Received: 30 June, 2010. Accepted: 12 March, 2011.

Invited Review
Fruit, Vegetable and Cereal Science and Biotechnology 2011 Global Science Books

Process Optimisation of Sweet Potato (Ipomoea batatas)
Puree as an Ingredient in a Formulation of Weaning Food

Ana Cristina Figueira
1,2
Olawumi Makinde
1
Margarida Cortez Vieira
1,3*

1
Food Engineering Department, Instituto Superior de Engenharia, Universidade do Algarve, Campus da Penha, 8005-139 Faro, Portugal
2
Research

Center for Spacial and Organizational Dynamics (CIEO), Universidade do Algarve, Campus de Gambelas, 8005-136 Faro, Portugal
3
Center of Investigation in Chemistry of Algarve (CIQA), Universidade do Algarve, Campus de Gambelas, 8005-136 Faro, Portugal
Corresponding author: * mvieira@ualg.pt

ABSTRACT
Sweet potato, a tuber crop widely spread across the World (Africa, Asia, America, Caribbean and Europe) and very affordable, supplies
the body with major and vital nutrients (carbohydrates, fat, proteins and vitamins) needed for good body development can be used as an
ingredient in weaning food for babies. For this purpose it needs to be boiled and mashed into a paste before being served to babies. The
cooking conditions of the Lira variety from a Portuguese region (Aljezur) were studied over a temperature range of 80 to 95C during 10
to 180 min to evaluate its effects on the physico-chemical properties (texture, non-enzymatic browning and ascorbic acid). From the
kinetics modelling study, it was concluded that a fractional conversion first order model fitted well the experimental data for these proper-
ties and the kinetic parameters estimated were: texture, (k
ref
= 0.1611 0.015 min
1
, Ea = 101.13 14.22 KJ/mol with R
2
adj
= 0.9719),
non-enzymatic browning (k
ref
= 0.0331 0.004 min
1
, Ea = 389.27 26.86 KJ/mol with R
2
adj = 0.9971) and ascorbic acid (k
ref
= 0.0244
0.023 min
1
, Ea = 101.70 14.66 KJ/mol with R
2
adj
= 0.9905). Based on these findings, the optimization of the cooking conditions to
obtain a high quality puree was determined as 90% colour retention, 70% ascorbic acid retention and moderately cooked at 90C, 7 min;
this result was further validated. The team that developed this work was composed by Nigerian and Portuguese researchers and though
performed in Portugal and with a Portuguese variety of sweet potato; the goal was to use these results in the industrial development of a
weaning food product in Africa, more specifically in Nigeria.
_____________________________________________________________________________________________________________

Keywords: ascorbic acid, -carotene, kinetics mathematical modeling, non-enzymic browning, physico-chemical properties, texture and
thermal degradation
Abbreviations: AA, ascorbic acid; Ea, activation energy; fwb, fresh weight basis; k
ref
, rate constant; R
2
, determination coefficient; R
2
adj
,
adjusted determination coefficient

CONTENTS

INTRODUCTION........................................................................................................................................................................................ 25
SWEET POTATO FROM ALJEZUR (PORTUGAL) .................................................................................................................................. 27
SWEET POTATO PURE PROCESSING OPTIMIZATION..................................................................................................................... 28
PHYSICAL AND CHEMICAL CHARACTERIZATION OF FRESH PRODUCT.................................................................................... 29
MATERIALS AND METHODS.................................................................................................................................................................. 29
RESULTS AND DISCUSSION................................................................................................................................................................... 30
CONCLUSION............................................................................................................................................................................................ 32
REFERENCES............................................................................................................................................................................................. 32
_____________________________________________________________________________________________________________


INTRODUCTION

Origin, varieties and distribution

Over 95% of the global sweet potato crop is produced in
developing countries, where it is the fifth most important
food crop. China is the largest grower of sweet potatoes,
providing about 80% of the worlds supply, followed by
Nigeria and Uganda which produce each about 2.5% of the
worlds supply, as can be observed in Table 1. Europe has
only a very small sweet potato production, mostly in Portu-
gal, which is the only European country that produces size-
able quantities of sweet potato, at 23,000 tons (International
Potato Centre (CIP) 2006). This tuber is very affordable,
mainly in Africa as it can be observed in Fig. 1, but excep-
tionally it can reach high prices such as the Portuguese one
(Fig. 2), due to its special characteristics which deserved a
highlight in this review.
Nutritional quality of sweet potatoes

Depending upon the variety, of which there are about 400,
the skin and flesh of the sweet potato may range from
almost white through cream, yellow, orange, and pink to a
very deep purple, although white and yellow-orange flesh
are most common. Sweet potatoes are grouped into two
different categories depending upon the texture they have
when cooked: 1
st
Group - Firm, dry, and mealy; 2
nd
Group -
Soft and moist. In both types, the taste is starchy and sweet,
with different varieties having different unique tastes. Usu-
ally sweet potatoes are high in carbohydrates and vitamin A
and can produce more edible energy per hectare per day
than wheat, rice or cassava (Baybutt et al. 2000). The yel-
low-orange flesh varieties also provide vitamins A and C.
The plants green leaves can also be consumed, providing
additional protein, vitamins and minerals (Woolfe 1992).
Besides simple starches, sweet potatoes are rich in complex

