Donal J. O'Callaghan

On-line sensing techniques for coagulum setting in renneted milks

Abstract In cheesemaking, the on-line measurement of clotting time, coagulum firmness and cutting point is useful for process monitoring but has proven difficult to carry out. This work presents a comparison of on-line sensors, more or less capable of allowing the determination of these data. An experiment was undertaken in which six on-line sensors, comprising of optical systems (NIR transmission X2 wavelengths and NIR reflectance), hot-wire, and vibrational systems (torsional and transverse), were used simultaneously to monitor milk coagulation in trials where protein levels and enzyme concentrations were varied and the sensor response characteristics were compared with oscillatory shear measurements. It was found that protein level mainly influenced curd firming rate, while rennet level mainly influenced gel time. Up to three coagulation parameters were derived using each sensor. While all sensors were able to detect the effect of rennet level, which is mainly seen in the gelation (or clotting) time, the sensors varied in their sensitivity to changes in protein level, which mainly affects the curd-firming time. For some sensors, a combination of response parameters was shown by multiple linear regression to be advantageous for predicting coagulum-firming time. By combining response parameters in this way the standard errors of prediction of coagulum cutting point (using oscillatory shear measurements as the reference) were better than 100 s for NIR reflectance, NIR transmission and low-frequency vibration systems as compared with ca.300 s for the hot-wire or torsional vibration systems.

Rennet coagulation properties of retentates obtained by ultrafiltration of skim milks heated to different temperatures

Skim milks were heated at temperatures ranging from 72 °C × 15 s to 100 °C × 120 s to give levels of whey protein denaturation (WPD) varying from ∼1 to 70%. The different milks were then ultrafiltered to yield retentates with protein levels ranging from ∼3 to 19%. Rennet was added on a volume basis and the coagulation properties of renneted retentates measured by low amplitude oscillation rheometry. For all heat treatments, the shear storage modulus, G′, increased more than proportionally with protein concentration (P), i.e. G′ ∞ Pn, where n ∼ 2, at times ⪢3600 s. A similar trend was noted for curd firming rates, with the power law index being ∼2.4. On heating under normal pasteurization conditions, i.e. 72 °C × 15 s, gelation time increased slightly with protein concentration. At the higher heat treatments, gelation time decreased markedly with protein concentration (especially at levels of WPD ∼70%), and approached those of the normal pasteurized milk at protein levels of 12–18%, depending upon the heat treatment and gel time definition.

The use of a simple empirical method for objective quantification of the stretchability of cheese on cooked pizza pies

Abstract An empirical, easy-to-use, tensile method for the objective quantification of stretch of molten cheese was developed. Shredded cheese was distributed uniformly, at a fixed loading (0.25 g/cm 2 ), on to a pizza base, pre-cut in half. After cooking in a thermostatically controlled electric fan oven at 280 °C for 4 min, the pizza pie was placed on the platform unit of the stretch apparatus. The platform unit consisted of fixed and rolling elements. Before stretching, the pizza pie was clamped (one half to the fixed element and the other half to the rolling element). The rolling element was then drawn along the rail system at a fixed speed, resulting in stretching of the molten cheese mass until complete failure of the extended string(s)/sheet connecting both halves of the pizza base. Stretch was defined as the distance travelled by the mobile element to the point of complete strand breakage. The stretch values obtained for low-moisture Mozzarella of different ages were influenced by cheese load and holding time before stretching of the cooked pizza pie, but were independent of stretching speed in the range 0.033–0.100 m/s. The Stretchability of low-moisture Mozzarella cheese (2–17 weeks old) was greater than that of Cheddar cheese (2–25 weeks old). Analogue pizza-type cheeses, with compositions similar to that of low-moisture Mozzarella cheese, showed large inter-product variation in stretchability and generally had inferior stretch compared with low-moisture Mozzarella cheese.

Process viscometry for the food industry

Real-time monitoring and control of food processing applications have grown significantly over the past decade. Food processors are increasingly demanding automated and real-time techniques, which are capable of withstanding the process environment coupled with meeting sanitary requirements. As a result new process monitoring techniques are emerging and traditional techniques are being modified to meet these demands. Commercial instrument designs are reviewed specifically for the food industry with a focus on their advantages and limitations combined with novel modifications developed to overcome such limitations. Emerging technology, which has potential within the food industry, is also discussed.

A comparison of on-line techniques for determination of curd setting time using cheesemilks under different rates of coagulation

Abstract An experiment is described where six on-line techniques for monitoring milk coagulation (NIR diffuse reflectance, NIR transmission (two wavelengths), hot-wire, torsional vibration and tuning-fork vibration) were compared with rheological measurements of curd firming (elastic shear modulus) under a range of coagulation rates, effected by varying enzyme level in fresh whole milk. Viscous and elastic moduli were measured during renneting and were analysed to determine their usefulness for determination of a gel point. G ′ and G ′′ each gave a suitable measure of gel time as indicated by compliance with Holters Law. The correlation of gel points determined by G ′′ with those determined by G ′ showed that a sensor which detects changes in viscosity per se works in principle as a gel point detector in cheesemilk. Gel times determined from G ′′ were slightly longer than those determined from G ′ which in turn were slightly longer than those determined from tan δ , with the differences becoming larger at lower enzyme levels, i.e. longer gel times. Gel times based on G ′′ were just as good predictors of cutting time as those based on G ′ . In addition the asymptotic behaviour of tan δ showed that a viscosity sensor can determine an optimum point of curd-cutting, which is usually defined in terms of a large value of G ′ . All on-line techniques modelled rheological gel time (by rheometer) with an accuracy of ca. 1 min. and predicted curd cutting time with an accuracy of ca. 2 min.

