
and easiest method of preservation, but food drying in Ghana is typically for
fish, cereals, grains and some vegetables. Little knowledge exists for drying
of tropical fruits in Ghana. Drying methods include oven drying, solar drying
and dehydrators, which are used for removing water or moisture from a food product.
Removing moisture from foods makes them lighter and smaller and hence takes
much less storage space than canned or frozen foods (Andress
and Harrison, 2006). Food scientists have found that by reducing the moisture
content of food to between 10 and 20%, bacteria, yeast, mold and enzymes are
all prevented from spoiling it. The flavor and most of the nutritional value
is preserved and concentrated (Dennis,
1999). For successful drying, heat, dry air and air movement are needed.
The temperature should be high enough (about 60°C) to force out moisture,
but not too high to cook the food. The dry air is needed to absorb and release
moisture and air movement to carry the moisture away. When drying foods, the
key activity is to remove moisture as quickly as possible at a temperature that
does not seriously affect the flavor, texture and color of the food. If the
temperature is too low in the beginning, microorganisms may grow before the
food is adequately dried. If the temperature is too high and the humidity is
too low, the food may harden on the surface. Temperature ranges between 37 and
71°C will effectively kill bacteria and inactivate enzymes, although temperatures
around 43°C are recommended for solar dryers (Kendall
and Allen, 1998). Rapid dehydration is desirable. The higher the temperature
and the lower the humidity, the more rapid the rate of dehydration will be.
Air drying of fruits especially pineapple has been reported to be limiting in
some factors especially the quality of the dried fruits (McMinn
and Magee, 1997). Experience with drying of tropical fruits has showed that
sole dependence on air/sun drying leads to great deterioration of the fruits
to be dried (Abano
and Sam-Amoah, 2007). McMinn and Magee (1997) reported
that quality deterioration can be categorized as nutritional, physical and chemical
in nature. The removal of moisture during drying is affected by the nutritional,
physical and chemical changes on the dried products (Karim,
2005). Browning and disruption of tissues of dried fruits owing to presence
of polyphenols are the physical characteristics and one of the major set backs
in fruit drying (Karim,
1983). Other physical changes affecting dried fruit quality are shrinkage
of the cells, loss of redrying ability, wettability and case hardening (Dalgleish
and Andy, 1988; McMinn and Magee, 1997). Dried foods
are tasty, nutritious, lightweight, easy-to-prepare and easy-to-store and use.
The energy input is less than what is needed to freeze or can.
Pre-treatment of fruits prior to drying is highly recommended because it helps
keep light-colored fruits from darkening during drying and storage and it speeds
the drying of fruits with tough skins, such as grapes and cherries. Pineapples
and apples are peeled, cored and sliced. Fruits should be cut into uniform pieces
or slices of not more than 10 mm thick so that it will dry more evenly. Thin
pieces dry faster than thick ones (Troftgruben, 1977).
Different pre-treatment methods have been developed for fruit drying, among
which are lemon juice, salt solution, honey dip, ascorbic acid, sulfuring, osmotic
pretreatment and blanching (Karim,
2005). If no pre-treatment is done, the fruits will continue to darken after
they are dried. For long-term storage of dried fruits, sulfuring or using a
sulfite dip are the best pre-treatments. However, sulfites may cause asthmatic
reactions in a small portion of the asthmatic population (Susan
and Williams, 1993). Large differences are found on the reported data on
the nutritive value of dried fruits and are reported to be due to wide variations
in the preparation procedures, drying temperature and time and the storage conditions
and Kaushal, 1990). Dried fruits are a good source of energy because they
contain concentrated fruit sugars. Dried foods are high in fibre and carbohydrates
and low in fat, making them healthy food choices. The aim of this work was to
assess dried pineapple slices under different pre-treatments. Specifically,
the work aimed to investigate the significance of pre-treatments on rate of
drying, assess the physio-organoleptic properties of the dried pineapples and
the overall acceptability of the dried pineapples. In addition, the work was
expected to provide an alternative way of processing pineapple for human consumption,
curtail postharvest loss of fresh pineapples and promote entrepreneurial opportunities
through the sale of dried pineapples in Ghana.
MATERIALS AND METHODS
Fresh ripe pineapple fruits of the sugar loaf variety with a mean moisture content of 86% were sourced from Ataabadze, a commercial pineapple community in the Central region of Ghana for the research in February 2009. The pineapples were brought to the laboratory, washed thoroughly, peeled and all core removed. It is then sliced into 6.5 mm thickness and the core removed prior to pre-treatment. The Completely Randomized Design (CRD) was used for the experiment with four treatments, one control, each replicated four times. Three thousand milligram (30 tablets) of ascorbic acid was dissolved in 660 mL of water in a bowl. Also, 7.25 g of salt was dissolved in 660 mL of water in another bowl. In addition, 255 mL of lemon juice was mixed with 255 mL of water in a third bowl and finally 40 mL honey and half cup sugar was measured and added to a 495 mL lukewarm water to mix.
