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Marek Sady, Jacek Domagala, Tadeusz Grega, Maria Walczycka, Dorota Kalicka, Dorota Najgebauer-Lejko ©
Department of Animal Product Technology, Food Technology Faculty, University of Agriculture in Krakow, Poland
SENSORY AND PHYSICOCHEMICAL PROPERTIES OF COMMERCIALLY AVAILABLE PLAIN YOGHURTS
Abstract
Continuous monitoring of food quality sold in the market is very important. The purpose of this work was to evaluate the quality of commercial yoghurt, obtained from the local market. Yoghurts from leader brand manufactures were characterized by the best sensory quality. Other products were characterized by much lower sensory quality except yoghurt T1 sold as own brand in hypermarket.
Fat content in evaluated yoghurts showed that in five dairies the standardisation of fat was done imprecisely. As a result of protein, total solids and SNF levels analysis it was concluded that all manufacturers have done standardisation of those components. Yoghurts from leader brand manufacturers D1, D2, D3 and product T1 from own brand hypermarket were characterised by the highest value of most evaluated rheological parameters. Rheological and sensory analysis showed that consumers prefer yoghurts rather with high viscosity which is correlated with high level of milk solids components.
Key words commercial yoghurt, quality, sensory properties, rheology
Introduction
Nowadays, because of still rise consciousness of society in the field of healthy nutrition, growth interest in high nutrition value food and food which have additional health effect on human organism (functional food). The plain yoghurt is typical product belonging to this group of products. Besides high value protein, minerals, vitamins, yoghurt contain live and active microflora which help to restore natural balance of gut microflora and enhance of calcium and magnesium absorption. Yoghurts with probiotic microflora stimulate immunosystem, inhibit pathogen, decreasing of cancer risk or even inhibit growth of cancer cell (Jakubczyk and Kosikowska 1994). With all these advantages plan yoghurts are low-calory products contain relatively small amounts of fat and carbohydrates. To keep the high consumption of yoghurt it is very important that consumer could possibility to buy the high quality product. To meet demand of market, manufacturers have to put attention on technological parameters and row material quality as a main factor affecting quality of final product. Nowadays there are a lot of dairies producing fermented milks on Polish market. Besides the big producers of well known leader brand there are small manufacturer and factories producing own brand made to order
© Marek Sady, Jacek Domagala, Tadeusz Grega, Maria Walczycka, Dorota Kalicka, Dorota Najgebauer-Lejko, 2008
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for hypermarket companies. It is very important that despite the tendency to price reduction, the quality of product was continuously the same.
There is a lot of consumer information concerning a low quality of commercial yoghurt. The main reason of this fact is that industry does not use a traditional technology with supplementation of milk with milk solids like skim milk powder or other milk powders, milk evaporation or membrane technique. Instead of milk solid concentration the addition of non milk components is used, like hydrocolloids. These substances are used as stabilizers and thickeners and because of relatively low price they are commonly used by manufacturers. The result is that final product changes sensory properties such as: taste, flavour or consistency and decreased its nutritive properties as well.
Task, the aim of the article Continuous monitoring of food quality sold in the market is very important. Especially it should concerns products with high nutritional value as the products having special consumer confidence, like plain yoghurts. The purpose of this work was to evaluate the quality of commercial yoghurt, obtained from the local market.
Material and methods Yoghurts produced by eight manufacturers were analysed. Three of them were classified as the brand leader, produced in factory with large scale of production and signed: D1, D2, and D3. Two yoghurts marked: M1, M2 originated from the local dairy cooperatives. Three evaluated product signed T1, T2, T3 was sold as own brand in hypermarket. All yoghurts were bought on the local market, a long time before the expiration date. Cooled products were directly transported to a laboratory and analysed. The sensory assessment of yoghurts was carried out on a 5-point scale. The analysed attributes were: overall appearance, aroma, flavour, consistency. Each of these attributes was multiplied by the proper index of importance. This assessment was done by 5 trained panellists, whose sensory sensitivity had been checked.
The chemical composition of yoghurts was done. Fat content was determined by the Gerber method (PN-A-86130, 1975). The protein content was determined by the Kjeldahl method (AOAC, 1975) using a Buchi digestion system and distillation unit Buchi 322 (Switzerland). A multiplication factor of 6.38 was used to convert per cent nitrogen to per cent protein. The total solids (TS) were determined using an air oven by drying samples at 105 for 3.5 h. The solids non fat (SNF) was calculated as a difference between total solids and fat (PN-A-86130, 1975).
The rheological characteristic of yoghurt were assessed using rotary viscometer Rheotest RV2 (Germany) with controlled shear rate in coaxial cylinder system s/s2 in the measuring range Ia. The analysis included the calculating of flow curves for shear rate from 1 to 437.4 s-1 and from 437.4 to 1 s-1. At the shear rate y=9 s-1 at rising curve, apparent viscosity of yoghurts was calculated. Flow curves were described by Ostwald de Waele and Casson models, which include such parameters as consistency coefficient K, flow behaviour index n, yield stress to, and Casson's viscosity nc. The hysteresis loop areas were calculated. This analysis was done using programme US 200 (Physics Messtechnik GmbH, Germany).
