Low fat and nonfat yogurts have gained popularity in recent years because of
the increasing demand for low calorie products (Mistry
and Hassan, 1992). Reduced-fat yogurts can be produced by replacing partially
the fat content of the milk base with low calorie products known as fat replacers.
The fat replacers Simplesse 100 (Microparticulated Whey Protein (MWP)) and Anhydrous
Milk Fat (AMF) become an integral part of yogurt microstructure, but the fat
replacer particles appear to be larger than the milk fat globules. This fact
can explain the higher serum separation and lower firmness of set-style yogurts
containing the MWP than those containing AMF (Sandoval-Castilla
et al., 2004). Fat replacers exhibit diverse functional, structural
and sensory characteristics so it is appropriate to consider their application
in the form of blends. However, the blends to be used should be carefully studied
prior to recommending their application (Sandoval-Castilla
et al., 2004). Non-dairy ingredients, especially polysaccharides
such as pectin and starches can also be used in yogurt in conjugation with dairy
ingredients or on their own (Oh et al., 2007).
The present investigation was carried out to examine the effect of modified
starch as a fat replacer in different proportions upon the texture and flavor
of reduced-fat yogurt, in comparison to the full-fat yogurt.
MATERIALS AND METHODS
Materials: Commercial grade wheat starch was purchased from Fars Glucosin Co., Shiraz, Iran.
Phosphoryl chloride (POCl3) was obtained from Sigma Chemicals (St. Louis, MO).
Modification of wheat starch
Acid Treated Starch (ATS): starch (50 g, db) was slurried for 2 h at 25°C
in 1 M hydrochloric acid (pH = 0), followed by addition of 1 M sodium hydroxide
until the slurry reached pH = 5.5.
Preparation of Acid Treated Cross-Linked Starch (ATCLS): after neutralizing
the starch slurry in earlier stage, sodium sulfate (1 g) was added, followed
by addition of 1 M sodium hydroxide until the slurry reached pH = 11. POCl3
(0.1% w/w, starch basis) was injected with a microliter syringe into the starch
slurry and after 1 h, the slurry was adjusted to pH = 5.5 with 1 M hydrochloric
acid. The modified starches were recovered by centrifuging (15000 x g, 10 min),
washing with water (100 mL, 2x) and drying at 40°C (Woo
and Seib, 2002).
Preparation of yogurt mix: In this Experiment, nonfat yogurts were made by blending skim milk, Skim Milk Powder (SMP) and modified starch in varying concentrations to produce mixes with 3.2% fat content in the control and 1.6 and 0% in the modified starch-supplemented mix. Control was made from raw whole milk with fat content of 3.2%, fortified with SMP to obtain approximately 14% total solids. Nonfat yogurts were made by blending skim milk, ATCLS (or ATS) and SMP in varying concentrations to produce mixes with fat content of zero, starch content of 3.2% and total solids of approximately 14%. For producing low fat yogurts a proper portion of skim milk, whole milk, ATCLS (or ATS) and SMP was used to get a mixture with fat content of 1.6% and approximately 14% total solids.
After blending, the mixes were homogenized with a hand-operated homogenizer (Fisher Scientific Co., Eden prairie, MN, USA). Each mix was pasteurized at 90°C for 10 min (12), cooled to 40°C and inoculated with 2% yogurt starter (commercial plain yogurt). Mixes were incubated at 40°C for approximately 3 h and then cooled to 4°C and hold for 24 h.
Chemical analysis: Yogurts were analyzed for total protein by the Kjeldahl
method, fat by Gerber method and total solids by oven drying. Titratable acidity
of yogurts was determined every three days by titrating 9 g of yogurt with 0.1
N NaOH to a pink endpoint using phenolphthalein indicator (AOAC,
Total calorie values (Cal) were calculated using the Atwater method. The following
equation was used for calculation (Cengiz and Gokoglu, 2005):
K = [ (Fp x P)+ (F1 x L)+(Fc x C)]
where, K is the calorie; F the multiplication factor for each component (Fp:
4.27 for protein, Fl: 9.02 for lipid, Fc: 4.10 for carbohydrate);
P the protein content (g/100 g); C the carbohydrate content (g/100 g) and L
the lipid content (g/100 g).
