Approaches to minimise yoghurt syneresis in simulated tzatziki sauce preparation
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doi: 10.1111/1471-0307.12238
ORIGINAL
RESEARCH Approaches to minimise yoghurt syneresis in simulated
tzatziki sauce preparation
PATROKLOS VARELTZIS,1 KONSTANTINOS ADAMOPOULOS,2*
E F S T R A T I O S S T A V R A K A K I S , 2 A T H A N A S I O S S T E F A N A K I S 2 and
ATHANASIA M. GOULA3
1
Department of Food Technology, Alexandrian Technological Institute of Thessaloniki, Thessaloniki GR – 574 00,
2
Department of Chemical Engineering, Faculty of Engineering, Aristotle University of Thessaloniki, Thessaloniki, and
3
Department of Food Science and Technology, Faculty of Agriculture, Aristotle University of Thessaloniki,
Thessaloniki, Greece
The phenomenon of syneresis becomes more profound in increased moisture yoghurt-based prod-
ucts. Such a product is the traditional Greek appetizer tzatziki that contains cucumber, a vegetable
with a high moisture content. During the manufacture of tzatziki, the addition of cucumber causes
the protein network to break up, provoking an increase in syneresis. The aim of this study was to
investigate a tzatziki manufacturing procedure that will lead to significantly decreased syneresis.
Two different manufacturing procedures were compared: the extra moisture coming from cucumber
was introduced before or after fermentation. The effect of adding whey protein concentrate (WPC),
albumin, sodium caseinate or a mixture of these was also studied. The results show that the addi-
tion of extra moisture before yoghurt fermentation leads to a significantly lower syneresis (7.5%)
and higher consistency (2000 cp) than those obtained in the case of addition after fermentation
(25% and 1500 cp, respectively). The use of albumin, WPC or a mixture of albumin, WPC and
sodium caseinate further decreased the phenomenon of syneresis to below 5%, without altering the
colour of the product (DΕ* < 2.3).
Keywords Syneresis, Stirred yoghurt, Tzatziki, Yoghurt, Casein, Whey protein concentrate.
maintained for 4–6 h. The acid development of
INTRODUCTION
yoghurt is carefully monitored until the pH
Yoghurt is defined as a fermented milk product reaches 4.0–4.7; then, the fermentation is
produced with thermophilic lactic bacteria, usu- stopped by rapid cooling (Connolly 1978).
ally Streptococcus thermophilus and Lactobacil- During milk fermentation, the casein becomes
lus delbrueckii ssp. bulgaricus (Mottar et al. unstable and coagulates to form a firm gel, com-
1989; Lucey et al. 1999; Ginovart et al. 2002). posed of strands of casein micelles, with whey
All yoghurt-manufacturing procedures are basi- entrapped within this matrix, which is inter-
cally the same. The milk is clarified and sepa- locked via hydrogen bonds, forming a protein
rated into cream and skim milk and then matrix. Yoghurt structure is the result of disul-
standardised to achieve the desired fat and pro- phide bonding between k-casein and denatured
tein content. The mixture is homogenised using whey proteins and by aggregation of casein as
high pressures (10–20 MPa) at temperatures of the pH drops to the isoelectric point of the
55–70 °C (Tamime and Robinson 1999). The casein proteins during fermentation (Damin
milk is pasteurised by heating for 30 min at et al. 2009).