Fruit, Vegetable and Cereal Science and Biotechnology 5 (Special Issue 2), 25-34 2011 Global Science Books






















carbohydrates, dietary fiber, -carotene (a vitamin A equi-
valent precursor), vitamin C, and vitamin B6 (CIP 2006). In
1992, the Center for Science in the Public Interest, USA
compared the nutritional value of sweet potatoes to other
vegetables. Considering fiber content, complex carbohyd-
rates, protein, vitamins A and C, iron, and calcium, the
sweet potato ranked highest in nutritional value. According
to these criteria, sweet potatoes earned 184 points, 100
points over the next on the list, the common potato, as re-
ported by the North Carolina Sweet Potato Commission
(NCSPC) (2006). Sweet potato varieties with dark orange
flesh have more -carotene than those with light colored
flesh and their increased cultivation is being encouraged in
Africa, where vitamin A deficiency is a serious health
problem (Baybutt et al. 2000).
Despite the name "sweet", it may be a beneficial food
for diabetics, as preliminary studies on animals have re-
vealed that it helps to stabilize blood sugar levels and to
lower insulin resistance (Baybutt et al. 2000). Sweet pota-
toes have been used in Africa to fight a widespread vitamin
A deficiency that results in blindness and even death for
Table 1 Ranking of the top 20 countries that produce sweet potato in the World.
Yearly production (tonnes) Ranking
2007 2008 2009
1 China 75800197 China 80522926 * China 81212926 F
2 Uganda 2602000 Nigeria 3318000 Uganda 2766000
3 Nigeria 2432000 Uganda 2707000 Indonesia 2057913
4 Indonesia 1886852 Indonesia 1876944 India 1120000
5 Viet Nam 1437600 Viet Nam 1323900 Angola 982588
6 United Republic of Tanzania 1322000 United Republic of Tanzania 1322000 F Kenya 930784
7 India 1067200 India 1094000 U.S.A. 883099
8 Angola 949104 Kenya 894781 Rwanda 850000 F
9 Madagascar 890000 F Madagascar 890000 F Philippines 600000 F
10 Mozambique 875216 Mozambique 890000 F Cuba 437100
11 Burundi 873663 Burundi 873663 F Ethiopia 262854 *
12 Rwanda 841000 U.S.A. 836560 South Africa 62688
13 U.S.A. 819641 Rwanda 826000 Senegal 40000 F
14 Kenya 811531 Angola 819772 Portugal 27000 F
15 Papua New Guinea 580000 F Papua New Guinea 580000 F Argentina M
16 Philippines 573734 Philippines 572655 Burundi M
17 Cuba 414000 Ethiopia 526487 Guinea-Bissau
18 Ethiopia 388814 Cuba 375000 Madagascar M
19 Republic of Korea 352269 Argentina 340000 F Morocco M
20 Argentina 340000 F Republic of Korea 329351 Mozambique M
[*]: Official data,F :FAO estimate * = Unofficial figure | [ ] = Official data | F = FAO estimate | M = Data not available
Source: FAOSTAT database, 2011


Fig. 1 Prices of sweet potato in African countries for the period of 2004-2007. Source: FAOSTAT database, 2010.
Fig. 2 Prices of sweet potato in European countries for the period of
2002-2007. Source: FAOSTAT database, 2010.
26
Process optimization of sweet potatoes puree. Figueira et al.

African children (Bengtsson et al. 2008; Nandutu and How-
ell 2009). About two-thirds of the children developing
xerophthalmia, the blindness-inducing disease resulting
from lack of vitamin A, die within a year of losing their
sight (FAO/WHO 1988; Aguayo and Baker 2005). Sweet
potato has been proved to be an excellent source of vitamin
A (in the form of -carotene) and a very good source of
vitamin C; it also has healing properties as an antioxidant
food. Both -carotene and total vitamin C (Table 2) are
very powerful antioxidants that work in the body to eli-
minate free radicals (Grabowsky et al. 2008). Free radicals
are chemicals that damage cells and cell membranes and are
associated with the development of conditions like athero-
sclerosis, diabetic heart disease, and colon cancer (Grabow-
sky et al. 2008). This may explain why -carotene and vita-
min C have both been shown to be helpful for preventing
these conditions (Agarwal 2000). The human body needs
energy to work and play. Energy comes from the nutrients
in foods. The main nutrients that provide energy are carbo-
hydrates, protein and fat. Energy is measured in kilocalories
(kcal), calories or kilojoules (kJ), and 1 kcal = 4.2 kJ. With
the exception of cassava (590 kJ/100 g), sweet potato (465
kJ/100 g) is a higher source of energy than other root and
tuber staples including potato (335 kJ/100 g), yam (434
kJ/100 g) and taro (432 kJ/100 g) (United States Depart-
ment of Agriculture (USDA) 2009). The starch making up
sweet potato dry matter is easily digestible and therefore a
useful ingredient in the preparation of good weaning meals
(Hamilton et al. 1986).
Sweet potato roots are valuable sources of ascorbic acid.
They may contain more than 30 mg/100 g (fwb) (Haytowitz
and Matthews 1984). As little as 100 g of boiled sweet
potato can supply 85% of a childs recommended daily in-
take of ascorbic acid and over 50% of that of an adult
(Woolfe 1992).