Six hundred gram each of the sliced pineapples was soaked in the solution for
4, 15, 10 and 4 min, respectively and allowed to drain according to recommended
guidelines. Each of the treated samples was divided into four equal weights
of 150 g for drying. Another 600 g of the sliced pineapples was divided into
four and used as the control. The pre-treated pineapples were put on a brown
paper and placed onto the racks in the Gallenkamp (Sanyo OMT oven) cabinet dryer.
The samples were dried at a temperature of 65°C for 16 h. The weights of
the drying pineapples were determined every 2 h until drying was complete. When
drying was completed the samples were allowed to cool and packed into a transparent
tightly zipped bag and labeled for analysis and storage. A 30-member semi-trained
panel evaluated the dried products for overall quality (taste, flavor color).
Panelist were asked to evaluate the dried pineapple slices employing a structured
hedonic test (scale from 1 = poor to 5 = excellent) (Larmond,
1977). The data was analysed using Analysis of Variance (ANOVA) and where
significant differences were observed, Duncans Multiple Range Test (DMRT)
was used to separate the means.
RESULTS AND DISCUSSION
Figure 1 shows the drying characteristic curves of the various
pretreated pineapples used for this study. Table 1 shows the
mean moisture content of the treated pineapple slices as a function of drying
time. The drying characteristics curves show that though drying of the pineapple
started at the same moisture content and time, there was a sharper drop in moisture
of the ascorbic acid and salt treated pineapples than the others in the first
2 h. There was a significant difference (p = 0.05) between ascorbic acid and
salt solution treated pineapple in the first 2 h of drying.
|| Drying characteristic curves of different pretreated dried
|| Mean moisture content (%) of treated pineapple slices as
a function of drying time
|Means in a column bearing the same letters are not significantly
different at p = 0.05
Lemon, honey dip and the control lost moisture steadily with no significant
difference in moisture content in the first 2 h. However, after 6 h of drying
there was significant difference between the mean moisture loss of ascorbic
acid, salt and lemon juice solution treated samples.
A Similar trend occurred after 8 h of drying. The honey dip treated pineapples
and Control recorded no significant difference (p = 0.05) in mean moisture loss
in the first 10 h of drying. However, there was a significant difference (p
= 0.05) between 12 and 16 h of drying. At 16 h of drying, there was no significant
difference (p = 0.05) between the ascorbic, salt, lemon juice and the control
products. Hence, it is likely that when drying time is increased at constant
temperature, there may not be any significant differences in moisture content
of the ascorbic, salt, lemon juice and the control samples. Conversely, there
may be significant difference in the honey treated sample. However, honey treated
samples showed significant difference (p = 0.05). The degree of drying after
16 h indicated a moisture content in the increasing trend from ascorbic acid
treated products, followed by salt solution treated pineapples, lemon juice,
control, to honey treated pineapples. Abano and Sam-Amoah
(2007) observed similar trend in dried mangoes chips under salt, acetic
acid and sugar pre-treatments. The drop in ascorbic acid treated product could
be due to the leaching effect of acid in the ascorbic acid solution which caused
the water content in the pineapple to lose more moisture at a faster rate in
the early hours of drying.
The evaporation rate of salt solution could also account for its sharp fall
in the first 4 h. Common salt made of sodium chloride in an acidic and base
reaction on heating evaporates rapidly. This may have resulted in the relatively
high loss of moisture in the samples pre-treated with the salt solution. The
delay in drying for honey might be due to its hygroscopic nature, which when
exposed to air naturally forms a coat that impedes moisture loss. It is also
possible that the delay in the honey treated sample could be due to the increased
sugar to acid ratio. Sugar when heated forms a gelatinous coat around the pineapples,
which is impermeable to moisture loss. This sticky coat also prevented moisture
loss from the products treated with honey. Sugar also becomes more viscous when
melted and this viscous fluid expanded to fill the pore spaces in the pineapple
slices on heating, which prolonged the rate of moisture removal from samples
pre-treated with honey dip. Since samples pretreated with honey exhibited a
viscous rather than an elastic behavior, it indicates that infusion of sugars
caused plasticity of the pineapple slices. The degree of increased sugar impregnation
by the samples pre-treated with honey dip also might cause the delay in drying.