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Results and discussion
The chemical composition and sensory properties of commercial yoghurts are shown in table 1. Most of the analysed yoghurts were characterized by satisfactory sensory quality, but only the product D1 got the note maximal 5.0. Four of eight assessed products (M1, M2, T2 and T3) have gained score below 4. The lowest sensory quality was stated for yoghurt M2 from local factory (3.20). All products from leader brand manufacturers got the very high note (4.75-5.00). Other products were characterized by much lower sensory quality and gained score below 4, except yoghurt T1 sold as own brand in hypermarket. Unexpectedly it obtained 4.55 point. Most commercial yoghurt is produced by the action of two dissimilar organisms, S. thermophilus and L. delbrueckii subsp. bulgaricus, which grow simultaneously to give the desired flavour, mouthfeel and body to the yoghurt.
Table 1
Chemica composition and sensory assessment of commercial yoghurt
Type of Total solids Solids non- Sensory
yoghurt Fat [%] Protein [%] [%] fat [%] assessment [points]
D1 2.85 4.56 13.75 10.95 5,00
±0.15 ±0.25 ±1.25 ±0.95 +0.15
D2 2.96 4.49 14.93 12.03 4,82
±0.11 ±0.18 ±1.07 ±1.00 +0.05
D3 2.90 3.83 15.00 12.10 4,75
±0.09 ±0.30 ±1.31 ±0.98 +0.12
Ml 2.20 4.89 15.80 13.60 3.60
±0.13 ±0.26 ±1.12 ±0.85 +0.06
M2 1.71 4.84 15.78 14.08 3.20
±0.20 ±0.19 ±1.10 ±1.02 +0.06
T1 1.75 4.94 13.90 12.20 4.55
±0.17 ±0.38 ±1.22 ±1.19 +0.10
T2 1.60 4.16 13.75 12.15 3.65
±0.21 ±0.30 ±1.30 ±0.93 +0.09
T3 2.23 4.53 14.70 12.50 3.70
±0.17 ±0.25 ±1.25 ±0.88 +0.15
It is generally agreed that lactic acid, acetic acid and acetaldehyde are among the most important contributors to yoghurt flavour and the symbiotic action of both bacteria is necessary to obtain the desired flavour profile and acid balance (Marsili 2003).
Fortuna et al. (2001) investigated the sensory quality of yoghurts available on Polish market. They concluded that the highest note has obtained yoghurt Danone which was characterized by attractive appearance as well as smooth and homogenous consistency. High score gained yoghurt Campina which has consisted very high level of fat which has positive effect on consistency. According to Szczepaniak and Gorecka (2002) consumers prefer plain yoghurts characterized by sweet, creamy and
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milk powder flavour, but they do not like products with high level of sour and astringency flavour.
The percentage chemical composition of yoghurt mainly depends on such factors as quality of row milk and standardisation of milk during technological process. Analysed yoghurts contained 1.60-2.96% of fat. The fat content in commercial fermented milks depends on type of products and is affected by standardisation process and ranged between 0 - 10%. The industrially manufactured fermented milks contain usually 1.5-3.1% of fat (Libudzisz and Piatkiewicz 1990). Fat content in evaluated yoghurts showed that in five dairies the standardisation of fat has been done imprecisely. In all these products real fat level was below value declared on product label. The highest discrepancy between the fat level declared by producer and determined was stated in yoghurt T2.
As a result of protein, total solids and SNF levels analysis it was concluded that all manufacturers have done standardisation of those components, fortifying milk to 3.83-4.94% of protein and 10.95-14.08% SNF. Obtained results revealed that during production of plain yoghurts fortification of the milk with milk solids is only possible way to achieve high quality of yoghurt, as there is forbidden by the law to add non-milk components to these products. Milk fortification in protein or solids-non-fat enhances the consistency, flavour and aroma of final product (Ziajka 1997). There are three most popular systems used for milk fortification: the addition of skim milk powder to liquid milk, the evaporation of water from liquid milk under vacuum and the removal water from liquid milk by ultrafiltration. Usually the process milk for yoghurt has around 4,0-5,0% protein or 13-14% of total solids content. This level of parameters provides a suitable base for most commercial fermented milks. For some niche markets, total solids levels of 16-18% may be used, but the additional expense is rarely justified for routine production (Robinson 2003).
Figure 1 shows flow curves of evaluated yoghurts. All curves have shape of hysteresis loop. The highest value of shear stress at maximal shear rate had yoghurts: D3, D2, D1 and T3. These products were characterized by the highest value of apparent viscosity as well. Figure 1 shows that in different range of shear rate particular yoghurts have different rheological characteristic. At the range of shear rate 0-25 s-1 yoghurts D1 and D3 revealed higher value of shear stress than yoghurt D2, but at the scope of >80 s-1 D2 had the highest value of shear stress. The lowest values of shear stress at the full measured range of shear rate gained the yoghurt M1 from the local manufacturer.