*Author for 85 °C or 10 min at 95 °C. Next, the milk is An important aspect of a milk gel is whey
correspondence. E-mail: cooled to approximately 43 °C, which is an separation, which refers to the appearance of a
costadam@eng.auth.gr
optimum growth temperature for the yoghurt liquid on the surface of milk gel. It is a common
© 2015 Society of starter culture that is added to the milk in a defect in fermented milk products such as
Dairy Technology fermentation tank. A temperature of 43 °C is yoghurt (Lucey et al. 1998). Syneresis is defined
Vol 68 International Journal of Dairy Technology 1Vol 68
as the shrinkage of gel, and this occurs concomitantly with moisture content of tzatziki. During the manufacture of tzat-
expulsion of liquid or whey separation and is related to ziki, migration of water from the high moisture content
instability of the gel network resulting in the loss of the ingredient (cucumber) to the low moisture content ingredient
ability to entrap all the serum phase (Walstra 1993). (yoghurt) occurs. Thus, the gel matrix of the yoghurt is bro-
According to Lucey et al. (1998), some possible causes of ken and cannot hold the extra water, resulting in increased
wheying-off in acid gels are very high incubation tempera- syneresis.
tures, excessive treatment of the mix, low total solids con- The aim of this study was to investigate a tzatziki manu-
tent (protein and/or fat) of the mix, movement or agitation facturing procedure that will lead to significantly decreased
during or just after gel formation and very low acid produc- syneresis. For experimental purposes, a simulated tzatziki
tion (pH > 4.8) (Magenis et al. 2006; Donato and Guyom- sauce preparation was used. This preparation was strained
arc’h 2009). Magenis et al. (2006) reported that factors yoghurt with added moisture where, instead of adding
influencing yoghurt texture and syneresis include total solids whole cucumber, garlic and dill, only the extra moisture
content, milk composition (proteins, salts), homogenisation, coming from cucumber was added. As stirring of yoghurt
type of culture, acidity resulting from the growth of bacte- breaks up the protein network leading to an increased syner-
rial cultures and heat pretreatment of milk. esis, we tested the hypothesis that extra moisture can be bet-
Traditionally, the solid content of milk is increased for ter retained by introducing it before the formation of
yoghurt production. The three main systems available nowa- yoghurt. To this end, two different ways of manufacturing
days are good options to achieve desired protein and solids were compared: the extra water coming from cucumber was
contents: (i) addition of protein ingredient powders (skimmed added (a) before and (b) after fermentation and compared to
milk, whey protein concentrates, caseinates); (ii) evaporation tzatziki made from a commercial readymade strained
of water from milk under vacuum; or (iii) removal of water yoghurt. The effect of adding whey protein concentrate,
by membrane filtration (Tamime et al. 2001). Fortification albumin, sodium caseinate or mixtures of these protein
with skim milk powder (SMP) is the common practice to ingredients on syneresis was also studied.
increase the solid content in conventional yoghurt manufac-
ture, but when enrichmenting the protein content is the main
MATERIALS AND METHODS
target, the amount of SMP that can be added to provide extra
protein content becomes limited, since too high levels of Materials
SMP can lead to a powdery taste and high lactose content, Homogenised, pasteurised fresh cow’s milk (72 °C for 15 s)
which ultimately results in a highly acidic product (Abd El- (Mevgal S.A., Thessaloniki, Greece) was used as a stock of
Khair 2009; Supavititpatana et al. 2009; Marafon et al. raw material throughout the experimental work to avoid var-
2011a,b). An alternative approach of fortification of the milk iation in milk composition.
for yoghurt manufacture is by ultrafiltration. Protein concen- Globulal 70 N whey protein concentrate (70% protein
trates produced by ultrafiltration have better nutritional value content) and Emulac sodium caseinate (50% protein con-
than that produced by traditional methods. Another advan- tent) were purchased from Meggle AG (Wasserburg, Ger-
tage of UF milk is that it contains a higher level of protein many), whereas hen egg albumin powder (78% protein
with a lower level of lactose than normal milk (Karlsson content) was obtained from Kallbergs Industri AB (T€ ore-
et al. 2005). However, UF concentration of milk for yoghurt boda, Sweden).
manufacture leads to an excessively firm coagulum and a Streptococcus thermophilus and Lactobacillus delbrueckii
more viscous product than conventional SMP fortification ssp. bulgaricus were obtained from Aristomenis D. Phikas
(Schkoda et al. 2001). & Co (Thessaloniki, Greece). The cultures were stored at
The phenomenon of syneresis becomes more profound in 20 °C in a concentrated form before use.
