Module 6. Process control and automation
Lesson 17
SPECIAL INSTRUMENTS FOR THE DAIRY INDUSTRY
17.1 Introduction
This lesson will discuss some the special instruments used in dairy industry such as e-tongue, e-nose, robotics, etc. The topics discussed in this lesson will be useful to students to know about the state of the art technology used in industry of processing of food products.
For increasing efficiency and optimizing various processing operations in either dairy or the food industry, many special instruments with state of art design and applications are increasingly finding place today. E-nose, E-tongue and robotics are few of such devices which amalgamate advances in electronic hardware and software to bring out novel applications.
17.2 E-Nose
E-Nose (Electronic-Nose) or artificial olfaction use multisensory arrays and neural computing to analyze odour or aroma as a whole as perceived by human nose. The system mimics the human olfactory system and thus finds routine application for odour and aroma analysis in the food, beverage, chemical and packaging industries. The modular sensing system comprising of the array of different sensors transduce the chemical quality into electric signals followed by appropriate signal conditioning and processing to classify known odours or identify unknown odours. The sample identification is carried out through the use of pattern recognition routines operated by artificial neural networks (ANNs) to evaluate data from sensor arrays. Once the ANNs are trained for odour or volatile compound recognition, the operation consists of propagating the sensor data through the network. Briefly, sensors in E-Nose act in response of a number of organic and inorganic compounds in the ppm range. The response can be highly reproducible. Application of E-Nose permits many sorts of diverse sample to be examined.
17.2.1 Principle of E-nose
The functions of human olfactory system comprises of operations such as sniffing, reception, detection and cleansing. The odour sensation depends upon the interaction of volatile molecules with a group of specialized nerve cells called olfactory receptors, situated just above the nose out of the main air stream. The first step in the olfactory recognition is the dissolution of the chemicals in the sample in an aqueous mucous layer covering the olfactory receptor cells. The interaction of odourant molecules with olfactory receptors results in excitation of the receptors cells thus producing electrical signals which pass to the olfactory bulb. The electrical signals are further processed and finally transferred to the brain.
The E-Nose mimics the human olfactory system and each chemical sensor functions like a group of olfactory receptors resulting in a time dependent electrical signal in response to an odour. The number of odorous substances the system can recognize depends on the number of sensors used. Electronic noses usually utilize a data pre-processor, which is analogous to the olfactory bulb in the human olfactory system. The pre-processor compresses the signals and amplifies the output in order to reduce noise and improve the sensitivity of the sensor. The digital signals are then ready for analysis by the computer.
17.2.2 Sensors
The chemical sensors are detectors that measure changes in the conductance when they come in contact with volatile chemical compounds. These compounds react with the sensitive materials on the surface of the sensors interfaced to a transducer (signal transforming device). The interaction brings about some physical changes that are sensed by the transducer and converted into output signals which can be captured and interpreted. The measurement principles to detect the chemicals are: electrical, thermal, optical and mass changes. Some of the popular sensors for the e-noses are:
17.2.2.1 Chemo resistor sensors
17.2.2.2 Metal oxide semiconductors (MOS)
MOS measures changes in electrical-resistance occurring as a result of vapours of the odorous substances getting adsorbed onto the semiconductor surface. When oxygen is adsorbed on to the sensor surface, electrons are removed from semiconductor and thus its electrical resistance is increased. When reducing gases interact with the surface adsorbed oxygen, it decreases trapped electron leading to increase in electrical conductivity of the sensor. MOS are generally operated at higher temperatures (up to 400 OC). Metal-oxide sensors are fairly sensitive. Metal oxides such as SnO2, ZnO, Fe2O3 and WO3 respond to reducible gases such as H2, CH4, CO, C2H5 or H2S and increase their conductivity. Contrary to these semiconductors such as CuO, NiO and CoO respond to oxidizable gases such as O2, NO2 and Cl2. The major limitation of these conductors is that they operate at elevated temperatures and hence energy requirements are very high.
17.2.2.3 Conducting polymers (CP)
Conducting polymers are usually synthesized by chemically or electrochemically oxidizing of the corresponding monomers. There is reversible adsorption of molecules to the films inducing a temporary change in the electrical conductance of the film by altering the population of active charge carriers in the polymer structure. When these sensors are exposed to a particular vapor, each polymer layer undergoes a characteristic swelling, drawing the conducting particles away from one another and thus increasing the measured resistance across the capacitor. Compared with metal oxides, organic polymers are much more diverse. CPs has high sensitivities, short response time and can operate at room temperature. Furthermore, conducting polymers have good mechanical properties which allow flexibility in fabrication of sensors.