SWEET POTATO FROM ALJEZUR (PORTUGAL)

History of Aljezur

The town of Aljezur was founded in the tenth century by
Arabs and later seized from the Moors in 1249. According
to legend (although this clashes with history and the date of
the sweet potatos introduction into Europe), the Knights of
the Order of St James of the Sword, led by Paio Peres Cor-
reia, drank a potion made from sweet potato before every
important battle. The force of the invasion and the speed
with which they took the castle of Aljezur stunned the Moors,
who were unable to react to such a sudden charge. The con-
quest took place in 1249 and, according to the legend, the
potion that was the determining factor in the victory was no
other than the famous feijoada de batata doce de Aljezur
(the Aljezur sweet potato bean stew). Whatever the truth of
this story, the sweet potatos origins in Aljezur have been
lost over time, which serves to illustrate that it has a long
tradition in the region and in the diet of the local people
(Ministrio da Agricultura do Desenvolvimento Rural e das
Pescas (MADRP) 2007).

Description and production and uniqueness of
Batata Doce de Aljezur

The Batata doce de Aljezur (Aljezur sweet potato) is the
adventitious, fleshy, tuberous root of the plant Ipomoea
batatas L. (syn. Batatas edulis) of the Lira variety and Con-
volvulaceae family. Elongated and pear shaped, its skin is
purple or reddish brown and its flesh yellow. Its size varies
between 8.5 4.0 cm and 16.5 7.1 cm and it reaches a
weight of between 50 and 450 g (Fig. 3). It has a sweet taste
and its texture is not very fibrous. When eaten raw, its taste
is similar to that of chestnuts (MADRP 2007). The product's
physical, chemical and organoleptic characteristics stem
from the farmers cultivation practices (expertise) and the
natural conditions present in the region (Fig. 4): the land
and sandy coverings that have an underlying layer of clay,
and the particularly oceanic climate affecting the narrow
coastal strip, which is irrigated by the Mira River. These
conditions are found only within the boundaries of the pro-
duction area. Even in neighboring areas, the climate and the
Fig. 3 Sweet potatoes from the Aljezur region right after harvesting.
Available online: http://www.barlavento.online.pt
Fig. 4 Location of Aljezur, in the Southeastern Atlantic Coast of
Portugal. Available online: http://www.cm-aljezur.pt
Table 2 Nutritional composition of raw sweet potato (per 100 g).
Parameter Amount
Energy 360 KJ/86 Kcal
Carbohydrates 20.1 g
Starch 12.7 g
Sugars 4.2 g
Protein
Vitamins
1.6 g
Vitamin B6 0.2 mg (15%*)
-carotene 8509 g (79%*)
Thiamine (vitamin B1) 0.1 mg (8%*)
Riboflavin (vitamin B2) 0.1 mg (7%*)
Niacin (vitamin B3) 0.61 mg (4%*)
Panthotenic acid (vitamin B5) 0.8 mg (16%*)
Folate (vitamin B9) 11 g (3%*)
Source: *% of RDA, USDA National Nutrient Database (2009)

27
Fruit, Vegetable and Cereal Science and Biotechnology 5 (Special Issue 2), 25-34 2011 Global Science Books

type of soil are significantly different. Aljezur has an out-
standing reputation as far as the sweet potato is concerned,
which is reflected by the price that this product reaches on
the market, the attempts to infringe on the name and abuse
it, the numerous references to it in literature and the many
events dedicated to the product. The sweet potatos repu-
tation in Aljezur is not a recent trend, resulting from spora-
dic and dubious demand for the product. It is still eaten
today because consumers of this type of product recognize
that these sweet potatoes have unique characteristics (sweet,
smooth and not particularly fibrous) and prefer sweet pota-
toes of the Lira variety produced in this region to those
imported from other parts of the world (MADRP 2007).

Cultivation

The soil is lightly tilled and harrowed and organic fertilizer
is put down. The nursery area is set up in February, when
roots are planted 10-15 cm deep in the ground, each one in
an area 50 60 cm. The move to the final growing area
takes place in April/May when a piece of the stem mea-
suring 25 cm in length is planted. Each one is planted in an
area 20 65 cm and lightly watered. Weed control is car-
ried out by weeding. In most cases, watering is not neces-
sary. The plant remains in the ground for about four months.
Most of the harvesting is done in October, when the roots
have reached maturity and there is not too much moisture in
the soil. After harvesting, the sweet potatoes are dried above
ground for around eight days, to allow suberisation and so
that any cuts can heal. The sweet potatoes are then washed
to remove excess soil and other impurities and are stored in
a cool, ventilated area with an average temperature of 13-
14C. Selection and packaging takes place in the region of
origin, which is justified not only by reduced production
costs and more effective and efficient quality control and
product traceability, but also and mainly because the prod-
uct is extremely sensitive to handling/ transport: sweet pota-
toes develop necrosis easily and this has a significant im-
pact on preserving the product and on its quality and ap-
pearance.