increased-moisture yoghurt-based products. Such a product Soft potable water was used for tzatziki manufacture.
is the traditional Greek appetizer tzatziki. Tzatziki is made
of strained yoghurt mixed with cucumbers (in a ratio of Yoghurt–Tzatziki manufacture
4:1), garlic, salt, olive oil and sometimes lemon juice, and Tzatziki is made of strained yoghurt mixed with cucumbers
dill or mint or parsley. Strained or Greek-style yoghurt is in a ratio of 4:1. The moisture contents of strained yoghurt
traditionally made by straining fermented yoghurt curd in a and cucumber are about 82 and 96% w/w, respectively
cloth bag to reach the desired solids level by removing acid (Trichopoulou and Georga 2004). For experimental pur-
whey. This step is achieved by mechanically separating the poses, a simulated tzatziki sauce preparation was used. This
whey from the curd using either a centrifugal separator or preparation was strained yoghurt with added moisture,
membrane filtration (Nsabimana et al. 2005) or by fortifica- where instead of adding whole cucumber, garlic and dill,
tion of milk with milk protein concentrates (Bong and only the extra moisture coming from cucumber was added.
Moraru 2014). Cucumber is a vegetable with a high mois- Thus, during tzatziki manufacture, 1.2 kg of water was
ture content (~96% w/w), which practically increases the added per 5 kg of yoghurt/yoghurt formulation.
2 © 2015 Society of Dairy TechnologyVol 68
Two different yoghurt-manufacturing procedures were fol- ready strained yoghurt, with the same total solids and fat
lowed as depicted in Figure 1. In the first case (Figure 1a), contents.
water was introduced after the incubation of condensed milk The milk was concentrated form an initial solid content of
with the micro-organisms, whereas in the second one (Fig- about 11.2% to a final concentration of approximately
ure 1b), the water was added before the incubation. In both 21.5% in a rotary vacuum evaporator (Model R 114, Buchi
cases, different mixes of protein ingredients were added as Laboratoriums-Technik, Flawil, Switzerland) at 45 °C. A
agents for improving texture and reducing syneresis. The quantity (500 mL) of the milk samples was prewarmed to
protein ingredients used were (w/w) whey protein concen- 45–50 °C in a microwave oven and heated in a water-boil-
trate 1% and 5%, sodium caseinate 1% and 5%, and albu- ing bath to 80 °C. When this temperature was reached, the
min 1% and 5%, as well as a protein mixture 5% consisting samples were kept for 30 min in the bath and protein ingre-
of 2% albumin, 2% whey protein concentrate and 1% dients were added. In the case presented in Figure 1b,
sodium caseinate. The percentages were based on prelimin- where the water was added before the incubation, soft pota-
ary experiments. Both preparations were compared to a con- ble water in a quantity that accounts for the extra moisture
trol sample, which was manufactured using a commercially content of the tzatziki was also added. After cooling to
Fresh milk (3.5% fat)
Condensation by boiling (continuous stirring)
Heating at 80 ºC for 30 min
Protein ingredients addition
(a) (b)
Addition of water corresponding to
Cooling to 42 ºC (temperature of incubation)
the cucumber of tzatziki
Inoculation and incubation for 5 h Cooling to 42 ºC (temperature of incubation)
(till pH reached 4.5)
Cooling of yogurt at room temperature (1.5 h) Inoculation and incubation for 5 h
(till pH reached 4.5)
Refrigerated storage (4 ºC) Cooling of yogurt at room temperature (1.5 h)
Addition of water corresponding to Refrigerated storage (4 ºC)
the cucumber of tzatziki
Refrigerated storage (4 ºC)
Figure 1 Tzatziki manufacturing procedure (a) water added after fermentation and (b) water added before fermentation.
