Characterisation of the water-holding capacity of fresh cheese samples by means of low resolution nuclear magnetic resonance

Abstract 

New process steps in the production of fresh cheese were introduced and varied in order to reduce the undesirable phenomenon of syneresis without adding milk-foreign addtitives. Native and modified whey protein concentrates (WPC) with apprx. 12% protein were used as ingredients in the production of fresh cheese spreads. During production the fresh cheese samples were submited to an extra flow-induced mechanical stress. The aim of the presente study is to detect changes of the water-holding capacity of different treated fresh cheese by means of the non-destructive low resolution nuclear magnetic resonance (LR NMR). Furthermore, a newly developed measuring method based on NMR, the so-called wash-out-test, was implemented in order to simulate syneresis of fresh cheese. The obtained data were correlated with results of the viscosity and the firmness of the samples. Syneresis data were determined by decanting experiments as a function of the storage duration.

The presente study shows that the shearing procedure during processing has an influence on the final product, its firmness and the syneresis behaviour of the fresh cheese samples. The main result is that good syneresis properties seem to be correlated with a softer mechanical consistency of the products. The presented data indicate that NMR measurements and, especially, wash-out-tests are useful techniques to characterise the water-holding capacity (WHC) and the structure of differently treated improve the production processsing and the final, consumer-relevant properties of fresh cheese products. © 2004 Elsevier Ltd. All rights reserved.

Keywords: Fresh cheese; NMR; Water-holding capacity; Wash-out-test; Whey protein concetrate.

1. Introduction 

Due to pasteurisation and modern packaging techniques, the shelf life of dairy products has been extended. As a consequence of lengthened storage durations, the problem of syneresis has increased. Syneresis indicates the phase-separation of the serum, mostly at the top of the product, and is presumably caused by a contraction of the gel-matrix combined with sedimentation during storage. One possibility to tackle this problem is the use of additives. Additives, especcially gelatina, however, gained a negative reputation in consumers’ opinion because of epidemics like mad cow disease or foot-and-foreign additives in fresh cheese, new process steps were introduced. Native whey protein concentrate (WPC), which has been used in fresh cheese manufacturing before, was modified via heating and shearing (Hinrichs et al., 2004). The “finisched” fresh cheese types were additionally sheared in order to enhance the water-holding capacity (WHC) of the final products.

Full fat fresh cheese is a spreadable, unripened curd-like type of product that can be consumed immediately after manufacturing. It is made of pasteurised and homogenised milk, which is coagulated by lactic acid-producing bactéria cultures and, in some cases, by addition of a small amount of rennet. When a pH of 4.6 – 4.7 is reached, the coagulum is broken by stirring. Afterwards curd and whey are separated by centrifugation or by ultra-filtration. Finally salt and, in some cases, spices and extra cream are added. The fat content is approx. 70% fat in the dry matter and the water content is above than 60%. The cheese tastes slightly sour and the consistency is pasty and spreadable.

Generally, fresh cheese has a limited shelf life in the refrigerator of about 3 – 4 months when stored in sealed packages after a heat-treatment and hot filling (Belitz; Grosch, 1999; Kessler, 2002; Walstra, 1999).

The composition of proteins in fresh cheese, that means, especially, the casein-whey protein ratio, strongly depends on the production process. Classically, fresh cheese is manufactured from pasteurised milk and is not ultra-filtrated. It generally contains casein as the main protein component. The yield of cheese increases when whey proteins are cross-linked by the heat-treatment of the milk (denaturation of whey proteins) or the ultra-filtration of the whey. The composition of the proteins and the characteristics of the fresh cheese can be changed by means of the corresponding applied processing method. Besides the quantity of whey proteins, the pH-value, the temperature and the ionic strength can influence the properties of the final product. It is possible to change the properties like sensory behaviour, firmness or syneresis of fresh cheese products adding differently treated (e.g., shearing or heating) WPCs (Hinrichs, 2001; Kessler, 2002; Piyasena; Chambers, 2003; Walstra, 1999).

