Light Backscatter Applications in Milk and Dairy Foods Processing

Transcription

1 Light Backscatter Applications in Milk and Dairy Foods Processing Fred Payne a, Manuel Castillo b, Mary-Grace Danao c a Univ. of Kentucky, Biosystems and Agricultural Engineering, Lexington, KY, USA ([email protected]) b Univ. Autònoma de Barcelona, Food Science Department, Barcelona, Spain ([email protected]) c Univ. of Illinois, Agricultural and Biological Engineering, Urbana, IL, USA ([email protected]) ABSTACT Development of processing control and automation in the food industry improves production efficiency and quality and provides for more consistent production of food products. Optical sensors, and especially light backscatter sensors, have excellent sanitary configurations, are non - destructive and are appropriate for many inline applications in the dairy industry. Light backscatter applications are divided into two classes. The first class is a single waveband backscatter which is used for monitoring milk coagulation. The second class is a single waveband backscatter with two detectors spaced at different radial distances used to measure optical extinction. A light sensor design was used to measure light extinction of homogenized cream. Typical reflectance profiles for coagulation of milk for soft cheese (cottage) and hard cheese are presented. The selection of time parameters from the derivative curves is illustrated. One time parameter, t max, was found to be a precise measure proportional to the enzymatic reaction rate and was shown to be a strong predictor of cutting time. A typical response for a soft cheese culture is shown and discussed. The data shows that light backscatter sensors have applications in the dairy industry for online process control applications. They are relatively low in cost, robust, sanitary and easily maintained in the dairy processing environment. The result is that light backscatter sensor technologies offer a very sensitive tool for inline measurement and control applications, especially for milk coagulation. Keywords: Milk; coagulation, sensor; optical; backscatter INTODUCTION Further automation is needed in food processing facilities to maintain competitiveness in the world economy, improve product consistency and quality, and increase processing efficiency. The lack of suitable sensors for characterizing the properties of liquid particulate food materials is hindering the implementation of modern process control technologies. Improved sensors will allow for tighter production tolerances, increased food processing consistency, increased information for real time process optimization, and increased saving through reductions in the use of raw materials, energy and wastes. The cheese-making process is a series of steps that starts with the enzymatic coagulation of milk. The enzymes break down casein and gradually convert the liquid milk into a gel (coagulum) over a 30 minute period. When the gel has sufficient textural strength, it is cut into small pieces. Determining the optimal time to cut the coagulum for hard and soft cheese (cottage cheese culture) was the objective of the single waveband backscatter technology. There is a specific need for a low cost in-line sensor technology to control milk fat content of creams (35 to 45 wt% milk fat) from separators. A sensor that provides separator control will give better product consistency and minimize heating and cooling costs while improving downstream pumping operations. The goal of this work was to confirm the potential of a light backscatter sensor that utilizes a single waveband with two detectors spaced apart for measuring fat content in cream. MATEIALS & METHODS A sensor with two optical fibers, as shown in Figure 1, was used to monitor the coagulation step. An LED light source directs light into an optical fiber that terminates in the milk and an adjacent fiber separated by a distance r 1 receives the light backscatter. The uniqueness of this light backscatter configuration is that the light must traverse through a finite distance of fluid (free of specular reflectance) and be backscattered by particulates to the receiving fiber. This configuration was used to measure the change in light backscatter during milk coagulation.