© 2015 Society of Dairy Technology 3Vol 68
42 °C, the mixes were inoculated with the active starter at 100 rpm. The sample temperature was 4 °C. For a rela-
(9 g/53.5 kg), divided into three equal portions in plastic tive comparison between treatments, viscosity reading was
cups and incubated at 42 °C in a fermentation store until a taken at the point of the 30th s and torque was maintained
4.7 pH was reached in about 5 h. The yoghurt was then at all times between 10% and 100%.
cooled to room temperature for 1.5 h by placing the cups in
a bath of cold water and stored at 4 °C in the cold refrigera- Moisture content
tor. In the case presented in Figure 1a, where the water was Moisture was determined by an Ohaus moisture analyzer
added after the incubation, the quantity of water that (Model MB35 Halogen, Ohaus Co., Pine Brook, NJ, USA).
accounts for the extra moisture content of the tzatziki was
added during storage. All samples were prepared in tripli- Colour
cates. Formulations used for tzatziki manufacture are listed Colour was determined using a Hunter Lab colorimeter. L*,
in Table 1. a* and b* values were determined, while the total difference
Tzatziki samples were drawn at different cold storage in colour (DΕ) was calculated using the following equation:
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
intervals (2, 9, 14, 16, 18, 22, and 25 days) and character-
DE ¼ ðL0 Ls Þ2 þ ða0 as Þ2 þ ðb0 bs Þ2 , where Lo*,
ised for viscosity, syneresis, pH and colour.
ao* and bo* are the values for the control sample, and Ls*,
Physicochemical characteristics as* and bs* are the values for the sample. DΕ ~ 2.3 corre-
sponds to just noticeable difference.
Solubility
Milled protein ingredient powders were dispersed (1% w/w) Statistical analysis
in deionised water and the pH adjusted with 2 N NaOH to Data were analysed by a general linear model procedure of
7.0. Dispersions were centrifuged at 20000 g for 15 min. the Fisher’s protected-least-significant-difference test using
Protein concentrations were determined according to Lowry SAS (SAS Inst., Cary, NC, USA). This test combines analy-
et al. (1951). Solubility was obtained from the concentration sis of variance (ANOVA) with comparison of differences
ratio of the supernatant and the dispersion before centrifuga- between the means of the treatments at the significance level
tion. of P < 0.05.
Syneresis RESULTS AND DISCUSSION
Syneresis was measured according to Keogh and O’Ken-
nedy (1998). Briefly, yoghurt (30–40 g) was centrifuged (T Effect of protein ingredients addition on syneresis rate
52.1, VEB MLW Zentrifugenwerk Engelsdorf, Germany) at of tzatziki
222 g for 10 min at 4 °C. The clear supernatant was poured The protein ingredients used as agents for improving water-
off, weighed and recorded as syneresis (%). holding capacity and decreasing syneresis were whey pro-
tein concentrate (WPC), albumin and sodium caseinate.
Viscosity Their measured properties are presented in Table 2. The
Viscosity measurements were obtained using a Brookfield choice of protein ingredients was based on their ability to
viscometer (Model DV-II+; Brookfield Engineering Labs, decrease syneresis in tzatziki manufactured using commer-
Inc., Middleboro, MA, USA) with a No 4 spindle rotating cial strained yoghurt. Figure 2a,b show the effect of the
Table 1 Formulations used for tzatziki manufacture
Ingredients (%)
Treatment Yoghurt Milk Water Whey protein concentrate Albumin Sodium caseinate Active starter
Control 80 – 20 – – – –
1% whey protein concentrate – 77.6 21.4 1.0 – – 0.02
1% albumin – 77.6 21.4 – 1.0 – 0.02
1% sodium caseinate – 77.6 21.4 – – 1.0 0.02
5% whey protein concentrate – 74.5 20.5 5.0 – – 0.02
5% albumin – 74.5 20.5 – 5.0 – 0.02
5% sodium caseinate – 74.5 20.5 – – 5.0 0.02
5% protein mixture – 74.5 20.5 2.0 2.0 1.0 0.02
4 © 2015 Society of Dairy TechnologyVol 68
syneresis, while a more profound effect on syneresis was
Table 2 Protein solubility and pH of solutions at 25 °C (avg SD,
exhibited by increasing albumin concentration from 1 to
n = 4)
5%.