These results fully agree with those reported by Panagiotou
et al. (1998) for osmotic dehydration of banana, apple and kiwi pieces
in sucrose solution. It was observed that ascorbic acid treated samples had
the highest rate of drying in terms of moisture content at 20.01%, hence had
the highest reduction of moisture after drying was completed. Moisture content
was highest in honey treated dried pineapples (25.03%), followed by the control
(21.12%), lemon juice (20.55%), salt solution (20.10%) and then ascorbic acid
treated products (20.01%). The mean final moisture content of all the products
|| Hedonic test of dried pineapple slices under different pre-treatments
|Five point scale: 1: Poor, 2: Satisfactory, 3: Good, 4: Very
Good, 5: Excellent
|| Overall preference of the dried pineapple
A sensorial analysis was done to determine the quality of the dried pineapples
in terms of taste, flavor and color. On the basis of taste, statistical analysis
of the hedonic test showed that honey dip treated pineapples scored the highest
(3.97±1.066), followed by ascorbic acid (3.17±0.699), lemon juice
(2.80±0.887), control (2.70±0.794) and then salt solution treated
pineapples (2.47±1.008). The taste scores by hedonic test indicated that
honey and ascorbic treated dried product ranged from good to very good whereas
the rest ranged from average to good. Similarly, in terms of the flavor of the
dried product, the same decreasing trend was recorded. Honey dip treated dried
pineapple slices scored the highest (3.47±0.973), followed by ascorbic
acid (3.23±0.858), lemon juice (2.90±0.759), control (2.80±0.961)
and salt solution (2.60±1.003) as shown in Table 2.
However, there was a different trend of decreasing score for color, from control
(3.77±1.382), honey dip treated dried products (3.70±1.291), lemon
juice (3.57±1.455), salt solution (3.30±1.149) to ascorbic acid
(3.17±1.289) as shown in Table 2. Overall, consumers
had the highest preference for honey dip treated dried pineapples (50%), followed
by ascorbic acid treated products (33%). Lemon juice treated product and the
control had the same percentage consumer preference (7%) whilst salt treated
product scored the lowest (3%) as shown in Fig. 2. This overall
high preference trend for honey treated products could be attributable to the
fact that honey contains about 75-80% sugar, which resulted in the sweet taste
and subsequent preference by panelist. Also, the preference of panelist for
ascorbic acid treated products could be due to the taste of vitamin C tablet,
which positively influenced the original taste of the pineapple slices. The
taste of lemon treated products was influenced by the sour nature of the lemon
juice, which affected the taste of the pineapple slices negatively. The taste
of salt treated products, which was least preferred, might be due to the chemical
reaction of the salt solution with the sugary nature of the pineapple slices.
Unsurprisingly, many individuals dislike such combination of salt and sugar.
Pineapple contains sucrose, fructose and glucose in concentrations, which make
it tastes good, which in combination with acids and other compounds, determine
the typical flavor of the fruit (Gherardi et al.,
1994). Conversely, the position of the control in the hedonic test scale
indicates that pre-treatments had tremendous influence on the taste of the dried
products. More so, it was revealed in Table 2 that the flavor
of the dried products had an influence on taste. This is because salt, which
scored least in taste by the hedonic test also, had the least score for flavor
preference. There was however no direct correlation between the ranking for
color and flavor as shown by the hedonic test. Nevertheless, the color of the
dried pineapples should not be taken for granted. Nutritionally, the taste of
dried foods is related by their flavor and texture.
The texture of the dried pineapples was described as rough. It was tender and
quite chewy and generally soft. Changes in the texture of dried foods are important
cause of quality deterioration. It has been explained that loss of texture is
caused by gelatinization of starch, crystallization of cellulose and localized
variations in the moisture content during drying, which set up internal stresses
leading to rupture (Fellows,
1998). The rupture compresses and permanently distorts the relatively rigid
cells, to give food a shrunken shriveled appearance. This may be responsible
for the variation in the quality of texture and the collapsed cell structure
of the dried pineapples. The dried pineapples had shrunk cells with dense tissue.
On hydration, the samples absorbed water more slowly and did not regain the
firm texture associated with the fresh pineapple (Maskan,
2001). It has been established that heat not only vaporize water during
drying but also causes loss of volatile components from the food.
This study focused on the drying characteristics and physio-organoleptic properties of dried pineapple slices under different pretreatment. It is evidently clear from the study that drying of pineapples is possible and has tremendous prospects of reducing postharvest losses. It is important to note that pretreatment, drying time and temperature have great influence on the general outlook and acceptability of the dried pineapples. At a drying temperature of 65°C for 16 h, honey treated products had the highest score (50%) for general acceptability, followed by ascorbic treated product with a score of 33%, then lemon and control with a score of 7%, with salt treated products scoring the least of 3%. All the treated dried products have stored five months now without any sign of deterioration. The best drying time for pineapples of the sugar loaf variety is 14 h at a temperature of 65°C.
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