Measurement of syneretic properties of rennet-induced curds and impact of factors such as concentration of milk: a review

Background The rate or extent of whey expulsion or syneresis from cheese curds during stirring in-vat determines curd moisture levels, which subsequently influences cheese moisture content. The outward migration of whey depends on curd contraction and on the structure of the pores permitting whey movement. Curd syneretic properties are one of the least understood areas of cheese science, particularly when milk of varying composition is used. Scope and Approach This review provides an insight into the mechanisms of curd formation and curd syneresis, and factors influencing syneretic properties in unconcentrated and concentrated milk and appraises syneresis measurement methods in terms of their relative strengths and weaknesses. Key Findings and Conclusions Direct measurement of moisture content of curds is recommended as a simple and reliable method for measurement of syneresis of industrial relevance and, although inline measurement for curd moisture prediction has been a significant development in the last decade, its application to commercial production is still limited. A review of previous studies found that experimental conditions and methodologies used to measure syneresis vary widely, making it difficult to compare data between studies. Overall, interactions between process variables employed determines whether syneresis is accentuated or inhibited, and this can be exploited by cheese producers to attain target curd moisture contents by varying process parameters, M AN US CR IP T AC CE PT ED ACCEPTED MANUSCRIPT particularly when milk is concentrated prior to cheese-making. Furthermore, further studies should be focused on endogenous syneresis and casein network rearrangement to clearly elucidate this mechanism and its influence on macrosyneresis under dynamic conditions.


M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT
particularly when milk is concentrated prior to cheese-making. Furthermore, further studies should be focused on endogenous syneresis and casein network rearrangement to clearly elucidate this mechanism and its influence on macrosyneresis under dynamic conditions.

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT
1. Introduction 21 In cheese-making, the rennet-induced coagulum is cut into small cubes of curd (5 to 15 22 mm) and subsequently stirred in-vat to release a large proportion of the occluded whey 23 (syneresis). Expulsion of whey from the curds during stirring influences curd moisture content 24 prior to drainage, and thereby the final moisture content in cheese (Everard et al., 2008;Giroux, 25 Bouchard, & Britten, 2014). For instance, in making hard-type cheese, a lower level of curd 26 moisture content is targeted than in making soft-type cheese, i.e., higher moisture varieties. 27 Whey expulsion from cheese curds during in-vat stirring is a complex phenomenon which 28 is influenced by multiple conditions prevailing in the cheese vat, e.g., milk composition, curd 29 firmness, curd particle size, stirring speed, stirring time, temperature, acidity, enzyme  When a renneted milk sample is monitored by rheometry, at the early stages it does not 73 show any consistency in signal, which continues for the time taken by rennet to hydrolyse the 74 casein micelles. The sample first exhibits some strain, with an apparent increase in storage 75 modulus (G'), when hydrolysed casein micelles start forming aggregates through Ca ++ bridges. 76 Although various studies have defined rennet coagulation time (RCT) as the time when viscous 77 modulus (G'') is equal to elastic modulus (G') of a gel, the first consistent increase in G' can also 78 be taken as the RCT. The storage modulus of the gel increases continuously due to increased 79 networking of hydrolysed casein micelles in the 3-dimensional protein network. Mellema, 80 Walstra, van Opheusden, and van Vliet (2002) proposed that movement of casein micelles or 81 clusters by detaching at weak bonds at one junction and connecting to another junction 82 eventually leads to increased G' values. Therefore, an increase in G' at a given time is due to 83 increased strength of the bonds between micelles, which is due in part to the rearrangement of 84 casein micelles in the network. 85 The firmness of gels for cheese-making is an important criterion. The gels should be capable of Bijgraat, 1988). However, when the shrinkage is constrained, the network rearrangement 112 produces an inhomogeneous structure, resulting in increased gel permeability, which favours 113 whey migration within the gel, inducing microsyneresis (Van Dijk, 1982). 114 The endogenous syneresis pressure has been reported to increase with the gel permeability and    187 Calcium is added to cheese milk for promoting protein-protein linkages, which provides   are applied externally to the cheese curds (curd size, stirring speed, temperature, cooking, etc).              Curd particle size (and curd particle size distribution) can also be determined using mesh 429 sieve of different sizes (12, 7.5, 5, 3 mm) stacked one above another, using a cascade approach. 430 The highest mesh size is at the top and particles are allowed to follow downwards to the lower     455 Zviedrans and Graham (1981) developed a tracer method to determine syneresis in curd.

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A high molecular weight blue dextran (2000 kDa) was added to whey/curd after cutting; the 457 concentration of blue dextran in whey decreased during stirring, due to dilution by exuded whey.

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The concentration of blue dextran was measured by absorption at 620 nm over the syneresis 459 time, and the amount of whey released could be predicted as: real-time in industrial cheese-making. However, validation of this method is necessary. 477 Whey measurement has been carried out by centrifuging curds/gels at different stages.   where Rt is the light backscatter ratio at time t, R α = is the light backscatter ratio at infinite time,

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Ro is the light backscatter ratio at time zero, and k is the kinetic rate constant (min -1 ).

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Prediction of moisture content using NIR technology has been reported to be highly