Protein (% w/w) Solubility (%) pH The effects of protein and total solids (TS) are difficult to
5% whey protein concentrate 33.0 1.5b 6.70 0.08c study separately, as the two variables cannot be modified
5% albumin 37.9 1.7a,b 7.24 0.02b independently. Increasing TS improved yoghurt texture,
5% sodium caseinate 31.7 0.7b 6.91 0.01a,c based on sensory and instrumental evaluations. Generally,
Means in the same column followed by the different superscript let-
higher TS caused an increase in density and reduced pore
ters are significantly different (P < 0.05).
size in the protein matrix of the yoghurt gel. This led to a
reduction in syneresis and improvement of the water-hold-
(a) 30 ing capacity of the gel. Keogh and O’Kennedy (1998)
explained that casein and b-lactoglobulin interact chemically
25 on heating. This effectively increases the concentration of
Syneresis (% w/w)
gel-forming protein in the yoghurt matrix and reduces syner-
20
A
esis through increased entrapment of serum within the inter-
15 B stices of the whey protein molecules attached to the surface
C of the casein. The effects of different levels of TS were
10 D
studied by Tamime and Robinson (1999), and they found
that the consistency was greatly improved as solids
5
increased from 12 to 20 g/100 g. The greatest change was
0 observed from 12 to 14 g/100 g, whereas levels above
9 14
16 g/100 g resulted in less pronounced change. According
16
18 to Prentice (1992), increase in protein levels is the principal
22
Time (days) 25
factor influencing texture and enrichment of milk with milk
powder results in the development of chains and aggregates
(b) 30 of casein micelles. Wu et al. (2001) demonstrated that
25
water-holding capacity was related to the ability of the pro-
teins to retain water within the yoghurt structure. These
Syneresis (%w/w)
20 researchers further suggested that the fat globules in the
A milk may also play an important role in retaining water.
15 B
C Tzatziki samples containing albumin had lower syneresis
10 D rates than the others, whereas syneresis was reduced with
increased proportions of sodium caseinate. This may be
5 because the high whey protein-to-casein ratio induced
0
shrinkage of the gel, which led to increased whey drainage
9
or whey separation (Lucey 2004). Keogh and O’Kennedy
14 16 (1998) explained that casein and b-lactoglobulin interact
18
22
Time (days) 25 chemically on heating. This effectively increases the con-
centration of gel-forming protein in the yoghurt matrix and
Figure 2 Effect of protein addition on syneresis rate during storage of reduces syneresis through increased entrapment of serum
tzatziki-simulated samples. Addition level of (a) 1% (w/w) and (b) 5%
within the interstices of the whey protein molecules attached
(w/w). (A) control sample, (B) whey protein concentrate, (C) albumin
to the surface of the casein. Saint-Eve et al. (2006)
and (D) sodium caseinate.
observed that sodium caseinate provides a gel with a hetero-
geneous structure with large pores. When yoghurts were
level of protein addition on the syneresis of tzatziki during enriched with whey protein, the protein network was more
cold storage at 4 °C. Each data point in the figure represents uniform and the pores of the gel were smaller than those of
average values of three replications. The repeatability for yoghurts to which sodium caseinate had been added.