Syneresis measurements: The extent of syneresis with some modification
was measured as described by Olson and Aryana (2008).
A funnel was lined with cheesecloth and placed on a 10 mL graduated cylinder
and a 15 g sample of yogurt at approximately 5°C was placed in the funnel.
After 20 min, the volume of whey drained into the graduated cylinder was measured.
The amount of syneresis was determined every 3 days.
Texture: Firmness of fresh and 2-weeks-old yogurts (4°C) was measured with a penetrometer (model 431, Precision Scientific Co., Chicago). The device uses a cylindrical probe with 7.7 mm diameter and 77 mm length. The force (expressed as grams) needed for the penetration of probe into the sample at the 0.2 mm sec-1 speed and 3 mm penetration was reported in firmness. The lower value indicates the softer texture.
Sensory evaluation: Yogurt samples were examined for color, flavor, texture and overall acceptability by 12 experienced panelists selected from members and students of Besat Education and Research Center food science department. The panelists were asked to evaluate fresh and 2 weeks old samples. A hedonic scale of 1 (dislike extremely) to 10 (like extremely) was used for flavor, 1 (very soft) to 7 (very hard) for body and texture, 1 (poor) to 5 (excellent) for color and 1 (poor) to 100 (excellent) for total acceptance. Panelists were also asked to list defects, if any were detected.
Statistical analysis: The experiments were performed in triplicates. Analysis of Variance (ANOVA) was performed using the Duncans multiple-range test to compare treatment means. Significance was defined at p<0.05.
RESULTS AND DISCUSSION
Energy values: Proximate composition of the yogurts manufactured with
different concentrations of modified starch and their energy values are shown
in Table 1. Energy values of the yogurts ranged from 58.3
to 74.1 Cal/100g. The effect of fat contents on energy values of the yogurts
was significant (p<0.05). Energy values of the yogurts decreased significantly
(p<0.05) with reduced fat levels. Fat reductions of 50 or 100% in yogurts
were obtained with a consequent decreases in energy values of 10.66 and 21.32%.
The lowest energy value was found in the ATCLS and ATS (3.2%).
||Yogurt samples composition and energy content for nonfat,
low fat and high fat yogurts
||Acidity values of non fat, low fat and full fat yogurts made
from ATS and ATCLS during storage. Mean of triplicates. Column means containing
a common letter are not significantly different from each other (p<0.05)
Energy values of the yogurts with added ATCLS and ATS at 1.6% were significantly
(p<0.05) lower than those of control yogurts (FFY). Addition of ATCLS and
ATS significantly (p<0.05) decreased energy values. Woo
and Seib (2002) indicated that cross-linked wheat starches could be counted
as dietary fiber when assaying a food by the AOAC procedure. Therefore, the
energy value of cross-linked wheat starch acted as resistant starch is lower
than what is estimated in Table 1.
There were no significant differences in acidity between control and the treated yogurts throughout storage (p<0.05). The acidity of all yogurts increased during storage, but this increment was not significant for all samples until 12 days. After 12 days (Fig. 1), the increase in acidity was significant (p<0.05).
Syneresis: Syneresis was significantly affected by the addition and concentration of starch. The amount of syneresis significantly decreased when the concentration of ATCLS was increased from 0 to 3.2. Syneresis was significantly increased by the storage time, for the FFY and ATCLSY (1.6%). However, the amount of whey separation from the ATCLSY (3.2%) was the least and did not change during two weeks of storage. This may arise from high water binding capacity of starch which reduces free releasable water. The starch may also retrograde that slightly decrease water binding capacity of the molecules but this phenomenon appear to be not important in comparison to the syneresis of protein network during the storage time (Fig. 2).