Comparison of nutritional and functional
properties of sweet potatoes (Ipomoea batatas)
and regular potato (Solanum tuberosum)

Although partly sharing a common denomination, potatoes
and sweet potatoes are biologically unrelated plants. In fact,
while there are over 200 species of wild potato that differ in
size, shape, colour, starch content and flavor, the sweet
potato is not one of them (Navarre et al. 2009; KLICK edu-
cao). Most cultivated potato varieties belong to the Sola-
num tuberosum species (Burlingame et al. 2009; Olson et al.
2010a). Other vegetable crops belonging to the Solanaceae
family include tomato, pepper, eggplant and tomatillo
(KLICK educao; Olson et al. 2010a). The sweet potato
(Ipomoea batatas L. Lam) is a member of the Convolvula-
ceae family, which includes other species, such as water
spinach, water convolvulus and kangkong (Roesler et al.
2008; Olson et al. 2010b).
The weight, length and width of potato and sweet potato
tubers also vary (Table 3), with the latter reaching conside-
rably higher sizes and weights (Tabatabaeefar 2002; Aina et
al. 2009). Visual flesh colours of potato pulps usually range
from white to yellow shades, whilst sweet potato pulps
show colours from white to yellow, orange, or orange-red
(Table 4) (Aina et al. 2009; Cabezas-Serrano et al. 2009).
The chemical composition of potatoes and sweet potatoes is
presented in Table 5. Sweet potatoes show higher fat, ash,
fructose, glucose, sucrose, total soluble sugars and amylase
contents, when compared to common potatoes (Lisiska and
Leszcyski 1989; Takahata et al. 1996; Aina et al. 2009;
Burlingame et al. 2009; Cabezas-Serrano et al. 2009). Total
phenols, antioxidant capacities and selected vitamin con-
tents are shown in Table 6. Again, sweet potatoes present
higher values, for most of the parameters, when compared
to potatoes.

SWEET POTATO PURE PROCESSING
OPTIMIZATION

As already mentioned sweet potatoes (Ipomea batatas) are
an economical and healthy food crop containing high -
carotene, substantial amounts of ascorbic acid and minerals
(Woolfe 1992). Globally, the sweet potato is an important
staple food or base material for a variety of food and in-
dustrial applications (Tian et al. 1991; Ravi et al. 1996). In
many developing countries, infant foods are prepared from
locally available sources, mainly cereals, roots and tubers,
which are served as a thick porridge (Sanni et al. 1999).
There is obviously a need for nutritionally balanced, energy
Table 3 Physical properties of potato and sweet potato tubers.
Width (cm) Length (cm) Weight (g)
Potato 2.6 - 15.8 2.2 - 7.8 9.0 - 445.0
Sweet potato 11.1 - 22.3 17.2 - 29.3 1100 -3100
Sources: Tabatabaeefar (2002); Aina et al. (2009)

Table 5 Proximate composition of potato and sweet potato tubers.
Parameter Potato Sweet potato
Moisture (%) 62.68 87.0** 64.1 83.8
Protein (%) 0.85 4.2** 1.9 4.4
Fat (%) 0.05 0.51** 0.2 1.8
Ash (%) 0.44 1.87**** 1.9 4.4
Fructose (mg/g ) 0.00 0.18** 0.420 12.2***
Glucose (mg/g ) 0.015 0.34** 0.090 11.6***
Sucrose (mg/g ) 0.08 1.39** 11.7 37.2***
Total soluble sugars (%) 1.4 2.3 fw* 1.8 4.7
Amylose (%) 3.5 4.3** 15.3 30.2
Sources: Aina et al. (2009); * Cabezas-Serrano et al. (2009); ** Burlingame et al.
(2009); *** Takahata et al. (1996); **** Lisiska and Leszczyski (1989).

Table 4 CIE Tristimulus L, a, b and other colour parameters of the pulp of potato and sweet potato.
L a b Hue angle Chroma
Potato 65.0 - 71.3 - 5.1 - (- 3.7) 16.7 - 30.5 1.72 - 1.79 17.1 - 31.0
Sweet potato 60.8 - 84.0 - 2.4 - 27.8 9.9 - 28.8 - 89.0 - 85.3 5.9 - 40.0
Sources: Aina et al. (2009); Cabezas-Serrano et al. (2009)