syneresis expressed as the average standard deviation of the
three replications was 0.3%. All three protein ingredients at Effect of way of added moisture on syneresis rate of
both levels (1 and 5%) significantly decreased syneresis tzatziki
compared to the control sample. However, comparing the Although the phenomena occurring during syneresis are not
effect of the different concentrations of the same protein fully understood, it is agreed that increased syneresis with
ingredient, it was concluded that increasing the concentra- storage time is usually associated with severe casein net-
tion of WPC and sodium caseinate did not further decrease work rearrangements (van Vliet et al. 1997) that promote
© 2015 Society of Dairy Technology 5Vol 68
whey expulsion. As stirring the formed gel would add an the role of milk fat, protein, gelatin and hydrocolloids
extra destabilising effect, we tested the hypothesis that water (starch, locust bean gum/xanthan mixture) on the rheology
would be better retained in stirred yoghurt products if it was of yoghurt. These authors showed that the consistency index
added before fermentation. (K) and syneresis were more frequently influenced by the
Syneresis was significantly lower (P < 0.05) for all tzat- composition than the behaviour index (n) and the critical
ziki samples in which water was added before fermentation strain (cc). Lee and Lucey (2006) investigated the structural
(Figure 3), except for the sample with 5% added albumin, breakdown of the original (intact) yoghurt gels that were
in which there was no difference in syneresis. This was also prepared in situ in a rheometer, as well as the rheological
true for the control sample, which exhibited an almost con- properties of stirred yoghurts made from these gels. The
stant syneresis around 7%, throughout the storage period. same authors found that the rheological properties of
However, the control sample in which the water was added yoghurts were greatly influenced by the physical properties
after fermentation had a much higher syneresis, at a level of the original intact (set) yoghurt gels. Water in milk gels
around 25%. Samples with 5% albumin and 5% of the pro- is physically trapped within the gel network, meaning that
tein mixture exhibited the least syneresis. the tendency for whey separation is primarily linked to
Rheological properties of yoghurt have been well studied. dynamics of the casein network rather than mobility of the
Yoghurts have flow properties that are characteristic of a water molecules (van Vliet and Walstra 1994).
non-Newtonian and weakly viscoelastic fluid (Ramaswamy
and Basak 1991, 1993; Benezech and Maingonnat 1993; Effect of protein addition and way of moisture addition
Skriver et al. 1993). Keogh and O’Kennedy (1998) studied on viscosity, pH and colour of tzatziki
Protein addition increased yoghurt viscosity. Similar results
were found by Abu-Jdayil (2003), who found greater vis-
A B C D cosity in yoghurts of the labneh type, with higher protein
(a) 35
content. Yoghurts containing WPC had lower viscosity val-
30
ues than the others. These results are in agreement with
Syneresis (% w/w)
25 those reported by Modler et al. (1983) and Guzman-Gonz-
alez et al. (1999), who found that casein-based products
20
tended to produce firmer gels with less syneresis than
15 yoghurts fortified with whey protein. Modler and Kalab
10
(1983), using electron microscopy studies, showed that
yoghurts prepared with casein, SMP and MPC exhibited a
5 high degree of fused micelles when compared to yoghurts
0 stabilised with WPCs.
2 4 6 8 10 12 14 16 18 20 Adding the extra moisture to the system before fermenta-
tion and setting probably allows casein and whey proteins
Time (days)
to arrange themselves in a network and trapping the extra
moisture, rendering yoghurts less susceptible to syneresis
A B C D
(b) 35 compared to samples in which the extra moisture is added
after fermentation. This is further strengthened by the obser-
30 vation that the viscosity of tzatziki samples with water
added before fermentation was also found to be higher than
Syneresis (% w/w)
25
that of all the respective samples in which the water was
20
added after fermentation (Figure 4). For yoghurt products,
15 steps such as mixing result in a viscosity reduction that is
only partially restored after shearing is stopped. Recovery of
10
structure is called ‘rebodying’ and is a time-dependent phe-
5 nomenon. Structural recovery also affects the apparent vis-
cosity of yoghurts (Lee and Lucey 2010). Arshad et al.
