Effect of various proteins on characteristics and synerisis of tzatziki

Effect of various proteins on characteristics and synerisis of tzatziki
Athanasios G.Stefanakis1, Efstratios K.Stavrakakis1, Konstantinos G. Adamopoulos1, Patroklos K. Vareltzis1,
                                            Athanasia M. Goula2

      1
          Department of Chemical Engineering, Faculty of Engineering, Aristotle University of Thessaloniki,
                                 Thessaloniki, Greece (costadam@eng.auth.gr)
2
    Department of Food Science and Technology, Faculty of Agricultural, Aristotle University of Thessaloniki,
                                Thessaloniki, Greece (athgou@agro.auth.gr)

ABSTRACT
One of the main problems in tzatziki quality is the phenomenon of synerisis, which is observed when the gel
structure collapses and water gets squeezed out. The aim of this project was to minimize synerisis and
increase the shelf life of the product during storage at refrigerating temperatures without changing product
texture and flavor. This was achieved by the addition of hydrophilic proteins that trap the water within the gel
structure minimizing the synerisis effect. Low expressible moisture content prevents grow of microorganisms
and, thus, improves the overall quality of the product.
Firstly, different samples consisted of yogurt and solutions of albumin, whey proteins, sodium caseinate
added in yogurt were prepared. The water content of the samples was that of a real tzatziki system (83%
water). The samples of protein-yogurt-water solutions where prepared by adding proteins to yogurt solutions
in ratios of 1% and 5% w/w. Measurements of water-holding capacity, moisture content and pH were taken
and showed a reduction 5% to 10% of synerisis compared to the plain yogurt solution depending on the type
and ratio of added proteins as well as the storage time. The used proteins were analyzed for water holding
capacity, solubility and pH. The type and concentration of the added proteins were the factors investigated.
Samples were characterized in terms of pH, synerisis, water holding capacity and storage stability.
It was found that protein addition (i) improved water holding capacity, (ii) decreased the synerisis effect, and
(iii) stabilized the pH of the samples during storage. The optimum conditions (protein type and ratio added
protein/yogurt) that yield minimum synerisis without affecting significantly product texture and flavor were
determined. Thus, proteins addition seems to be an effective way of producing a stable tzatziki gel system.

Keywords: Tzatziki: Synerisis: Albumin: Sodium Caseinate: Whey Powder Concentrate

INTRODUCTION
Tzatziki is a well known Hellenic delicatessen consisted mainly of yogurt, pieces of cucumber garlic and
olive oil. Industrialy it is produced by mixing milk and bacteria culture (Streptococcus thermofilus and
Lactobacillus delbrueckii ssp. Bulgaricus [4]) with cucumber (Cucumis sativus) and garlic (Allium vineale)
in pre-ordered ratio of the ingredients. The type of yogurt that is used is strained/ concentrated yogurt. It is
cow originated and has high concentration of fat (10%) and proteins (8%) [1].Since tzatziki consists mainly
of yogurt, it shares the same quality profile and problems of product failure.
Yogurt is the result of acidic fermentation of milk creating a network of coagulated proteins. It is the result of
disulfide bonding between k-casein and denatured whey proteins as well as casein-casein aggregation [5].
The common problems yogurt-based products face, are related with protein gel network stability, variations
of acidity, microbiological infection during storage and the synerisis phenomenon. Synerisis is defined as the
shrinkage of gel and this occurs concomitantly with expulsion of liquid or whey separation and is related to
instability of the gel network resulting in the loss of the ability to entrap all the serum phase [4]. Whey
separation begins with the appearance of water at the surface of the gel and is a macroscopic evidence of the
syneresis effect. The apparent moisture content results to microbiological infection of the product, loss of the
nutritious value, altered texture and eventually the collapse of the gel structure. Altered texture and Humidity
that enhances the acidity also affect the taste. Therefore good quality of yogurt-based products is directly
related to the higher possible resistance in synerisis.
Addition of dried dairy ingredients is a common practice in yogurt manufacture. The addition of dried dairy
ingredients causes an increase in density of the protein matrix in the gel microstructure and reduction of

synerisis in yogurt [2]. In this research 3 dried dairy types of proteins were tested: sodium caseinate, whey
protein concentrate (WPC) and albumin which is mainly originated from egg white serum.
The primary aim of this study was to examine the effect of the above proteins, incorporated in yogurt
individually at different concentrations, on the pH and synerisis of a tzatziki gel during storage at 5 oC. The
water holding capacity (WHC) of each protein-enriched sample in different ratios was measured. Solubility
of the added proteins was determined, since it plays a crucial role in delaying synerisis in yogurt. Preventing
or significantly delaying synerisis, without significant change in pH to avoid textural and organoleptic
changes in the product is necessary for promising results of this investigation.

MATERIALS & METHODS
Preparation of tzatziki
Cucumber was added to strained yogurt (10% fat) in a ratio of 1:4 cucumber to yogurt. The moisture content
of cucumber was measured and the appropriate amount of water was added to yogurt in order to achieve the
same moisture content with tzatziki. Whey powder concentrate (WPC), albumin and caseinate sodium were
individually added to the samples at concentrations of 1% and 5% w/w for each protein. The proteins were
dissolved in water at 25°C before added to the strained yogurt. One extra sample without added protein was
used as the control sample. All samples were stored at 5°C.

