ASSESSMENT OF SELECTED PHYSICOCHEMICAL PARAMETERS OF UHT STERILIZED GOAT’S MILK

Izabela Dmytrów¹, Anna Mituniewicz-Małek¹, Jerzy Balejko^2
1 Department of Dairy Technology and Food Storage, West Pomeranian University of Technology, Szczecin, Poland
² Department of Food Process Engineering, West Pomeranian University of Technology, Szczecin, Poland

ABSTRACT

Selected physicochemical characteristics and organoleptic attributes of UHT sterilized goat's milk were evaluated. Two kinds of UHT goat's milk were obtained in the retail market and stored at room temperature (21°C±1°C) for 6 months. The examined milk was analysed for water, fat and protein content, titratable acidity, active acidity and hydroxymethylfurfural content. The experimental goat's milk was also subjected to rheological analysis based on evaluation of viscosity.
It was found that during storage, UHT sterilized goat's milk was characterized by fairly stable organoleptic attributes and proper chemical composition, which slightly differed from manufacturer specifications. The results obtained confirmed significant differences in active acidity, titratable acidity and viscosity of the examined samples. Moreover, significant increases in titratable acidity, viscosity and total hydroxymethylfurfural content were observed in UHT goat's milk during storage.

Key words: sterilization, acidity, hydroxymethylfurfural, viscosity.

INTRODUCTION

One of the more interesting segments of the food market in Poland is the dairy industry, which offers high-value food products that provide proper amounts of valuable and easily assimilated animal protein in the daily diet [18]. The nutritive value of milk cannot be equalled by any other food product or replaced with another single product [36]. Milk components vary in quantity according to mammalian species. Milk-producing mammals include cows, goats, sheep and buffaloes. One of the best known and most important animals in terms of productive value are goats, which can be used for milk, meat and wool depending on the breed. Raising goats for milk production is the most popular, making goat's milk one of the most popular types of milk produced in the world [33]. In Poland, interest in goat's milk has increased due to the current trend for ecology, agritourism and health food [47]. Goat's milk is mainly consumed directly and any surpluses are used for production of yoghurt, cheese and dry whole milk [45]. The shelf life of raw goat's milk is similar to that of cow's milk. Its acidity does not change for 12 h at 20°C and for 48 h after cooling to 5°C. Forty-eight-hour storage of goat's milk at 4–5ºC increases its viscosity and content of citric acid, soluble calcium, magnesium and phosphorus [13]. Because goat's milk is perishable and subject to seasonality, efforts are constantly underway to find methods for extending its shelf-life. The shelf life of milk can be greatly extended by destroying pathogenic and saprophytic bacteria as a result of pasteurization or sterilization. Research concerning heat stability of goat's milk has shown that it is extremely sensitive to heat treatment. Some researchers go so far as to hold that goat's milk is not suitable for UHT (Ultra High Temperature) processing [37]. This may be due to differences in structure of casein micelles or balance of mineral salts between goat's and cow's milk. The heat instability of goat's milk is also attributed to its high content of ionic calcium and lower hydration of casein micelles [38]. It has also been reported that stored UHT goat's milk undergoes rapid and unfavourable changes in organoleptic characteristics [45].

Today's consumers also believe that heat treatment (particularly sterilization) leads to unfavourable changes in the composition and nutritive value of milk. In addition, it is known that characteristics of and changes in the finished product depend largely on parameters of the technological process, good production practices and manufacturer's experience. Therefore, the aim of the present study was to evaluate some physicochemical characteristics of two types of UHT sterilized goat's milk obtained in the retail market.

MATERIALS AND METHODS

Analysis was made of sterilized goat's milk originating from 2 different manufacturers (A and B). Milk was packed in Tetra Pak cartons and bought from retail stores in Szczecin, Poland. After arrival in the laboratory, the samples were stored at room temperature (21 ± 2ºC) for 6 months. Analyses were performed on the day of purchase and every 30 days during 6 months. In accordance with information found on the packages, the analysed samples differed slightly in the content of individual chemical components. Chemical composition of the analysed milk, as specified by the manufacturers, is given in Table 1. The expiry date of both milk types was 6 months.