Table 6 Total phenolic content, antioxidant activities and vitamin concentrations in potatoes and sweet potatoes.
Parameter Potato Sweet potato
Total phenolics (mg/g dw) 4.0 6.5 **** 1.4 4.7
Antioxidant activity (mg/g dw) Trolox equivalent 0.43 0.892 *** 1.0 4.6
Chlorogenic acid (g/g dw) 0.00 1446.0 *** 25.9 422.4
Caffeic acid (g/g dw) 0.10 1570.0*** 0.5 20.5
Ascorbic acid (mg/100 g) 16.1 34.8 fw* 5.2 31.2
Folate (g/g dw) 0.46 1.37**** 0.11 ****
-Carotene (g/g fw) 0.00 1.11***** 0.60 231.1******
Sources: Aina et al. (2009); * Cabezas-Serrano et al. (2009); ** Padda and Picha (2008); *** Burlingame et al. (2009); **** Navarre et al. (2009); ***** Wu et al. (2008);
****** Bengtsson et al. (2008).
28
Process optimization of sweet potatoes puree. Figueira et al.

dense, easily digestible weaning foods, especially for the
first six months, in the case of infants who cannot be breast-
fed or when breast-feeding alone becomes insufficient
(Hansen et al. 1981). Weaning from the breast is a process
during which mothers gradually introduce their babies to
culturally assigned foods as they continue to breastfeed.
Weaning begins with the introduction of sources of food
other than breast milk and ends with the last breastfeeding
(Riordan 2005). Weaning is also described as the transition
from suckling to eating of non-milk food, occurring during
the first year of life - based on cultural beliefs and practices,
physicians beliefs, mothers perceptions of their infants
needs and economic realities (Chao et al. 2007). Because
weaning typically occurs during a period of rapid growth,
attention to both nutritional and developmental issues
during this period is warranted. Weaning practise can affect
infant health, particularly in developing countries like
Africa, or in inner city areas in which weaning diarrhoea is
prevalent (Slome 1960; Riordan 2005).
Pureeing of sweet potato is carried out on a large scale
in the United States with most of it being utilized to manu-
facture baby foods (Woolfe 1992). A sweet potato-proces-
sing plant in Snow Hill, N.C., USA produces aseptic sweet
potato puree using a continuous-flow microwave heating
technology developed by Coronel et al. (2005).
During thermal treatment, carbohydrates undergo a
series of physical transitions and chemical transformations
that are manifested by changes in their physical properties,
including heat capacity, enthalpy and crystalinity (Biliaderis
1990). The objective of this study was to determine the best
time-temperature combination that gives rise to high quality
sweet potato puree for weaning food formulation by a opti-
mization procedure and consequent validation.

PHYSICAL AND CHEMICAL CHARACTERIZATION
OF FRESH PRODUCT

Mathematical modelling of experimental data can accurately
predict the progress of a chemical reaction taking place in a
homogeneous liquid or semi-solid during thermal proces-
sing and/or storage, being very useful in many engineering
applications, including process optimization. Therefore,
experimental studies and application of various simplified
models to predict and interpret kinetic parameters (reaction
order, rate constant, and activation energy) are needed. The
kinetic parameters for a reaction should be independent of
the process and instrument used. Most food quality attrib-
utes changes during thermal preservation or during storage
follow a first order reaction model (Eq. 1).

(1)

where C
0
is the initial concentration, C the concentration at
time t and k the reaction rate constant (min
-1
). However, for
the so called psychophysical attributes such as colour or
texture, the attribute doesnt disappear totally; instead often
it decreases just to a certain extent, reaching a plateau. If the
fraction reactant that has been converted to yield a product
at a given time, f is used (Eq. 2), a convenient variable
often used in place of concentration (Levenspiel 1974;
Zimerri and Tong 1999) and a modified version of Eq. 1 is
obtained, a fractional conversion first order reaction (Eq. 3).
For irreversible first-order reaction kinetics, this model also
applies.

(2)



(3)

with C

the measured non-zero equilibrium parameter val-

ue(s) at infinite time. The dependence of the rate constant
on temperature is defined by the Arrhenius equation (Eq.
4):

(4)

with k
0,
the frequency factor (min
-1
), Ea, the activation
energy (kJ/mol), R, the universal gas constant (8.314 J/mol
K) and T, the absolute temperature (Kelvin, K).
Due to the large difference in magnitude between the k
0
value and the Ea value, a reference temperature, T
ref
was
introduced to rescale the parameters (Nunes et al. 1993)
(Eq. 5). This transformation not only brings stability to the
numerical integration and parameter estimation, but also, by
using a reference rate constant, k
ref
, it is enhanced that the
equation can be applied in the most interesting range of
temperatures including the reference temperature, T
ref
.

(5)

where k
ref
, is the reaction rate at the reference temperature,
T
ref
, the reference temperature and the other parameters, Ea,
k and R, same as defined above.

MATERIALS AND METHODS

Sample preparation

Fresh samples of sweet potatoes (Ipomoea batatas, var.
Lira) from Aljezur were analysed. The sweet potatoes were
purchased from the market and washed thoroughly. The
skin was removed and washed again thoroughly to expose
the yellow fleshed skin. This was followed by cutting the
washed tuber into cylindrical discs of 2 cm in diameter and
0.5 cm in thickness.