2 COAGULATING MILK I(r) - LIGHT INTENSITY AS A FUNCTION ADIAL DISTANCE POBE r FIBE LED LIGHT SOUCE, 880 nm I F, PHOTODETECTO ESPONSE Figure 1. Schematic of a single waveband backscatter sensor used for cutting time determination in cheese processing. Figure 2 shows the fiber optic measurement of intensities at two radial distances, r 1 and r 2, from the emitting fiber. The sensor employed three optical fibers: one that delivered light to the scattering media and two others that measured light intensity (I 1 and I 2 ) at spatially separated points r 1 and r 2, respectively ( 1 mm apart). PATICULATE FLUID I = f(r) LIGHT INTENSITY AS A FUNCTION OF ADIAL DISTANCE r FIBE OPTIC POBE r 1 r 2 FIBE LED LIGHT SOUCE I 1 I 2 F 1 F 2 PHOTODETECTO ESPONSE F 1 AND F 2 Figure 2. Schematic of the optical fiber configuration tested for measuring composition showing a LED focused on a fiber and two receiving fibers at radial distance r 1 and r 2 focused onto detectors with outputs F 1 and F 2. A relatively simple empirical correlation between the distribution of backscattered light intensity and the particle concentration was utilized by adapting a widely used diffusion approximation equation presented by Bolt and ten Bosch [1]: exp( β C r) I ( r) = I Equation (1) 0 m r where: I 0 = apparent intensity at radial center line of emitting fiber I(r) = Light intensity as a function of radial distance from the emitting fiber β = specific backscatter light coefficient C = concentration of particulates

3 m = exponent relating light diffusion in the radial direction r = radial distance of the receiving fiber (centerline to centerline), mm. The backscatter light coefficient, β, is based on the ability of the sample to scatter light and depends on the optical and radiative properties of the particles in the sample. The value of m depends on whether the detector is placed in the intermediate area (m = ½) or the diffusion area (m = 2). The diffusion area is defined as the area in which sufficient multiple scatterings have taken place, so that the diffusion approximation is valid. In the development of a sensor, the use of signal ratios has the advantage of normalizing the resulting response. This isolates the signal ratio from changes in light intensity and some changes to the physical system (optics, mechanical connections, etc.). For the fully developed diffusion area, the ratio of the intensities at two radial distances (r 1 and r 2 ) using Equation (1) reduces to the following equation: I ( r ) I ( r r m 1 2 = exp 2 ) r 1 ( K ( r r )) 2 1 Equation (2) where r 1 and r 2 are radial distances for fiber 1 and fiber 2, respectively, and K is a constant for a specific fat content. This equation predicts an increasing signal ratio with increasing milk fat. Light scattering is dominant for high concentrations of fat and the widely used diffusion approximation is valid for this case. ESULTS & DISCUSSION A consistent light backscatter pattern (reflectance ratio ratio of light backscatter signal to initial signal at time zero) was found and is shown in Figure 3. The reflectance ratio is initially relatively steady, traverses through a sigmoidal increase and then gradually increases at a decreasing rate. The first chemical reaction during the enzymatic coagulation of milk for cheese making is the reaction of chymosin and casein. This enzymatic reaction phase destabilizes the colloidal nature of casein micelle but does not significantly affect light backscatter. This reaction proceeds as a first order chemical reaction when the substrate concentration (casein) is in excess supply in relation to the enzyme concentration. The second phase is the aggregation phase of the destabilized casein micelle into flocks which grow and consume the entire casein particulate system. It is during this phase when larger particles are formed and the reflectance of light increases rapidly. The aggregation reaction follows a second order chemical reaction. The third phase is the gel forming phase which results from a micro consolidation of protein strands that result in stronger protein strands and an increasing space within the protein matrix. This phase follows a first order chemical reaction. These three chemical reactions overlap with the resulting reflectance profile as shown in Figure 3. The reflectance of milk at 880 nm typically increases 15% to 40% during coagulation providing a strong response signal. 1.4, IO T A 1.2 E C N A T 1.0 C E L F E 0.8 Light Backscatter Profile for Milk Coagulation tmax TIME (min) Figure 3. A typical light backscatter profile,, for coagulating milk is shown with the first derivative,, the second derivative,, and the time-based parameter, t max. tcut ' ''