Both variants of sterilized goat's milk were each time subjected to an organoleptic assessment using a five-point scale. Eight tasters evaluated both brands of sterilized goat's milk for flavour, aroma, colour, structure and consistency [34]. The samples used for the analysis were collected randomly. The organoleptic assessment was performed on the basis of methods using scales, categories and centre-punching. The obtained results for particular variants of milk were summed and expressed as an arithmetic mean. Examination was carried out in a room free from foreign odours, where each of the panellists had a separate test stand and distilled water for mouth rinsing.

The goat's milk was each time analysed for active acidity (pH) using a pH meter, titratable activity (°SH) [35] and hydroxymethylfurfural (HMF) content [20]. In addition, at the start, in the middle and at the end of the storage period the samples were analysed for dry matter content (reference method), fat content (Gerber method), protein content (according to Kjeldahl), lactose content (Bertrand method) and density (aerometric method) [35]. All the determinations were performed in 6 replications. The studied milk was subjected to rheological analysis which included measurement of milk viscosity using the steady-state flow test. An AR 2000 EX rheometer (TA Instruments) was used for analysis. The samples analysed for viscosity were subjected to coagulation with a stepwise increase in shear rate. Data sampling was conducted under equilibrium conditions (steady state). Samples were analysed using double-gap coaxial cylinders with a shear rate of 437.4 to 1ֿ¹s at a constant temperature of 22°C. Rheological determinations were performed in 12 replications. The obtained results were analysed statistically using Microsoft Excel 2000 based on tests of significant differences between two means for dependent samples and for independent samples, i.e. t-Student's and Cochran-Cox's tests [11,20]. All statistical tests were performed at α = 0.05 level of significance.

RESULTS AND DISCUSSION

The organoleptic evaluation showed that throughout its shelf life, UHT goat's milk was characterized by normal colour, structure and consistency (Fig. 1).

Both milk A and B was a homogeneous white liquid that had no visible signs of mechanical impurities and flowed freely from the carton. Even on the last day of analysis, both milk variants scored maximum 5 points for structure and consistency. The experimental samples differed considerably in flavour and aroma. Over the first 5 months of storage, the flavour of goat's milk A was described as typical, fresh and definitely "goaty" (4 points). Unfortunately, the flavour of this milk deteriorated considerably during the last month of storage at room temperature (3 points). Up to 4 months of storage, the flavour of sample A was described as typical but with perceptible "goat overtones" (4 points). After this period, its flavour clearly deteriorated. During this time, the evaluators of sample A described it as definitely goaty and untypically "musty" with a clear sterilization flavour. As a result, this sample was given the lowest score (3 points) at the end of storage. In the opinion of tasters, milk B was by far better. It had fresh, natural flavour and "goat overtones" that were much less perceptible (5 points). Because its aroma was described as natural, pleasant, and typical of sterilized milk, sample B scored 4.5 5 points.

Goat's milk with normal organoleptic characteristics should be a liquid with a uniform white colour, possibly with a cream shade. Right after milking, it has practically no aroma, while perceptible "goat overtones" are the result of hygienic and technological negligence [43]. Directly after manufacture, sterilized milk has the characteristic heated flavour. It is caused by the presence of free sulfhydryl groups but disappears during storage. Flavour changes that occur during further storage are the consequence of extensive changes that include Maillard reactions, lipase and protease activity, and oxidative and photolytic reactions [22,25,45]. The deterioration of organoleptic characteristics, observed in the milk samples analysed in the present study, is confirmed by the literature. Pieczonka [31] reported that high sterilization temperature may induce or increase "goat" flavour and aroma, which results from the considerably higher concentration of low-molecule fatty acids than in cow's milk. Simon and Hansen [42] attributed the unfavourable changes in consumer milk flavour to the presence of naturally occurring lipases, which can survive sterilization and, during milk storage, they release free fatty acids (which give a rancid off-flavour) from triacylglycerols. According to Szwocer et al. [43], uncontrolled lipolysis is considerably influenced by the enzyme lipoprotein lipase, about 45% of which is present in the fat phase of goat's milk compared to just 6% in cow's milk. Because fat globule envelopes are structurally more delicate in goat's milk than in cow's milk, intensive mechanical processing, which may destroy the envelopes and thus activate lipases, should be avoided. Burton [7] states that development of intensive rancid flavour and aroma of milk results from the oxidation of milk fat triglycerides in the presence of light and oxygen during storage. According to the same author, the rancid flavour and aroma only occurs if all ascorbic acid has been oxidized, and this phenomenon requires considerable amounts of oxygen. This is the reason for differences in the intensity of rancid flavour and aroma between products in opaque packages and those in transparent and non-airtight packages. A major role in milk flavour and aroma is also played by Maillard reaction products such as volatile carbonyl compounds that result in sterilized aroma. It is quite stable and intensifies during storage, with the rate of change dependent on the rate of these reactions, i.e. storage time and temperature [7,8]. According to Usarek et al. [44], changes of this type make UHT milk less acceptable. Other compounds that contribute to sterilized flavour include diacetyl, lactones, ketone alcohols, maltol, vanillin, acetic benzaldehyde and acetophenol [12].

