Impact of processing conditions on microstructure, texture and chemical properties of model cheese from sheep milk

https://doi.org/10.1016/j.fbp.2019.05.003Get rights and content

Highlights

  • Cutting and cooking settings affect cheese microstructure and texture properties.

  • A higher cooking temperature raised free fat formation and reduced cheese porosity.

  • The hardness and chewiness of cheese increased with higher cooking temperature.

  • A bigger curd grain size increased the globular fat volume in cheese microstructure.

  • A smaller curd grain size enhanced the fat loss and decreased cheese yield.

Abstract

Cutting and cooking settings have a strong effect on curd particle features, whey syneresis and cheese properties. In the present study, the impact of curd grain size and cooking temperature on the microstructure, texture and composition of cheese, whey losses and cheese yield was studied with specific focus on sheep milk. Cooking temperature especially affected cheese microstructure, texture and composition, while cutting process was largely responsible for fat losses in the whey. Additionally, cheese yield increased with a bigger curd grain size and lower cooking temperatures. Higher cooking temperatures reduced the moisture content of the curd grains and cheese and lead to cheeses with reduced porosity and more free fat in their structure, resulting in harder and chewier cheeses. Interactions between the microstructural arrangement of fat and textural parameters were also observed. These results contribute with new data on the relationships between curd grain size and cooking conditions on the microstructure and physico-chemical properties of cheese. In addition, reducing the compound losses in whey would have a direct effect on the improvement of processing, cheese quality and yield, and the ulterior by product management.

Introduction

The technological conditions used during cheese processing, particularly curd cutting and cooking process, have a direct effect on cheese composition, yield and compound losses in the whey. During cutting and cooking, syneresis of curd occurs and this is affected by several factors well documented in the scientific literature such as: milk composition, firmness of the gel at cutting, gel acidity, temperature, time of cutting, speed of rotation of the cutting and stirring tools and the surface area of the curd (Banks, 2007; Dejmek and Walstra, 2004; Everard et al., 2008; Johnston et al., 1991, 1998; Walstra et al., 2006).

While some authors (Johnston et al., 1991; Whitehead and Harkness, 1954) have suggested that the size of the curd grain plays an important role in the moisture of the final cheese product, others have found that different curd cutting intensities have little effect on the extent of syneresis, although enhanced losses of fat and casein fines were reported (Everard et al., 2008). The relationship between fat and protein losses from the curd and a reduction in cheese yield with curd grain size has been shown to be influenced not only by the cutting revolutions but by a combination of the total revolutions applied during cutting and cooking processes (Everard et al., 2008; Johnston et al., 1991, 1998). Even though cheesemaking processing conditions affect chemical properties of cheese, the studies about the interactions of cutting and cooking settings with microstructure and texture are scarce. Additionally, the comparison of cutting and stirring conditions in this prior studies is made difficult due to technical differences in the cheesemaking processes applied. In this study, the curd grain size was assessed by image analysis to provide information about the curd grain size obtained at the end of the cutting or cooking processes (Aldalur et al., 2019). This method may reduce some of the comparative issues described by other authors, such as the use of different vat designs and sizes, cutting knives, stirrers and cutting and stirring conditions (Everard et al., 2008; Johnston et al., 1998).

Temperature is another factor that enhances moisture expulsion from curd grains, as the permeability of the curd matrix increases at higher temperatures (Fagan et al., 2007). A rapid increase in temperature, however, may lead to a shrunken outer layer of the curd grain, impairing permeability and slowing syneresis (Dejmek and Walstra, 2004). Moreover, temperature greatly affects the properties of fat globules and the spatial arrangement of other components in cheese, which ultimately determine the structure of the cheese and its physico-chemical and sensory properties (Lamichhane et al., 2018; Lopez et al., 2006). As a result, the microstructural observation of cheese and samples taken during the cheesemaking process can be very useful to predict and control the properties of the final cheese product (El-Bakry and Sheehan, 2014). Confocal laser scanning microscopy (CLSM) is a powerful tool, which allows two-dimensional (2D) thin optical sections of a sample to be digitally captured and reassembled to obtain three-dimensional (3D) information (Gunasekaran and Ding, 1998). This imaging allows both fat and protein to be visualised and quantified, providing numerical information about the size, shape and distribution of key components of the curd and cheese matrix. In addition, cryo-scanning electron microscopy (cryo-SEM) provides a detailed image of the surface features of hydrated samples without chemical staining and conventional sample drying (Ong et al., 2011).

Cheese produced from raw sheep milk is particularly important in European countries such as France, Italy, Greece, Spain and Romania (EUROSTAT, 2016). This type of milk is suitable for cheesemaking due to the higher concentration of milk components and improved coagulation properties compared to cow milk (Kammerlehner, 2009; Park et al., 2007). While considerable differences have been observed, the effects of the conditions used during the cheesemaking process using sheep milk in the cheese product, compound losses and yield have been scarcely reported.

The objective of the current study was to investigate the effect of the processing parameters curd grain size, assessed by image analysis, and cooking temperature on the microstructure, texture, composition and yield of cheese from sheep milk.

Section snippets

Experimental design and sampling

A two-level full factorial experimental design was employed, which looked at the effects of curd grain size (CGS) and cooking temperature (CT) on the final cheese product (Table 1). Two levels of CGS were selected: big and small, and two levels of CT: 36 °C and 45 °C, all other cheesemaking conditions remained fixed. The four experimental cheeses were made in two consecutive days using the same bulk raw ewe milk collected from a local farm (Meredith Dairy, Truganina, Victoria, Australia) one

Curd grain size and cooking temperature

Cheesemaking experiments were performed where the curd grain size (CGS) and cooking temperature (CT) were varied to assess the impact of these parameters. The area of the large curd grains assessed by image analysis was significantly (P ≤  0.05) greater than the area of the small curd grains (Table 1). As expected, this difference was significant for grains assessed both after cutting (FCG) and after cooking (SCG), indicating the effectiveness of the manual wire mesh technique and number of

Conclusions

This study indicates that curd grain size and cooking temperature affect the structural and textural characteristics of cheese made using raw sheep milk, in addition to affecting the yield and composition of the whey generated during the cheesemaking process. The cooking temperature had a major effect on cheese composition, increasing the dry matter content of the cheese due to the enhanced syneresis during cooking. A higher cooking temperature significantly altered the texture of the cheese,

Acknowledgements

The authors thank Meredith Dairy for supplying the milk used in this study. Financial support was provided by The University of the Basque Country (PA16/04) and The Basque Government (IT944-16) A. Aldalur thanks The Basque Government for the research fellowship. The research was supported by The Australian Research Council (ARC) Industrial Transformation Research Program (ITRP) funding scheme (Project Number: IH120100005). The ARC Dairy Innovation Hub is a collaboration between The University

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