Thermal treatment

The isothermal temperatures of the sweet potatoes were
first determined to ensure the uniform heat circulation in the
samples. This was done by inserting a thermocouple con-
nected to a data logger (Delta logger), in the geometrical
centre of each cylindrical disc of the samples and fixed on a
basket support placed inside the water bath. Next, the sweet
potato samples were placed inside a beaker containing 100
mL of water at each study temperature, and placed in a
Thermostat water bath (model-Grant W 38) set at the same
temperature and exposure times ranging from 0 to 180 min.
Right after the heating time was reached the samples were
transferred to a beaker for each experiment and immedi-
ately cooled in a beaker with ice water, for 2 min. Trip-
licates were used for each time-temperature combination.

Ascorbic acid

Reagents: The extracting solution was 0.4% oxalic acid
containing 20% acetone. To a solution of 4 g oxalic acid
dissolved in 500 mL distilled water, 200 mL acetone was
added and diluted to 1 L with distilled water. A stock solu-
tion of 2,6-dichloroindophenol (DCIP) was prepared by dis-
solving 100 mg of dye in 100 mL warm water, and adding
84 mg NaHCO
3
. This was diluted to 500 mL with water and
filtered. The DCIP stock solution was further diluted with
water such that when 1 mL of extracting solution and 9 mL
of DCIP solution were reacted and an absorbance value (at
520 nm) of 0.300 to 0.350 was obtained.
Ascorbic acid standard curve: 100 mg of anhydrous as-
corbic acid (AA) was dissolved in 100 mL of extracting
solution. Appropriate aliquots of the stock AA solution (1.0
to 5.0 mL) were pipetted into 100 mL volumetric flasks
containing extracting solution, yielding concentrations of
10.0 to 50.0 g/mL. One millilitre aliquots of these dilu-
tions were then reacted with 9 mL DCIP and the reaction
read at 520 nm.




kt
C
C
=
0
ln
) (
) (
0
0

=
C C
C C
f
( )
kt
C C
C C
or f =
(
(

) (
ln ) 1 (
0
|
|
.
|

\
|
|
.
|

\
|
=
ref
ref
T T R
Ea
k k
1 1
exp
|
.
|

\
|
=
RT
Ea
k k
o
exp
29
Fruit, Vegetable and Cereal Science and Biotechnology 5 (Special Issue 2), 25-34 2011 Global Science Books

Sampling procedure

Ascorbic acid concentrations were evaluated by the spectro-
photometric method of Egoville et al. (1988). The method
is based on the ability of AA to reduce the dye 2,6- DCIP.
15 g of fresh or boiled sweet potatoes were extracted with
an oxalic acid and acetone solution (0.4 and 20%, respec-
tively) by homogenizing in a Ultra-turrax T25 (Janke &
Kunkel Ika-Labortechnik, Germany) during 5 min at 4000
rpm. The extract was filtered under vacuum through a filter
paper Whatman no. 2 (Whatman International Ltd., Maid-
stone, UK) and brought to 100 mL with the same extracting
solution. One milliliter of the extract was reacted with 9 mL
of 2,6-dichloroindophenol (1.6%) during 1 min and read at
520 nm on a Hitachi U-2000 spectrophotometer (Hitachi
Ltd., Tokyo, Japan). The AA concentration was quantified
through comparison with a standard curve of L-AA (Riedel-
de-Haen 33034).

Non-enzymatic browning

Browning measurements were carried out using the method
described by Baloch et al. (1973). Raw and cooked sweet
potato samples (2.5 g) were homogenized for 5 min in 50
mL acetic acid-formaldehyde aqueous solution (2-1%, v/v) .
The slurry was filtered and 0.5% lead acetate was added to
the filtrate. Then, the slurry was collected in a volumetric
flask, made up to 100 mL with acetic acid-formaldehyde
solution. After mixing, the extract was centrifuged at 2500
rpm for 5 min and the supernatant was mixed with an equal
volume of ethyl alcohol. The mixture was centrifuged again
and absorbance measurements were taken at 420 nm and
600 nm by using a Hitachi U-2000 spectrophotometer.
Browning was calculated from the difference between the
two absorbance values.

Texture determination

Texture measurements were performed in a Texture Analyser
Instron Model (1011) using a 50 N load cell and equipped
with a 8 mm diameter probe at a speed of 50 mm/min.
Forcedistance curves were recorded and firmness (maxi-
mum peak force, N) and energy (area under forcedistance
curve, J) were used as indicators of textural parameters.
Four measurements were done for each conditions tested.