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Figure 1 Flow curves of commercial yoghurt
0 20 40 60 80 100 120 140 160
Shear rate [1/s ]
—*—D1 —D2 —b—D3 —i—M1 —©—M2 —T1 -T2 -A-T3
Table 2
Rheological parameters of commercial yoghurt
Type of yoghurt Ostwald de Waele model Casson model Hysteresis loop area [W/m3] Apparent viscosity [Pa^s]
K [Pa^sn] n R2 To [Pa] nc [Pa^s] R2
D1 16,68 0,3155 0,959 12,973 0,2628 0,805 4,6807 45.85
D2 19,933 0,3289 0,992 14,337 0,3857 0,968 16,3885 40.33
D3 52,424 0,1333 0,991 50,126 0,0881 0,906 32,9725 81.58
Ml 7,8031 0,3473 0,972 5,484 0,1738 0,820 3,5728 21.08
M2 21,2629 0,1677 0,995 19,651 0,0689 0,966 10,2789 33.00
T1 20,4397 0,2451 0,972 17,312 0,1882 0,992 12,8182 35.75
T2 11,9418 0,3316 0,982 8,791 0,2329 0,863 4,1481 30.25
T3 30,6658 0,1877 0,986 27,729 0,1367 0,971 14,5167 47.66
The rheological parameters of the model applied for the description of flow curves are presented in table 2. Yoghurts from group of leader brand manufacturer D1, D2, D3 and product T1 from own brand hypermarket was characterized by the highest value of most evaluated rheological parameters: consistency coefficient K,
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yield stress, Cassons viscosity, and apparent viscosity. The highest values of hysteresis loop area were obtained for D3, D2 and T1. The hysteresis loop area can be interpreted as a measure of yoghurt structure breakdown during shear, and the slope of the flow curve indicates the resistance of yoghurt gel on the action of shear forces (Rhom 1992, Jaros et al. 2002).
Rheological and sensory analysis showed that consumers prefer yoghurts rather with high viscosity, which was correlated with high level of milk solids components. This tendency confirm Szczepaniak and Gorecka (2002), they concluded that consistency is main factor affected the overall sensory characteristics, and consumer prefer yoghurt with high viscosity. The main way to obtain the satisfactory level of products viscosity is process milk fortification with milk solids (Zbikowska and Zbikowski 1995). Obtained data showed that products with high level of total solids, solids-non-fat and protein were characterized by high viscosity. The use of exopolisaccharide producing starter culture increase rheological parameters of yoghurts (Hassan 1996). The parameters of heat treatment and homogenization of processed milk for yoghurt, influence on viscosity and stability of products as well (Tammime and Robinson 1999).
Conclusions
1. Yoghurts from leader brand manufactures were characterized by the best sensory quality. Other products were characterized by much lower sensory quality except yoghurt T1 sold as own brand in hypermarket.
2. Fat content in evaluated yoghurts showed that in five dairies the standardisation of fat was done imprecisely.
3. As a result of protein, total solids and SNF levels analysis it was concluded that all manufacturers have done standardisation of those components.
4. Yoghurts from leader brand manufacturers D1, D2, D3 and product T1 from own brand hypermarket were characterised by the highest value of most evaluated rheological parameters.
5. Rheological and sensory analysis showed that consumers prefer yoghurts rather with high viscosity which is correlated with high level of milk solids components.
References
1. AOAC (1995): Official methods of analysis of AOAC International (16th Ed.). Arling VA, USA
2. Fortuna T., Gol^biowska-Gajda A., Juszczak L., 2001, Sensoryczne i reologiczne wlasciwosci jogurtow naturalnych, Zeszyty Naukowe Akademii Rolniczej, Krakow, 389, 5-16;.
3. Hassan A.N., Frank J.F., Schmidt K.A., Shalabi S.I., 1996, Rheological properties of joghurt made with encapsulated nonropy lactic cultures, Journal of Dairy Science, 79, 12, 2098-3103;
4. Jaros D., Haque A., Kneifel W. (2002): Influence of starter culture on the relationship between dry matter content and physical properties of stirred yoghurt. Milchwissenschaft. 57 (8), 447-472
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Summary
Continuous monitoring of food quality sold in the market is very important. The purpose of this work was to evaluate the quality of commercial yoghurt, obtained from the local market. Yoghurts from leader brand manufactures were characterized by the best sensory quality. Other products were characterized by much lower sensory quality except yoghurt T1 sold as own brand in hypermarket.
Fat content in evaluated yoghurts showed that in five dairies the standardisation of fat was done imprecisely. As a result of protein, total solids and SNF levels analysis it was concluded that all manufacturers have done standardisation of those components. Yoghurts from leader brand manufacturers D1, D2, D3 and product T1 from own brand hypermarket were characterised by the highest value of most evaluated rheological parameters. Rheological and sensory analysis showed that consumers prefer yoghurts rather with high viscosity which is correlated with high level of milk solids components.
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