The results of our determinations confirmed that both variants of goat's milk had normal chemical composition that slightly differed from the information specified by the manufacturers on the packages. The chemical composition and density of the UHT goat's milk analysed are given in Table 2.

From the literature on the subject, it is apparent that density of raw goat's milk ranges from 1.027 g·cm^-³ to 1.029 g·cm^-³ and is similar to density of cow's milk [9,29,31]. The available sources and standards fail to specify density ranges for UHT sterilized goat's milk. It is a well-known fact, however, that the effect of sterilization on milk density is mainly associated with physical changes in the milk fat fraction [16]. In addition, density is a variable which depends on percentage and specific weight of all milk constituents. Except fat, an increase in the concentration of milk substances increases milk density unlike an increase in fat content, which reduces milk density [6]. Wszołek [46] reports that fat content of goat's milk is similar to that found in cow's milk, but the former contains more volatile fatty acids and slightly less monoenic fatty acids, with a higher level of polyenic acids. The lactose content of goat's milk is in principle the same as in cow's milk and ranges from 3.92 to 5.28 [17,23].

An important technological factor that determines the behavior of milk in various production processes is acidity. In the whole period analysed, the milk originating from producer B was characterized by significantly higher pH (Fig. 2, Table 3).

During the course of the analyses, despite the observed fluctuations in active acidity, there was a non-significant decrease in the pH of sample A and a non-significant increase in the pH of sample B (Table 4).

The results obtained during determination of potential acidity of the samples and their statistical verification showed that they were significantly higher for goat's milk originating from producer B (Fig. 3, Table 4). In addition, a significant increase in titratable acidity of both sample variants was observed (Table 4).

As reported in the literature, acidity of UHT milk reflects product freshness and, to a certain extent, the quality of raw material used for processing and the intensity of heat treatment used during the technological process [24,28]. Obrusiewicz [28] reports that the rate of thermal processes has a significant effect on UHT milk acidity because carbon dioxide is released during the initial stage of milk heating, which leads to reduced acidity. Beyond 100°C, changes in acidity result from changes in the balance of mineral salts and lactose, under whose conditions titratable acidity begins to increase. When investigating the effect of temperature storage conditions on physicochemical and sensory changes of UHT milk, Cais-Sokolińska et al. [8] found that a 24-week period of milk storage at both 3–5°C and 7–9°C caused active acidity to decrease but to a lesser extent than for samples stored at 19–21°C. Higher storage temperature also caused a higher increment in potential acidity. The latter authors attribute milk acidity changes to the course of enzymatic (mainly lipolytic) reactions, which result in the formation of free fatty acids. In another study, Cais-Sokolińska et al. [9] reported that during storage of UHT milk, titratable acidity of samples increased in proportion to time and temperature. A similar phenomenon, in which pH value decreased and potential acidity increased with the time of UHT milk storage was described by Bilińska et al. [5]. Panfil-Kuncewicz and Kuncewicz [29] demonstrated that in general, the quality of analysed UHT milk was satisfactory, but pointed to the relatively high acidity of the samples, both before and after storage. However, they failed to provide an unambiguous interpretation of this phenomenon. They claimed that changes in acidity of milk heated above 75ºC may result from changes in the balance of mineral salts (mainly calcium phosphates), which are less soluble in higher temperatures than in room temperature and precipitate in the form of colloidal tricalcium phosphate. This reduces the amount of soluble and ionic calcium, which negatively affects the heat stability of milk, and the hydrogen ions formed in this reaction increase milk acidity. A certain effect on the acidity of sterilized milk (mainly milk sterilized by the long-time method) can be exerted by small amounts of formic, acetic and other acids and Maillard-type reactions [14, 22].