Chemical analysis
The samples are analyzed for moisture content and pH. The water content was measured by a moisture meter,
model MB35 HALOGEN (OHAUS) using 5g of each sample heated at 105°C The pH value is measured by
a pH-meter WTW series (inoLab) .

Synerisis and water holding capacity
Both synerisis an water holding capacity were measured by a centrifuge method according to a modified
method of Keogh and O’Kennedy (1998). A 20g sample of tzatziki was centrifuged at 2.500rpm for 10min at
25°C. The whey expelled was removed and weighed. The synerisis was expressed as the percentage % of the
whey relative to the original weight of the sample. The water holding capacity was expressed as the
percentage % of pellet weight relative to the original weight of the sample.

Solubility of tested proteins
The solubility of protein water solutions was measured according to Markwell et al. (1978) [6]. The
absorbance of the samples was measured at 660nm using a Heλιos γ spectrophotometer (Thermo
Spectronic). Bovine serum albumin was used to obtain the protein standard curve in the range of 0-100 μg
protein/ml. The pH value of protein solutions was also measured

RESULTS & DISCUSSION
Water content
The water content of tzatziki was measured respectively at 83% w/w which is 5% higher than that of plain
strained yogurt and the percentage of the tested simulated samples of tzatziki are shown in Table 1.

Table 1. Moisture content of tzatziki samples with different levels of added WPC, albumin or casein
samples          Moisture %
0% protein            82,41
1% WPC                81,37
1% albumin            81,52
1% casein             81,47
5% WPC                77,91
5% albumin            78,19
5% casein             78,67

Total moisture content was reduced proportionally to the level of added proteins due to the increase of total
solids in the mixture.

pH value and stability
Variations of pH during storage time at 5oC are shown in Figure 1.

              Figure 1. Temporal variation of pH in different concentrations of tzatziki simulated samples

pH values of the protein-enriched samples appeared to be more stable through time compared to the control.
Samples containing 1% of added proteins exhibited a stable pH around 4, slightly higher than the control. .
However, samples containing 5% of added proteins stabilized at significantly higher pH values around 4.5.
These higher values of pH may be responsible for some changes in the sensory characteristics of the product
whereas samples containing 1% of added proteins were not expected to cause significant variations. The
significant changes in pH of the control sample might be attributed to the transformation of lactose to lactic
acid at different rates depending on the culture reaction. On the other hand, protein-enriched samples exhibit
a higher pH throughout storage, probably due to the dissolved proteins. Table 2 shows the pH values of the
1% and 5% protein solutions. Protein buffering capacity could also be the reason for the pH stability that
protein-enriched samples exhibit during storage. After 18 days of storage pH is comparatively more stable
in every sample than the early days of storage.

Table 2. Values of pH of protein water solutions at 5°C
samples                pH
1% WPC                 6.85
1% albumin             7.33
1% casein              7.05
5% WPC                 6.70
5% albumin             7.24
5% casein              6.91

Synerisis
Overall, protein-enriched samples displayed less synerisis than the control sample. Higher total solids could
cause an increase in density and reduce pore size in the protein matrix of the yogurt gel [3]. As shown in
Figure 1 the presence of added proteins decreased significantly the rate of synerisis until the 14th day of
storage. After the 14th day the rate gradually increased, but always it was lower than the control product. At
samples of 1% added protein, WPC exhibited the most stable results reaching an 8% difference compared to
the control and dropping at 4% difference on the 25th day. Casein reacted slightly slower until the 14th day
but the rate reached the lowest values from all samples against the control ranging from 10% the 14th day to
4% the 25th. Albumin showed no significant difference in synerisis rate compared to the control throughout
storage time.
Samples containing 5% of added proteins seemed to have synerisis rates stabilizing at significantly lower
levels than the control. At 5% level, albumin displayed the best behavior achieving 10 – 13% lower synerisis
followed by casein reaching 7-10% less synerisis effect. WPC had the minimum effect and displayed similar
level of synerisis as the 1% WPC sample. Figure 2 depicts the water holding capacity of proteins.

            Figure 2. Rate of synerisis of tzatziki simulated samples in different concentrations of proteins

Figure 3. Rate of WHC of tzatziki simulated samples in various concentrations of proteins

Solubility of tested proteins
The proteins were more soluble at concentration 1% than for 5% (w/w). This can be a pH effect. Table 2
shows the pH of solutions containing 1% or 5% (w/w) of the proteins tested. It is known that the closer a
protein solution to its isoelectric point is, the less solubility these proteins have [10]. The isoelectric points of
albumin, WPC, sodium caseinate are 4.8, 5.2, 4.6 respectively [7, 8, 9].. Albumin presented the larger
percentage of solubility for both concentrations and especially for 1%. Sodium caseinate recorded similar
solubility for both concentrations and was less soluble than the other proteins. WPC approached closer the
solubility of albumin for 1% but for 5% was slightly greater than the solubility of sodium caseinate.

Table 3. Solubility of protein water solutions at 5°C
samples            Solubility %
1% WPC                  42.6
1% albumin              52.9
1% casein               31.8
5% WPC                  33.0
5% albumin              37.9
5% casein               31.7

CONCLUSION
The addition of proteins to yogurt in order to avoid or delay synerisis seems to be an effective method to
improve product stability. However, more work is needed in order to determine the optimum conditions of
pH, temperature and concentration of the added proteins.

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