Article Type: Research Article Article Citation: Regiane Ogliari, Jaqueline Machado Soares,
Flávia Teixeira, Kélin Schwarz, Kátia Aparecida da Silva, Dalton Luiz
Schiessel, and Daiana Novello. (2020). CHEMICAL, NUTRITIONAL AND SENSORY
CHARACTERIZATION OF SWEET POTATO SUBMITTED TO DIFFERENT COOKING METHODS. International
Journal of Research -GRANTHAALAYAH, 8(10), 147-156. https://doi.org/10.29121/granthaalayah.v8.i10.2020.1881 Received Date: 01 October 2020 Accepted Date: 28 October 2020 Keywords: Tuber Nutritional Value Nutrients The objective was to evaluate different cooking methods effect on chemical, nutritional and sensory characteristics of sweet potatoes. Samples were subjected to four cooking methods: boiled in water, fried, microwaved and baked. In general, pH, Titratable Acidity and Soluble Solids contents were altered by cooking methods. Reducing, Non-Reducing and Total Sugars levels increased after cooking, regardless of the method used. Levels of red and yellow were lower in tubers after the methods of boiled and baked, while the light reduced in all cooking processes. Cooking by boiled and fried increased Total Carotenoid content, reducing it in other methods. Except for boiled, all other processes increased Phenolic Compounds content. Ascorbic Acid content increased in all cooking processes, being higher for fried. Sweet potatoes cooked by boiled had higher moisture content and lower protein, lipid, carbohydrate and energy, while fried was the process that most elevated the content of these nutrients. Sensory scores were higher for sweet potatoes cooked in fried method. It is concluded that the processes of baked, boiled, fried and cooking in microwave alter chemical and nutritional characteristics of sweet potatoes. The tuber submitted to fried has greater sensory acceptability, however it is the least suitable for consumption due to high levels of fat and energy.
1. INTRODUCTIONSweet potato (Ipomoea potatoes L.) belong
to family Convolvulaceae [1] and are considered one of the main food crops in the
world. The tuber is grown in tropical and subtropical regions, particularly in
Asia, Africa, and Pacific. Asia and Africa are responsible for 95% of all world
production [2]. Sweet potato trade occurs throughout all year, with the
harvest taking place mainly in winter beginning [3]. The tuber is the 5th most produced food in
the world, reaching a production of 106,601,602 tons/year. In Brazil, its
production reaches 525,814 tons/year, thus occupying the 17th place
as the most cultivated temporary crop [4]. Sweet potatoes are a basic crop in developing
countries, as they have high yield potential, resistance, low cultivation
requirements and high economic potential [5], [6]. The skin and pulp of the tuber may have different
colors, which vary from pure white to intense purple [7]. Due to its favorable nutritional profile, sweet
potatoes have been the focus research in recent years [8]. It has high levels of carbohydrate, dietary fiber,
vitamins, and minerals (K, P, Ca, Mg, Fe, Mn, Cu). In addition, there is
bioactive compounds presence, such as anthocyanins and phenolic acids, which
contribute to the skin and pulp color. Also, high concentrations of vitamin C
(6-10 mg 100 g-1) and β-carotene (273-400 μg 100 g-1)
are observed, especially in orange-fleshed sweet potatoes [6], [8]. In human health, studies show that the tuber can have
an antioxidant, hepatoprotective, anti-inflammatory, anti-tumor, antimicrobial
and anti-aging effect, reducing risk of developing diabetes mellitus and
obesity [8]. Sweet potatoes are usually cooked before
consumption. The most used methods are fried, boiled, dehydration, baked,
steaming, and cooking by microwaved [9], [10], [11]. Although cooking improves sensory, nutritional,
digestibility and bioavailability product aspects, and cooking can promote some
nutrients losses. Tian et al. [12] demonstrated that sweet potatoes submitted to fried
showed reduction in levels of vitamin C (83.4%) and carotenoids (75.7%), when