In this paper, the effects of different treatments on fresh cheese were screened by means of nuclear magnetic resonance spectroscopy (NMR), which is already used in food Science (Belloque; Ramos, 1999; Cavanagh; Fairbrother; Palmer; Skelton, 1996; Colquhoun; Goodfellow, 1994; Cornillon, 1998; Gidley; McArthur; Darke; Ablett, 1995; Ruan; Chen, 1998). Up to now only Hori (1982) used NMR for fresh cheese to study the influence of freezing and thawing of green curdo n the state of water in fresh cheese. NMR is based on the absorption of radiofrequency energy by spins of nuclei, in the presente case of the hydrogen nucheus 1H. Differently bound (more precisely: immobilised) water (hydrogen) and the mobility of these fractions can be detected. The results of these meassuremments are used to quantify the WHC, to characterise the structure of the gel-matrix and to estimate the grade of the expected syneresis. Furthermore, NMR is non-destructive and non-invasive. Therefore, NMR experiments can be repeated using one single sample, thus reducing systematic erros, for example in atorage studies.

4. Conclusions

NMR provides a poweful possibility to tackle practice-relevant problems like syneresis, to characterise the viscosity of complex systems and, therefore, to improve or develop appropiate processes. NMR can be realised as real online measuring method. Thus, it can be used as process control (e.g., T2-relaxation) and, by means of T2/n-correlation (Bloembergen et al., 1947, 1948) exactly for simple systems and qualitatively for heterogeneous systems as process rheometry, which is importante for food technology due to the high relevance of viscosities for food products. In order to correlate T2-relaxation times and characteristic viscosities previous calibration-tests are necessary. Hence, a partial substitution of conventional laboratory rheometry, which is not appropriate for online measurements, is possible. Besides the viscosity, NMR experiments could be used to determine the actual content of moisture, fat, solid fator proteins, for example by the application of chemometric methods (Massart; Vandeginste; Buydens; De Jong; Lewi; Smeyers-Verbeke, 1997). This would provide further useful data in order to optimise the processing of dairy products.

NMR allows one to obtain, by means of T1 and T2, microscopic information concerning the state of water and to quantify the influence of processing, process parameters, formulation (Table 1) and measuring parameters (e. g., temperature).

As expected, T2 relaxation times are more sensitive to the presence of several phases in a sample than T1 relaxation times. Interesting is the correlation between the two relaxation times T1 and T2 (Fig. 4), which provides further knowledge concerning the mechanical structure of the samples, for heterogeneous systems. This information cannot be obtained by T2 relaxation experiments alone.

In order to obtain further information about the water-holding capacity of the studied material it is useful to implemente the so-called wash-out-test (Fig. 1).

The results show which fraction of the serum in a gel-structure is: (i) bound or retained in closed pores and, therefore, does not contribute to syneresis processes or is (ii) in capillaries which are open to the surfasse of the sample. The fraction (ii), especially, contributes to syneresis. This hypothesis is supported by the obviously existing correlation (Fig. 7) between syneresis and the amount of detectable water or pr (Eq. 4).

A further objective beyond the scope of this study is the development of a rapid measuring methodology in order to optimise the production of dairy products with regard to their syneresis behaviour (that means extent and evolution).

The results of various types of studied processes and formulations for fresh cheese spread samples (Table 1) show that shearing of the finished product has an influence on the syneresis. The supposed dependence of T1 and T2 of fresh cheese as a function of the correlation time t shows an increase of viscosity during shearing to a maximum after 10 min of shearing at 80ºC. The shortest T2 relaxation times and the best water-holding capacity are shown by the sample fc10DSFC after 3–10 min of shearing. Highest firmness was also obtained by the sample fc10DSAC after 3–10 min of shearing at 80ºC. At the same time syneresis of fc10DSAC was wort with a maximum of 0.57 – 0.62%, respectively 39.13% + 4.17% of detectable water (Fig. 5(a)). High strength durability and pressure difference mean a high syneresis properties seem to have a softer consistency. Analogous results have been found by Schhkoda (1998) for differently treated gels. Rennet acid-gels, for example, have a coarser network structure with thicker protein strings than products without rennet. Rennet acid gels, however, are firmer and have a lower WHC than purê acid gels (Schhkoda, 1998). Based on data presented in this study na improved water-immobilisation can be realised by a better controlo f the pretreatment of single ingredients (here WPC), the formulations and the process and storring conditions. Based on the empirically found correlations (in contrast to T1, T2 – n (Bloembergen et al., 1947, 1948)) between the firmness, the syneresis and the water-holding capacity (Figs. 5(a) and (b)) even the later quantities could be determined online after previous calibration tests by means of T2 relaxation experiments.            

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[1] HINRICHS, R.; GÖTZ, J.; NOLL, M.; WOLFSCHOON, A.; EIBEL, H.; WEISSER, H. Characterisation of the water-holding capacity of fresh cheese samples by means of low resolution nuclear magnetic resonance. Food Research Internacional, v. 37, p. 667-676, 2004.