Modelling casein aggregation and curd firming in goats' milk from backscatter of infrared light

Abstract

A kinetic model was proposed for describing the curd assembly of skimmed goats' milk during enzymic coagulation. The enzymic coagulation of milk was monitored using an optical sensor that measured diffuse reflectance (light backscatter) at 880 nm. The appearance of a shoulder, at low temperatures and protein concentrations, in the diffuse reflectance ratio profile after the inflection point of the curve (Tmax) appeared to separate the aggregation and curd firming steps. The diffuse reflectance ratio profile after Tmax was attributed to the overlapping of casein micelles aggregation and curd firming reactions. The developed kinetic model combined a second order reaction model to describe aggregation reactions and a first order reaction model to describe firming processes reactions. A completely randomised block design with three replications was used to determine the effect of protein concentration and temperature on kinetic constants. Milk was adjusted to three levels of protein (30, 50 and 70 g/kg), and coagulated at five temperatures (20, 25, 30, 35 and 40°C) to test a wide range of processing conditions. Data points from each profile after Tmax were fitted to the proposed model using non-linear regression. The average R2 and standard error of prediction (SEP) for 45 tests conducted were in the range of 0·9975±0·0027 and 0·0081±0·0037, respectively. A significant increase in characteristic times for aggregation (τ2) and curd firming processes (τ1) were found when temperature decreased or protein increased. Theoretical asymptotic value of reflectance ratio, R∞, increased with increasing level of protein and temperature (P<0·05). The parameter β1, which represented the fraction of diffuse reflectance ratio attributed to aggregation, decreased with increasing temperature and decreasing protein.

The effect of temperature and inoculum concentration on rheological and light scatter properties of milk coagulated by a combination of bacterial fermentation and chymosin. Cottage cheese-type gels

“…On the other hand, Castillo et al (2003b) located the onset of aggregation in rennet-induced coagulation close to t max . These authors estimated the kinetic rate constant for the aggregation reaction by fitting the observed light backscatter data points between t max and t 2min to a second-order equation.…”

“…5a, 2) corresponding to t max and t max2 . Castillo et al (2003b) claimed that the appearance of a shoulder in the light backscatter profile after t max showed that gel formation during enzymatic goat milk coagulation consisted of two distinct and overlapping processes caused by the micelle aggregation and the protein network firmness development, respectively. In agreement with these authors, the first derivative peak (i.e., surroundings of t max ) was considered dominated by aggregation kinetics (Fig.…”

“…The correlation between t 2min2 and t gel was very strong (r ¼ 0:992, Po0:0001; Table 4) and the pH values at these two times were not significantly different ( Table 5). The parameter t 2min2 was first defined by Castillo et al (2003b) as the second minimum of the second derivative of the light backscatter profile versus time. These authors estimated the kinetic constant for the curd firming reaction assuming a first-order reaction for the disappearance of crosslinking sites and using as a start time for the curd firming reaction t 2min2 (in curves with two maxima in the first derivative, as was observed in our profiles).…”

“…Infrared light backscatter has been applied to monitor the cottage cheese fermentation process (Payne, Freels, Nokes, & Gates, 1998). More recently, a kinetic model, based on this technique, has been proposed to monitor gel assembly in goat milk rennet-induced gels (Castillo, Payne, Hicks, Laencina, & Lo´pez, 2003b). The objectives of this study were to: (a) monitor and analyze the mixed coagulation process using oscillatory rheometry and infrared light (880 nm) backscatter, and (b) study the effect of starter inoculum concentration and incubation temperature on pH, rheological and light backscatter parameters.…”

An optical method to monitor casein particle size distribution in whey

“…Since the work of McMahon et al [16] and Banon and Hardy [1], a lot of research has been carried out on the use of optical measurement systems for monitoring coagulum formation in cheese-making [3,4,8,14,17,18]. An interesting application of optical sensor technology has been proposed by Castillo et al [5,6].…”