compared to raw tubers. Baked methods (71.6% and 52%, respectively), boiled
(40.8% and 20.1%, respectively) and microwaved cooking (7.5% and 66.3%,
respectively) caused less losses of these compounds. In this context, the
objective of this research was to evaluate the effect of different cooking
methods on chemical, nutritional and sensory characteristics in sweet potatoes
submitted to different cooking methods. 2. MATERIALS AND METHODSVegetable Material Sweet potato samples were acquired in a
supermarket located in Guarapuava, Paraná, Brazil. It was used 15 kg of sweet
potato with purple skin and cream pulp, with a better visual aspect. Those with
defects and/or that had a hugely different size and appearance were excluded. Cooking Methods The samples were washed in running water, with
subsequent sanitization in sodium hypochlorite solution (150 ppm) for 15
minutes and bark and tips were discarded. The sweet potatoes were cut in the
form of a toothpick of approximately 1 cm x 1 cm, in which 1 kg of raw tuber
was reserved for the analysis of the chemical and nutritional compositions. The sweet potato remainder was separated into 4
groups, being subjected to four different cooking methods: a) boiled in water,
3.5 kg were immersed in 4 L of boiling water (100 ºC) and cooked for about 20
minutes ; b) fried, 3.5 kg were immersed in 2.7 L of soybean oil (180 ºC) and
cooked for about 10 minutes; c) microwaved, 3.5 kg were distributed in glass
bowls and cooked at power 8 for about 10 minutes and; d) baked, 3.5 kg were
distributed in an aluminum pan and baked in a conventional oven (180 °C) for 40
minutes. In all methods the sweet potato was cooked until the material did not
show resistance to perforation. Chemical and Nutritional Composition The raw and cooked sweet potatoes were dried in
dehydrator (Pardal®, Brazil) with air circulation (65 °C) for 48
hours. After drying, they remained at room temperature (22 ºC) until completely
cooled. Samples were crushed in domestic blender (Mondial®, Brazil)
and stored at - 20 ºC in aluminum containers until evaluation. The following chemical evaluations were carried
out: pH, measured using a bench pH meter (Tecnopon®, MPA-210 model,
Brazil); Soluble Solids (SS), obtained by reading directly on an ABBE bench
refractometer (Bel®, model RMI/RMT, Brazil) [13]. The values were expressed in °Brix; Titratable Acidity
(TA), using the titrometric method [13] and the results expressed in % citric acid; SS and TA
ratio, which was obtained by dividing the values of SS and TA; Reducing Sugars
(RS), Non-Reducing Sugars (NRS) and Total Sugars (TS), were evaluated using the
Lane-Eynon reductometric method [13]. The results are expressed in g 100 g-1;
Instrumental color, evaluated in quintuplicate on the surface of raw and cooked
peeled sweet potatoes. The Comission
Internatinale de E'clairage (CIE) L* (lightness), a* (red-green) and b* (yellow-blue) system
was used, with a colorimeter reading (Konica Minolta®, model Chroma
Meter CR 4400, Japan), D65 illuminant and 10º angle. To determine nutritional composition, moisture
content (g 100 g-1) was evaluated; ash (g 100 g -1);
protein (g 100 g-1) [13]; lipid (g 100 g-1) [14]; carbohydrate (g 100 g-1) by difference (%
Carbohydrate = 100 - (% moisture + % protein + % lipid + % ash + % fibers) and;
total energy value (kcal 100 g-1), using the values recommended by
Merrill and Watt [15] for lipid (9 kcal g-1), protein (4 kcal g-1)
and carbohydrate (4 kcal g-1). Ascorbic acid (Vitamin C) content was
determined by titrometric method with 2.6 dichlorophenolindophenol [13];, modified by Benassi and Antunes [16] and results were expressed in mg 100 g-1. Total carotenoids (µg g-1) were
obtained by spectrophotometric analysis (Agillent Technologies®,
model Cary 60 UV, Malaysia) at 450 nm [17]. Phenolic compounds were measured by Follin-Ciocauteau
spectrophotometric method [18], The reading was performed in spectrophotometer
(Agillent Technologies®, model Cary 60 UV, Malaysia) at 765 nm, the
results being expressed in mg of gallic acid equivalent (GAE) 100 g-1.