Modeling of the kinetics of thermal degradation of
relevant parameters during heat preservation

From the observation of the trend of degradation of ascorbic
acid, texture and non-enzymatic browning (Figs. 5-7), it can
be seen that the behavior of thermal degradation clearly
deviates from simple first order levelling off after a certain
time. Due to this fact, a fractional conversion model (Eq. 3)
was attempted to model the data (Levenspiel 1974). The use
of the fractional conversion model for determining the
effects of heat processing on sweet potatoes have been pre-
viously reported (Rizvi and Tong 1997; Stoneham et al.
2000) and first order kinetic model reported by other au-
thors (Nisha et al. 2006). The kinetic parameters were then
estimated based on the Arrhenius law (Eq. 4) using a two-
step analysis and the adequacy of the models studied. A one
step non-linear regression was next performed through all
the data points in order to calculate the activation energy,
Ea, and rate constant at reference temperature, k
ref
, from the
original data (Lund 1983; Arabshahi and Lund 1985).
In all this study, the statistical software STATA, version
4.0 (STATA Corp. 1995) was used. The parameter property
presented as the fraction of property change for fractional
conversion, f, at any time t, is defined as (Eq. 3): The use of
the fractional conversion model for determining the effects
of heat processing on sweet potatoes have been previously
reported (Rizvi and Tong 1997; Stoneham et al. 2000).
The reference temperature used was the average value
of the range considered (i.e. T
ref
= 87.5C). Parameters pre-
cisions were evaluated by confidence intervals at 95%, and
the quality of the regression was assessed by the coefficient
of determination (R
2
) by checking the randomness and nor-
mality of residuals, thus allowing best model selection.

RESULTS AND DISCUSSION

Effects of heating on the ascorbic acid content
and its degradation

Raw sweet potatoes contain between 17 and 35 mg/100 g of
ascorbic acid on a fwb (Bradbury and Singh 1986; Woolfe
1992). However, these levels are significantly reduced by
thermal processing (Bradbury and Singh 1986). The ascor-
bic acid values reported for the raw sweet potatoes in this
current study are much lower than the previously reported
values; this may be as a result of type of variety used and/or
some oxidation and hydrolysis over storage time before
analysis. Zee et al. (1991) have previously reported the
effect of storage on ascorbic acid in raw sweet potatoes and
they concluded that AA is sensitive to air, heat and water,
and can easily be destroyed by prolonged storage. Cooking
losses of L-ascorbic acid depend on the degree of heating,
leaching into the cooking medium, surface area exposed to
water and oxygen, pH, presence of transition metals, and
any other factors that facilitate oxidation (Eitenmiller and
Laden 1999).
The kinetic parameters of AA during heating at different
temperatures and processing times are shown in Table 6.
The reaction mechanism of ascorbic acid decomposition in
foods has been studied extensively (Archer and Tannenbaum
1979; Liao and Seib 1987). It should be emphasized that
different pathways, which give origin to different break-
down products, exist for the degradation of ascorbic acid.
As many parameters will influence the kinetics of vitamin
decomposition, it is difficult to establish a precise precur-
sor-product relationship. Various mechanism of deteriora-
tion might operate simultaneously.


Fig. 5 Effect of temperature and time on ascorbic acid degradation in
sweet potatoes, at the temperature range of 8095C. () experimental
values with heating processes; (
__
) model predicted values for heating pro-
cesses.
30
Process optimization of sweet potatoes puree. Figueira et al.

Non-enzymatic browning

Non-enzymatic browning is the formation of dark-colored
pigments in foods during processing and storage, which
results from reactions between reducing sugars such as
fructose and glucose on one hand, and protein and its deri-
vatives (amino acids and amides) on the other (Maillard
1912). As the sugar concentration decreased, an increase in
the degree of non-enzymatic browning was seen across the
time-temperature combinations studied. At an initial sugar
concentration of 17.35 0.057 g/100 g of sweet potato, the
non-enzymatic browning value was 0.007 (absorbance val-
ue at 420 nm) and gradually increased at increasing tempe-
ratures and times of exposure (Fig. 6). The sucrose hydro-
lysis observed at increasing temperatures caused an increase
in fructose and glucose contents and hence the consumption
of these reducing sugars, which was attributable to the
browning reactions. Therefore, the obtained increase of the
browning rate at high amplitudes can be attributed to the
decrease of sugar content (Yuan et al. 2009).

Texture degradation kinetics

Thermal texture degradation of sweet potatoes was investi-
gated over a varying time and temperature periods. Long
heating times were included in this study in order to esti-
mate C

correctly. In Fig. 7, the relative firmness (i.e. firm-

ness of the sample divided by the firmness of the time-zero
samples) is plotted as a function of the heating time. The
data was modelled using a fractional conversion kinetic
model (Eq. 3). This figure indicates that the fractional con-
version kinetic model is suitable for modelling thermal deg-
radation of sweet potato texture (R
2
adj
= 0.9719) (Table 7).
Rapid texture degradation was observed with increasing
temperature and heating times, followed by a saturation
phase where texture degradation rate slows down, until an
equilibrium texture property is achieved. Similar results
have been reported by Abu-Ghannam (1998) when model-
ling the textural changes of legumes and red kidney beans.
The texture degradation of sweet potatoes had an Ea of
101.13 14.22 KJ/mol which is close to the values reported
by Vu et al. (2004) as 117.56 10.55 KJ/mol for texture
degradation in carrots. However Cunningham et al. (2008)
reported for common potatoes soaked in water in the tem-
perature range of 20-80C a lower value of 41.12 kJ/mol.