0
(1993) reported that glucono-d-lactone (GDL)-induced gels
2 4 6 8 10 12 14 16 18 20
had only 30% recovery of the original value of the dynamic
Time (days) moduli even after allowing 20 h for recovery after shearing.
Figure 3 Syneresis during storage of tzatziki-simulated samples with As far as the effect of protein ingredients on pH changes,
added water (a) after and (b) before incubation. (A) control sample, (B) there was a small decrease in the pH from the time the fer-
1% whey protein concentrate, (C) 5% albumin and (D) 5% protein mix- mentation was stopped till 48 h after storing the product at
ture. 4 °C (postacidification) and slowly continued till the 10th
6 © 2015 Society of Dairy TechnologyVol 68
(a) A B C D Control 5%WPC
8000 4.9
5%Albumen 5%Caseinate
7000
4.7
6000
Viscosity(cp)
5000 4.5
4000
4.3
3000
pH
2000 4.1
1000
3.9
0
1 3 5 7 9 11 13 15 17 19 21 3.7
Time (days)
3.5
(b) 8000 A B C D 0 5 10 15 20 25 30
Time (days)
7000
Figure 5 Change of pH in tzatziki-simulated samples with 5% w/w
6000 addition of different protein ingredients.
Viscosity(cp)
5000
4000
(a) 3.5 A B C D
3000
2000 3.0
1000 2.5
0 2.0
ΔΕ*
1 3 5 7 9 11 13 15 17 19 21
1.5
Time (days)
1.0
Figure 4 Viscosity of tzatziki-simulated samples with added water (a)
after and (b) before incubation. (A) control sample, (B) 1% whey protein 0.5
concentrate, (C) 5% albumin and (D) 5% protein mixture.
0.0
0 5 10 15 20
day of storage. A decrease in pH during storage is expected Time (days)
as a result of the activity of LB (Tamime and Robinson
1999); this has been observed in lactic beverages (Oliveira A B C D
et al. 2001). Postacidification occurred independently of the (b) 2.5
ingredient or the concentration used and was 0.20 pH units
on average. A similar trend was observed by Damin et al. 2.0
(2009). There was no significant pH difference between the
control and the samples throughout the storage period for 1.5
ΔΕ*
the 1% level of addition. On the other hand, at the 5% level
of addition, a significant (P < 0.05) difference was observed 1.0
(Figure 5). This can be attributed to the higher buffering
capacity brought by the added protein ingredients. There 0.5
was a high correlation coefficient between the pH through-
out the storage period and the respective syneresis, that is 0.0
0.90–0.95. 0 5 10 15 20
Furthermore, there was no significant effect of the way of Time (days)
extra water addition on the pH. There was, however, a ten- Figure 6 Total colour difference of tzatziki-simulated samples with
dency for a slightly higher pH during the first 2 weeks of added water (a) after and (b) before incubation in relation to the control
storage for the samples with added water after fermentation sample. (A) control sample, (B) 1% whey protein concentrate, (C) 5%
(~0.1–0.15 pH units, results not shown). albumin and (D) 5% protein mixture.
© 2015 Society of Dairy Technology 7Vol 68
Protein addition significantly affected the colour of logical properties and structure of non fat stirred yogurt. LWT - Food
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Martınez et al. (2002), who studied the change in colour as
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Addition of water before fermentation also led to better Guzman-Gonzalez M, Morais F, Ramos M and Amigo L (1999) Influ-
colour retention of the tzatziki samples (Figure 6) for all ence of skimmed milk concentrate replacement by dry dairy products
treatments. In tzatziki samples in which water was added in a low fat set-type yoghurt model system. I: use of whey protein
before fermentation, DE* is below 2.3, that is the value that concentrates, milk protein concentrates and skimmed milk powder.
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though significantly improving syneresis, led to a greater Karlsson A O, Ipsen R, Schrader K and Ardo Y (2005) Relationship
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