Antiradical activity was evaluated using ABTS method (2,2-azinobis- [3-ethyl-benzothiazolin-6-sulfonic
acid]), in both versions (hydrophilic and lipophilic) [19] and results were expressed in mmol of equivalents to the
Trolox per gram of sample. Sensory Evaluation For the sensory test, sweet potatoes were cooked
as described above. All samples were evaluated using an acceptance test
applying a hedonic scale of nine points, with extremes ranging from very much disliked (1) to very much liked (9) [20]. Attributes related to appearance, aroma, taste, texture
and overall
acceptance were evaluated. Acceptability index (AI) was
calculated by multiplying the average score informed by consumers to product by
100, dividing result by maximum score given to product within the hedonic scale
of 9.0 points. Sixty tree untrained judges and sweet potatoes
usual consumers have participated in sensory analyzes. Consumers were aged
between 18 and 60 years old and were recruited among students and employees at
the State University of the Midwest, Guarapuava, Paraná, Brazil. Each sample
was served on white plates encoded with randomly selected 3-digit numbers,
sequentially monadic [21] and under fluorescent lighting. After consuming each
sample, evaluator was instructed to drink water to cleanse the taste. The
samples were evaluated in triplicate and in separate sessions. Statistical Analysis Results were analyzed using analysis of variance
(ANOVA). Data were compared using Tukey’s test, with a significance level of 5%
(p ≤ 0.05) and R software was used to perform the statistical
tests. Ethical Issues The
study was conducted in accordance with the protocol approved by the Ethics
Committee of Midwest State University, protocol number 2,201,325/2017. 3. RESULTS AND DISCUSSIONSChemical and Nutritional Composition Chemical and nutritional composition results of
raw and different methods of cooked sweet potatoes have been shown in Table 1. Table
1: Chemical and nutritional
composition average (± standard deviation) of raw sweet potatoes and samples
subjected to different cooking methods
Distinct letters in
row are significantly different by the Tukey’s test (p < 0.05);
Results reported in wet weight basis. The boiled and baking processes increased the
sweet potatoes pH value (p < 0.05), while frying and microwaved
cooking did not change this parameter compared to raw product (p >
0.05). A higher TA content was found in fried and cooked in the microwave sweet
potatoes, and lower for those cooked by boiled. In the water presence, there
was an increase in the phosphorylase enzyme activity that degrades starch. This
resulted in lower sugars availability sugars in sweet potato, necessary for
acids formation, reducing the product acidity. In addition, water causes damage
to the tuber cell wall, which causes loss and dilution of organic acids to the
water, thus increasing the pH of product [2], [22], [23]. Sweet potatoes cooking promoted an increase in SS
in all processes (p < 0.05). This happens due to tuber exposure to high
temperatures, which increases cell permeability, causing greater enzyme
activity such as invertase and phosphorylase, which act in breaking down starch
into sugars. Tubers submitted cooked in the microwave and baked showed higher
SS levels. This parameter was also significantly higher for fried compared to
boiled, corroborating the literature [24]. The large amount of water present in boiled method
causes greater starch solubility, facilitating its loss to aqueous milieu. In
the microwave and baked cooked methods, the tuber dehydrates due to the water
loss due to high temperatures, which facilitates the starch exposure to enzymes
that are responsible for its degradation. Higher SS/TA ratio was observed for
baked sweet potatoes and lower for fries. The presence of sugars and acids in
sweet potatoes is relevant to taste perception, so high SS/TA ratios can
increase consumer acceptance [25]. Reducing, non-reducing and total sugars levels
were increased after cooking sweet potatoes (p < 0.05), regardless of
the method used. This effect occurs because about 20% of sweet potatoes are
composed of starch. During cooking at high temperatures, formation of
monosaccharides and disaccharides occurs through the starch decomposition [26], [27]. In addition, sucrose, which is the main sugar present
in sweet potatoes [28] is converted into fructose and glucose, which increases
the reducing sugars content into the tuber [27]. Higher contents of RS, NRS and TS were observed in
cooking by microwaved/fried, baked and fried, respectively. The boiled process
was the one with the lowest levels in these evaluations, due to dilution and
loss of glucose, fructose and sucrose to cooking water [29]. Similar results were observed by Chan et al. [27] and Hou et al. [30], who reported an increase in RS, NRS and TS in baked
sweet potatoes, and by Yang et al. [29] and Murniece et al. [31] in sweet potato fries. Cooking promoted reduction in sweet potatoes
lightness, especially in boiled and baking methods, which made products darker.