4 The goal of signal analysis is the extraction of information which will allow accurate prediction of the cutting time from the profile. A successful cutting time prediction technology was developed by utilizing the strong correlation between two time events; t max and t cut. The correlation, however, needed to be corrected for protein content to develop a precise prediction. The following cutting time prediction equation was determined to predict the cutting time with an approximate precision of ± 1 min: t cut = β t max (protein correction) Equation 3 where β = a calibration constant (usually determined by the cheese maker) which typically varies from 1.3 to 2.5. Equation 3 requires only t max. (a time-based parameter) and protein content to make a cutting time prediction. The time-based parameter, t max, was shown by Tabayehnejad et al. [2] to be an exact measurement proportional to the enzymatic reaction rate. The change in reflectance does represent product changes; however, numerous attempts at extracting consistent information from reflectance changes have proved fruitless. Figure 4 shows observed and predicted cutting time for a broad based test of conditions typically encountered in cheese making as reported by Payne et.al.[2]. Figure 4. Measured and predicted cutting time using the cutting time prediction equation Eqn. 3 and data from Payne et.al [3]. The light backscatter was measured during the culture of cottage cheese. A typical reflectance profile is shown in Figure 5. Figure 5. A typical light backscatter profile,, for cottage cheese culture showing the first derivative,, the second derivative,, and the time-based parameters.

5 A cutting time prediction equation was determined for cottage cheese culture using time-based parameters and a multiple linear regression. The profile is more complicated in that there are two maxima in the first derivative. The second noteworthy difference is that light backscatter increases 200% during the process providing a very strong signal. The ratio of light intensity response, (I 2 /I 1 ), measured with two detectors spaced at different radial distances was referred to as signal ratio and was found as predicted by Equation 2 to increase with fat content. The signal ratio for homogenized whipping cream diluted in water is shown in Fig. 6. Figure 6. Light backscatter signal ratio as a function of homogenized whipping cream An LED with a wavelength (36% of 470 milk nm was fat) selected at 25 C for for these light experiments at 470 nm. because the response with signal ratio appeared to follow Equation 2 with a slight positive increase. A single waveband backscatter with two detectors spaced at different radial distances that measures optical extinction has several applications in the dairy industry where an inexpensive sensor is required to monitor and control product composition. A light extinction sensor is a response-based sensor normalized by dividing two signals. Further testing to optimize light wavelengths and fiber spacing is required before a light extinction sensor can be developed with the accuracy and precision required by the dairy industry. From experience, it is known that the attenuation of light varies significantly between products. Thus the determination of optimum fiber spacing and wavelength is required for different products. CONCLUSION Light backscatter sensor technologies offer a very sensitive tool for inline measurement and control applications, especially for milk coagulation. The two types of backscatter configurations tested were single waveband with one detector and single waveband backscatter with two detectors spaced at different radial distances. The single waveband backscatter provides time-based parameters and has shown to be useful in milk coagulation monitoring. Optical extinction measurement was tested for determining fat content of a homogenized cream with encouraging results. The consistence of particle size within a product could be the major limitation for measurement of a milk property using light backscatter. Optical sensors are relatively low in cost, robust, sanitary and easily maintained in the dairy processing environment. The future direction as viewed by the first author is that novel inline optical sensor technologies will employ multiple light wavelengths and, in essence, be simple dedicated spectrometers. The challenge for researchers will be the extraction of information from a host of available data to provide a sensor technology that measures or controls a process to the exact standards required by the dairy industry.

6 EFEENCES [1] Bolt,. A. and J. J. ten Bosch Methods for measuring position-dependent volume reflection. Appl. Optics 32: [2] Tabayehnejad, N., Castillo, M., and Payne, F. A Comparison of total milk-clotting activity measurement precision using the Berridge clotting time method and a proposed optical method. In review [3] Payne, F.A., Hicks, C.L., and Shen, Pao-Sheng Predicting optimal cutting time of coagulating milk using diffuse reflectance. J. Dairy Science, 76: DISCLOSUE Some of the technologies derived from this research are undergoing patenting processes and are the subject of licensing agreements between the University of Kentucky and a private entity, eflectronics, Inc., Lexington, KY, in which one of the authors, Fred Payne, has equity. A University of Kentucky Conflict of Interest Committee is overseeing his work relating to this project to ensure research integrity.