Thermal processes used in milk processing regulate the growth and activity of microorganisms as well as the course of biochemical and physicochemical processes [2,3]. Storage of sterilized goat's milk was associated with an increase in total hydroxymethylfurfural (HMF) content in the analysed samples, which on every occasion was higher in milk samples originating from producer B (Figs. 4 and 5). The increase in free HMF content was statistically significant only for milk A (Table 4) and this sample was characterized by a higher content of free HMF during the entire study period. However, statistical analysis did not enable the differences to be estimated conclusively (Table 4). The observed phenomena are evidence that Maillard reactions, which cause milk to brown and undergo changes in flavour and aroma, were already underway [8,24].

It should be noted that the content of both HMF forms observed in our study was always similar to the values reported in related publications. Data reported by Dzwolak and Ziajka [12] indicate that HMF content of UHT milk averages 13.81 µM·dm^-³. According to Akalin and Gönç [1], HMF content of sterilized milk varies between 5.2 µM·dm^-³ and 15.1  µM·dm^-³. Meanwhile, Fink and Kessler [14] and Berg and Boekel [4] report that the content of this chemical compound in UHT milk may reach 22 µM·dm^-³. Similar amounts of HMF in UHT sterilized milk were determined by Morales et al. [25] and Schmidt and Renner [41], who also showed a relationship between HMF content and milk storage temperature – the higher the storage temperature, the higher the HMF content. Other authors state that HMF content of milk sterilized by the long-time process ranges from 30 µM·dm^-³ to 140 µM·dm^-³ [25,39,40]. Therefore, some researchers claim that determination of HMF levels may serve to distinguish UHT milk from milk sterilized by the long-time method.

The most important physical changes that take place during the storage of market milk are changes in viscosity. Their extent depends on temperature and time of storage [44]. Both samples were characterized by standard viscosity of 1.73^-3 mPa·s for milk A and 2.77ֿ³ mPa·s for milk B. Statistical analysis of the results obtained during the determination of viscosity showed that both sample variants differed significantly in this parameter, with lower viscosity shown by milk originating from producer A (Fig. 6).

A significant increase in the viscosity of both samples was observed (Fig. 6, Table 4). As noted above, the extent of viscosity changes depends on temperature and time of milk storage and may conclude with complete gelification of milk. This problem is very important because gelified products are unfit for consumption. Dzwolak and Ziajka [12] report that storage gelification is reflected in a slight decrease in viscosity right after UHT milk production, after which it remains almost unchanged and suddenly increases. This phenomenon is the result of direct interactions between casein micelles and the formation of a three-dimensional network of bonds. Milk viscosity is also influenced by the total fat content and the size of fat globules.

In summary, the two brands of UHT sterilized goat's milk were characterized by relatively stable organoleptic characteristics, normal values of titratable and active acidity, and standard content of both total and free hydroxymethylfurfural. The observed differences in the broadly defined quality of samples support a well-known regularity that characteristics of the final product depend largely on the quality of raw material, good production practices and manufacturer's experience. Different parameters of the production process result in a product with different organoleptic and physicochemical characteristics.

CONCLUSIONS

1. The organoleptic evaluation confirmed that both variants of sterilized goat's milk were characterized by relatively stable organoleptic attributes except the flavour and aroma of the analysed samples, which showed some differences.

2. Over a 6-month storage period, UHT goat's milk from producer B showed better flavour and aroma.

3. The analysed samples were characterized by normal density, viscosity, and content of fat, protein, lactose and dry matter.

4. The titratable acidity of both milk samples was found to increase significantly, with higher values observed throughout the storage period for goat's milk originating from producer B.

5. During the entire period analysed, goat's milk from producer A was characterized by significantly higher pH value.

6. During the course of the analyses, total hydroxymethylfurfural content increased significantly in both milk samples.

7. The increase in free HMF content was significant only for milk obtained form producer A.

8. A statistically significant increase in the viscosity of both samples was observed.

9. Significantly higher viscosity during storage was characteristic of UHT goat's milk from producer B.

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Accepted for print: 10.05.2010

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