In these processes, greater enzymatic reaction occurs, favoring starch
hydrolysis and product luminosity reduction. Microwave cooking, on the other
hand, was one that least interfered in the L* value, since irradiation
reduces starch availability for enzymatic action, which reduces hydrolysis
while preserving the starch's crystallinity [29], [32], [33]. According to Feltran et al. [22], reduction in luminosity in fried method is explained by
starch conversion into sugars, which contributes to tuber browning. The raw
sweet potato has greenish color (negative a* value), which increases in
presence of oxygen and light. The enzymes peroxidase and polyphenol oxidase are
responsible for this effect, since they act on water-soluble oxidizing
compounds, such as ascorbic acid, for example [33], [34]. The red content (a*) was lower in samples
cooked by baked and, especially, in those submitted to boiled, compared to
fried process. There was a significant increase in the b* (yellow) value
of sweet potatoes after cooking with microwaved, while boiled and baking
methods reduced this parameter. As previously reported, enzymatic action is
more pronounced for baking and dipping methods in sweet potatoes. Thus, greater
food oxidation is observed, which causes a greener and less yellow color (lower
values of a* and b*, respectively) [29], [35]. In general, raw sweet potatoes cooked by different
cooking methods (boiled, fried, baked and microwaved) can be considered light
in color, since all L* values were greater than 50%, with a yellow hue (b*)
and green undertone (a*) [36]. The moisture content was higher in sweet potatoes
cooked by boiled compared to other methods, being lower for fried process (p
< 0.05). In general, cooking processes have increased ash content,
especially baked. However, in boiled method caused reduction in mineral content
compared to raw tuber. Sweet potato submitted to boiled process also had the
lowest levels of protein, lipid, carbohydrate, and energy, since the leaching
process into water occurs [23]. The other methods increased the content of these
nutrients in product, mainly for fried (p < 0.05). It is noteworthy
that when food is cooked on oil, there is significant increase in lipid and,
consequently, energy, so other methods are more suitable for consumption, since
a high fat intake may increase the non-chronic diseases communicable diseases
risk [37]. Cooking by
baked and microwaved methods promoted a decrease in carotenoids levels,
especially for baked process. In these processes,
it appears that the temperature rises rapidly, which destroys some nutrients
such as carotenoids [33], which are highly unsaturated
[38], as observed in other studies
[39], [40]. The methods of boiled and
fried increased the carotenoids content compared to raw sweet potatoes, being
higher for fried (p < 0.05), corroborating with the literature [39], [41]. According to Wu et al. [39], the possible increase in
carotenoid content is due to the greater capacity for extracting beta-carotene
due to changes in the cell wall structure caused by oil and increased temperature.
Furthermore, carotenoids linked to some proteins dissociate in water, which
increases their content. Higher levels of phenolic compounds were found in
fried process, followed by baked and microwaved, all of which are higher than
the phenolic content of raw sweet potatoes, as found by Bellail et al. [42]. Phenolic compounds have some stability when exposed to
high temperatures [43], in addition, heat increases the release of phenolic
compounds through the hydrolysis of glycosidic bonds. Another factor is that
heat, inactivates the enzyme polyphenol oxidase, which is responsible for
degradation of these compounds in fresh tuber [44]. The boiled method in water was one with the lowest
phenolic compounds content. including when compared to raw potatoes (p
< 0.05), results that are in accordance with the literature [45], [46]. The loss of these compounds by leaching is the main
responsible for this reduction since the phenolic compounds are hydrophilic [48]. All cooking
methods showed an increase in ascorbic acid content, being higher for fried (p
< 0.05). Cooking alters tuber cell wall matrix, facilitating vitamin C
release in the product. In addition, cooking inactivates enzyme ascorbic acid
oxidase [44], factors that may explain the
higher vitamin C content after cooking sweet potatoes. Similar results have
been observed in other studies [44], [47], [48]. Figure 1 shows the results
of the antioxidant capacity of sweet potatoes submitted to different cooking
methods. Baked and microwaved cooking increased lipophilic
antioxidant capacity in sweet potatoes, while boiled and fried processes
reduced this parameter when compared to raw sweet potatoes. When tuber is
cooked by oven and microwave, cell modification occurs due to dry heat, which
makes lipophilic antioxidant compounds more available [32], [40], [49]. Boiled method showed the lowest lipophilic antioxidant
capacity due to leaching [50], [51], [52]. In the fried case, there may be losses of fat-soluble
compounds to the oil, reducing lipophilic antioxidant capacity [10], [40]. Fried and baked processes increased hydrophilic
antioxidant capacity of sweet potatoes. In the case of fried, the heat of the