Texture: The different yogurt samples showed similar texture after two
weeks of storage as that of the zero time.
||Water release values of non fat, low fat and full fat yogurts
made from ATS and ATCLS during the storage time. Mean of triplicates. Column
means containing a common letter are not significantly different from each
||Texture analysis of non fat, low fat and full fat yogurts
made from ATS and ATCLS. Mean of triplicates. Column means containing a
common letter are not significantly different from each other (p<0.05)
ATSY exhibited the softest texture so that the device did not report any texture
for them. ATCLSY (1.6%) exhibited the highest firmness than ATCLSY (3.2%) and
FFY (Fig. 3), however FFY exhibited higher firmness than ATCLY
(3.2%) (p<0.05). The particle size of the starch in the yogurt depends on
the kind of modification. On the other hand, the concentration of the starch
in the yogurt samples differs and has an important relationship to the kind
of modification. The positive effect of the starch on texture is achieved until
a certain concentration and this is attained by the ability of starch to absorb
water. In fact the carbohydrate molecules of starch bind and orient water, that
increase the viscosity, but by increasing the concentration, the large size
of starch particles (and the high swelling property of them) prohibited formation
of a protein network composed of casein chains, which give a more open and loose
structure (Sandoval-Castilla et al., 2004).
||Sensory evaluation of non fat, low fat and full fat yogurts
made from ATS and ATCLS*
|*Mean values for a given parameter containing a common letter(s)
are not significantly different from each other (p<0.05)
This effect is more notable in the case of ATS. The higher water sorption capacity
of ATSY and then higher particle swelling and larger particles results in prevention
of casein micelles aggregation and formation of the protein network in yogurt.
This is why no acceptable curd was formed in the ATSY, even at 1.6% concentration.
Woo and Seib (2002) showed that a cross-linked wheat
starch with phosphorus oxychloride showed highly restricted swelling. Blends
of ATCLS (at low concentration (1.6%)), in addition to casein forming biphasic
gelled systems are claimed to possess fat-like characteristic as the yogurts
exhibit higher firmness (Sandoval-Castilla et al.,
2004), but high association of polysaccharide-polysaccharide chains, at
higher concentration of ATCLS (3.2%) that did not integrate unto the protein
network, inhibited milk protein chains-casein interactions and casein micelles
aggregation. Therefore, it results in the loss of network strength in the yogurts,
added with ATCL (3.2%).
Sensory evaluation: The sensory (flavor, body/texture, color and total acceptance) scores are given in Table 2. Flavor scores at zero time were significantly (p<0.05) higher than that of two weeks, but there were no significant differences in color, texture and acceptance scores (p<0.05). FFY received the best flavor score. After FFY, ATCLSY (1.6%) had significantly (p<0.05) higher flavor score than the other yogurt samples and ATCLSY (3.2%) placed at the third position. However, the starchy flavor was felt in these samples, which gave them the lower scores as compared to FFY. There were no significant differences in color scores between FFY and ATCLS (1.6 and 3.2%). The ATSY (1.6 and 3.2%) had significantly lower body/texture and color scores (p<0.05) than the others. The panelists did not recognize any significant differences between the texture of the FFY and ATCLSY (p<0.05). FFY received the best total acceptance score, which was followed by ATCLSY (p<0.05). ATSY were not acceptable at all (p<0.05). The higher total acceptance score of FFY in comparison to ATCLSY was due to starchy taste of ATCLSY.
The purpose of this study was to evaluate the effect of modified starch as a fat replacer in yogurt on quality. Good quality non fat and low fat yogurts can be made with ATCLS. Syneresis was reduced in these yogurts in comparison to the control. Use of ATCLS for the manufacture of non fat (and low fat) yogurts appears to be promising because it is available in a dried form and can be purchased in bulk. It can be stored for a long time and finally the low energy content of this starch encourages the application of it as it can act as resistance starch.