Optimization of time-temperature conditions for
the puree

The objective function of this optimization is to maximize
the retention of ascorbic acid, while reducing non-enzy-
matic browning and obtaining good firmness, necessary for
the weaning puree. The mathematical models previously
obtained and the respective kinetic parameters, activation
energy, Ea and rate constant k
ref
(at a reference temperature
of 87.5C) obtained for texture, non-enzymatic browning
and ascorbic acid (Table 6) were utilized in Equation 6.
These equations were derived from the fractional conver-
sion model to obtain time (t) at different temperatures that
will correspond to different percentage retentions (X) lim-
iting it to 60%.

(6)

Fig. 8 shows the graph of time (t), obtained plotted
against temperature. The coarse line represents the vitamin
C concentration at different retentions (60, 70, 80 and 90%),
the dash line representing non-enzymatic at different colour
retentions (60, 70, 80 and 90%) and the round dots rep-
resenting texture at different levels of cooking; A (well-
cooked), B (moderately cooked) and C (not cooked). There-
fore, a point was reached and determined to obtain a time
K
X
t
ln
=
Fig. 6 Effect of temperature and time on the degree of non-enzymic
browning in sweet potatoes, at the temperature range of 8095C. ()
experimental values with heating processes; (
__
) model predicted values
for heating processes.
Fig. 7 Effect of temperature and time on peak load/firmness of sweet
potatoes, in the temperature range of 8095C. () experimental values
with heating processes; (
__
) model predicted values for heating processes.
Table 7 Kinetic parameters of properties utilized for optimization.
Kinetic
parameters
Ascorbic acid Non-enzymic
Browning
Texture
C0 0.9966 0.017 1.0550 0.032 1.0202 0.029
C 0.2412 0.011 2.1261 0.043 0.0509 0.0020
Ea (KJ/mol) 101.70 14.66 389.27 26.86 101.13 14.22
Kref (min
-1
) 0.2444 0.023 0.0331 0.004 0.1611 0.015
R
2
adj 0.9905 0.9971 0.9719

31
Fruit, Vegetable and Cereal Science and Biotechnology 5 (Special Issue 2), 25-34 2011 Global Science Books

and temperature combination where there is maximum ret-
ention of ascorbic acid, high colour retention and good
firmness, this point represents Z as can be seen in Fig. 8
and this point corresponded to 90C and 7 min.

Validation of optimized conditions of the puree

The firmness, non-enzymatic browning and ascorbic acid
were tested for at the optimized conditions obtained (90C
for 7 min) and the values obtained and its percentage reten-
tion compared with the predicted values are presented in
Table 8. It can be seen that at a temperature of 90C, the
sweet potato sample is well cooked (54 6% firmness loss),
enough to allow for easy mashing to obtain the puree during
the weaning food preparation, although slightly lower than
the predicted (60 0.02%), also the percentage of ascorbic
acid retention (70.2 5%) obtained was slightly higher than
the predicted (60 0.31%) and a good colour retention
(80.8 3%) was observed, which is also slightly lower than
the predicted (90 0.015%).

Potential use of sweet potato pure in baby food
in developing countries

Based on the result obtained from the optimization, the best
cooking conditions were found (a time and temperature
combination) minimizing therefore the degradation of the
main ingredient in a weaning food, the sweet potato puree.
According to USDA (2002), the recommended daily allow-
ance (RDA) of vitamins and proteins for adequate growth
and development of babies are vitamin A 800 g, vitamin
B1 1.4 mg, vitamin B2 1.6 mg, vitamin B6 2 mg, vita-
min B12 1 g, vitamin C 60 mg, vitamin D 5 g, vita-
min E 10 mg. In order to meet these requirements, the
necessary ingredients which will fortify and supplement
some nutrients that might be lost despite the use of the
optimized cooking conditions of the sweet potatoes puree
will be: the addition of coconut milk, soy milk and orange
juice, which are rich in vitamin A, C and E to a formulation
would fulfill these requirements with an additional protein
supplementation by soy milk. The vitamin C derived from
the addition of orange juice, apart from making up for the
RDA required of vitamin C for babies, also served to im-
prove the flavor and colour of the puree, to give it an attrac-
tive and appealing look. In addition, it also served as an
anti-oxidant to reduce the risk of microbial contamination
of the finished product, whilst the fructose acted as a sweet-
ener to make it more enjoyable. To complement this recipe,
it is very important to use boiling/warm water to mix the
formulae composition, before feeding the babies with it, so
as to reduce or maintain the microbial load to the minimum
limits specified for their body system. In Fig. 9 a flowsheet
for a production line of a dried weaning porridge that in-
cludes sweet potato is proposed.

CONCLUSION

The optimized cooking conditions for the puree, which are
a temperature of 90C and a duration of 7 min, obtained
from this study for a weaning food formulation was effici-
ent for the control of the firmness, colour retention and as-
corbic acid. The consumer acceptability and market survey
of this formulation will be a major area for future research.

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