oil is transferred to food surface and, when it reaches the center, moisture
mobilizes to the surface, making hydrophilic antioxidant compounds more
available [53], [40]. The baked method causes a structural change in tuber,
due to dry heat, which increases the availability of hydrophilic antioxidant
compounds [40]. There was a reduction in hydrophilic antioxidant
capacity in cooking process by boiled, similarly to that verified for
lipophilic antioxidant capacity. In microwave cooking, there was also less
hydrophilic antioxidant capacity, which is due to dryness of the tuber and
consequent dehydration [32], [40], [49]. Figure
1:
Lipophilic and hydrophilic antioxidant capacity of raw sweet potatoes and
submitted to different cooking methods. Distinct letters in
column are significantly different by the Tukey’s Test (p < 0.05); Results
reported in wet weight basis. Sensory Evaluation Table 2 describes the sensory analysis results of
sweet potatoes submitted to different cooking methods. Table 2: Sensory scores
(average ± standard deviation) obtained in sweet potatoes evaluation submitted
to different cooking methods
Distinct letters in
row are significantly different by the Tukey’s test (p < 0.05); AI: Acceptability Index. There was a significant difference (p <
0.05) in sweet potatoes acceptability submitted to cooking methods. Higher
grades for appearance were obtained for fried compared to baked and microwaved
cooking processes. The other methods were not statistically different (p
> 0.05). Fried process dehydrates the starch granules, forming a thick, dry
and crunchy surface. In addition, it also gives golden color to sweet potatoes,
which is expected in fried products [12], [24]. This effect does not occur in microwaved cooking and
baked processes, since heat causes water evaporation and modifies the tuber
cell structure, leading to dehydration [32]. For attributes of aroma and color, greater
acceptability (p < 0.05) was observed for sweet potatoes submitted to
fried, compared to other samples. A similar result was verified for taste and
global acceptance. However, the average score for sweet potatoes cooked by
boiled was lower (p < 0.05) than other processing. In fried method,
oil incorporates different types of fatty acids into the product, which improve
palatability, taste and aroma. Other compounds such as aldehydes, ketones,
hydrocarbons and alcohols are also formed in this process, promoting fatty
aroma notes in the product, which can be pleasant to consumers. However,
research has already shown that some individuals prefer foods with high fat
levels [24], [54]. Factors related to creaminess, spreadability and
fluidity that fat produces in the mouth, associated with the sensation of
pleasure and satiety released by the brain, are considered the main responsible
for this type of preference [54], [55]. Sweet potato
fried showed higher notes for texture, compared to that submitted to boiled and
microwaved methods, which did not differ from each other (p > 0.05).
Similar results were observed by Caetano et al. [24] evaluating baked sweet potato
chips, fried in oil and by air fryer. In the water cooking process, the fibers
soften [29], in contrast to microwaved
cooking, which makes the food stiffer and drier. In this case, the product
loses moisture through evaporation, since there is a migration of water from
the sweet potato core to extremities, which limits the starch
gelatinization. These changes in sweet potato texture generally
reduce acceptability by consumers [29], [32], as seen in the present study. Although the fried method
has shown better sensory acceptability, the baked process has also received
good acceptance by consumers, since it obtained AI ≥ 70% [56], for attributes of aroma, taste, texture and global
acceptance. Furthermore, this method is also more favorable to consumption
since it retains a greater amount of minerals, vitamin C, carotenoids and
contains a lower fat content compared to fried [12], [32]. A diet with reduced fat content and associated with
healthy habits can decrease the risk of developing chronic non-communicable
diseases such as high blood pressure and diabetes mellitus [57], [58]. 4. CONCLUSIONSIt is concluded that processes of baked, boiled, fried and cooking in the microwave alter chemical and nutritional characteristics of sweet potatoes. Fried and microwaved cooking processes are the ones that most interfere in chemical composition of sweet potatoes, reducing pH levels and increasing titratable acidity concentration, reducing sugars and red content. The fried tuber has the highest sensory acceptability by consumers and the highest nutritional content. However, it is the least suitable process for human consumption since it increases the fat and energy content of the product. The boiled method promotes the greatest losses in relation to chemical and nutritional composition of sweet potatoes, in addition to being the least accepted sensorial in general. SOURCES OF FUNDINGThis research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. CONFLICT OF INTERESTThe author have declared that no competing interests exist. ACKNOWLEDGMENTNone. REFERENCES
[20] Meilgaard M, Civille GV, Carr BT. Sensory
evaluation techniques. Florida: CRC Pres; 1999.
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