Milk protein standardization by ultrafiltration for Cheddar cheese manufacture

Skim milks were concentrated by ultrafiltration. Cream was added to the retentates to give cheesemilks standardized to a casein :fat ratio of ∼ 0.74 with protein levels ranging from 30 to 46 g/l. Pasteurized cheesemilks were renneted on a volume basis (22 ml single strength calf rennet/100 l) and converted to Cheddar cheese in 5001 cheese vats. Set temperatures were reduced from 31 to 28 °C with increasing milk protein level to normalize curd firming rates and prevent curd shattering on cutting the curds. Cheesemaking was otherwise as normal for Cheddar cheese. The proportions of milk fat and protein lost in the cheese whey were not significantly influenced by milk protein level. Moisture-adjusted cheese yields increased with milk protein at a rate similar to that predicted by the Van Slyke cheese yield equation. However, owing to the negative correlation between cheese moisture and milk protein concentration, actual yields increased at a lower rate with respect to milk protein than moisture-adjusted yields. Increasing milk protein levels resulted in significant (P < 0.01-0.001) decreases in the concentrations of moisture, moisture-in-non-fat cheese solids and fat-in-dry matter in the cheese and increases (P < 0.05-0.001) in the levels of protein, salt-in-moisture, Ca and P. While increasing milk protein concentration resulted in significant (P < 0.05-0.01) reductions in the levels of water-soluble N at all stages of the 270 d ripening period, it had little influence on the sensory scores awarded for aroma/flavour at 180 or 270 d.

Prediction of Inorganic Salt and Moisture Content of Process Cheese Using Dielectric Spectroscopy

The development of on-line sensors for compositional analysis during cheese manufacture is desirable for improved quality control. Dielectric properties of a food product are principally determined by its moisture and salt content. This indicates that dielectric spectroscopy may offer a rapid, on-line and non-destructive method for the determination of moisture and salt content of process cheese. However limited information is available in the literature on the dielectric properties of process cheese. Therefore the aims of this study are to investigate the dielectric properties of process cheese samples over a range of compositional parameters and to assess the potential of dielectric spectroscopy to improve process control during process cheese manufacture. Dielectric spectra of process cheese samples were measured using a coaxial line probe between 300 MHz and 3 GHz. A clear tend was observed between higher moisture content and increases in the dielectric constant. Inorganic salt content was found to have a major influence on the loss factor. The dielectric data obtained was used to develop chemometric models for the prediction of moisture and inorganic salt content of two experimental sets of process cheese samples (exp A and exp B). The root mean square error of prediction (RMSEP) for the models developed to predict moisture content were 0.524% (w/w) (exp A), and 0.423% (w/w) (exp B), while the RMSEP of the inorganic salt models were 0.220% (w/w) (exp A), and 0.263% (w/w) (exp B). It was concluded that dielectric spectroscopy has potential application for compositional analysis in process cheese manufacture.

Correlation Between Process Cheese Meltability Determined by Sensory Analysis, Computer Vision Method and Olson and Price Test

The meltabilities of 14 process cheese samples were determined at 2 and 4 weeks after manufacture using sensory analysis, a computer vision method, and the Olson and Price test. Sensory analysis meltability correlated with both computer vision meltability (R2 = 0.71, P < 0.001) and Olson and Price meltability (R2 = 0.69, P < 0.001). There was a marked lack of correlation between the computer vision method and the Olson and Price test. This study showed that the Olson and Price test gave greater repeatability than the computer vision method. Results showed process cheese meltability decreased with increasing inorganic salt content and with lower moisture/fat ratios. There was very little evidence in this study to show that process cheese meltability changed between 2 and 4 weeks after manufacture.

Comparison between red-green-blue imaging and visible-near infrared reflectance as potential process analytical tools for monitoring syneresis.

The current work focuses on the comparison of 2 on-line optical sensing systems; namely red-green-blue imaging and visible-near infrared reflectance, for monitoring syneresis during cheese manufacture. The experimental design consisted of 3 temperature treatments carried out in an 11-L cheese vat in triplicate. Both systems were shown to predict syneresis without significant differences in prediction accuracy. However, a single-wavelength near infrared model was the most parsimonious (standard error of prediction=4.35g/100g) for predicting syneresis. This technique was also the simplest in terms of parameters in the model (standard error of prediction=4.15g/100g with 2 parameters), when time after gel cutting and process parameters (temperature and cutting time) were included in the models. The study showed that either system could be employed to control syneresis in cheese manufacture and improve the control of moisture content in cheese.

A Novel Sensor to Monitor Colour Changes during Cheese Curd Syneresis in a Vat

Syneresis is a critical processing step in cheese manufacture, follows the cutting of milk coagulum into cubes and is promoted by stirring. During the syneresis phase shrinkage of the casein micelle network induces whey expulsion from the curd grains. The rate and extent of syneresis plays a fundamental role in determining the moisture, mineral and lactose content and texture of drained curd and hence that of the final cheese.