17.2.2.4 Chemocapacitors (CAP)
The CAP sensors work on the following principals. In the first state, only air present in the sample environment is incorporated into the polymer and the baseline value of capacitance is obtained. In the second stage, gaseous analyte molecules that are present in the sampling environment are adsorbed onto the sensitive polymer layer resulting in change in its electrical (e.g. dielectric constant) and physical properties (e.g. volume). This deviation over the baseline value is measured.
17.2.3 Electrochemical sensors
17.2.3.1 Metal oxide semiconductor field effect transistors (MOSFET)
The micro-chemosensors are based on the structure of a MOSFET. Catalytic metals or metal alloys (e.g., Pd, Pt or Ir alloys) are used as materials of construction of gates of the transistor which are then left exposed to air. The adsorbed gases interact with the sensor and change the surface-charge density resulting in change of the potential of the device. Selectivity of MOSFET sensors is achieved by the choice of the operation temperature, the metal on the gate and by varying the microstructure of the metal.
17.2.3.2 Amperometric sensors
An amperometric sensor has a working, counter, and reference electrodes that are dipped in an electrolyte. The sensors record the current in the electrochemical cell between the working and counter electrodes as a function of the analyte concentration. The molecules of the analyte diffuse into the electrochemical cell and to the working electrode surface through a porous membrane. Then, the analyte reacts electrochemically, and through either oxidation or reduction process produces or consumes electrons at the working electrode.
17.2.4 Optical odor sensors
17.2.4.1 Surface plasmon resonance (SPR)
SPR works based on an optical phenomenon. The incident light excites a charge-density wave at the interface between a highly conductive metal and a dielectric material. The conditions for excitation are determined by the characteristics of the metal and the dielectric material. Optical SPR sensors are sensitive to the change in the refractive index of a sample surface and thus measures small changes in the refractive index of a thin region adjacent to the metal surface.
17.2.4.2 Fluorescent odor sensors
The optical sensing element is composed of a reagent phase immobilized at the fiber tip by either physical entrapment or chemical binding. This reagent phase contains a chemical indicator which interacts with analyte gases or vapours and experiences the change in optical properties. The responses depend upon the nature of the organic vapor and the strength of its interaction with the different polymer systems used. The sensitivity of optical chemosensor is generally not high (detection limits is 1000 ppm).
17.2.4.3 Gravimetric odour sensors
Gravimetric odour sensors use acoustic wave devices which operate by detecting the effect of sorbed molecules on the propagation of acoustic wave. Two types of acoustic wave odour sensors viz., bulk acoustic wave (BAW) sensor and Surface acoustic wave (SAW) sensor are used. In both the types, the basic device consists of a piezoelectric substrate such as quartz, lithium niobate and ZnO coated with a suitable sorbent coating. The sorption of vapour molecules onto the sorbent membrane coated with the substrate affect propagation of the acoustic wave leading to changes in the resonant frequency and the wave velocity.
17.2.5 Data analysis in E-Nose
For sample identification, different pattern recognition techniques are used to analyze data derived from an E-Nose. Pattern recognition is a process of identifying structure in data by comparing it to known structure. The different pattern-recognition modules are used to obtain instrumental correlation to sensory tests for evaluating the data recorded from the array of sensors. These techniques could be statistical methods, proprietary pattern recognition methods or Artificial Neural Networks (ANNs). The statistical or chemometric approach, complementary to ANNs includes principal components analysis (PCA), partial least squares (PLS), discriminate analysis (DA), discriminate factorial analysis (DFA) and cluster analysis (CA).
17.2.6 Application of E-Nose in food industry
E-Nose has many applications which include its uses in evaluation of sensory quality of foods, detection of environmental toxins, carcinogens and pollutants, air quality as well as in medicine. Some specific functions related to food industry are listed below:
· Identification of adulteration of many raw and processed foods
· Presence of contaminants such as diacetyl detection in orange juice, identification of milk from mastitis-affected cows etc.
· Quality control applications like recognizing sensorily acceptable and rejectable samples to evaluate raw materials and finished products.
· Product and formula matching for comparing one’s product with a competitors’ sample
· Classify various types of cheese, various strains of bacteria, and to determine the odour from paper based packing material for foods.
· Detecting aroma differences among samples of dairy products like ice cream
17.3 E-Tongue
E-tongue refers to an array of sensors that are immersed in liquids in order to identify their “tastes”. It has widespread applications in dairy and food industries to monitor the quality of products. The major advantage of e-tongue is that unlike human beings the sensitivity does not decrease even during prolonged exposure. It also permits evaluation of toxic substances which cannot be tasted by human beings.
17.3.1 Sensors
Depending on the task in hand, the number of sensors in the sensor arrays may vary from 4 to 40. Sensors based on different principles of signal transduction (e.g., potentiometric and amperometric) may be used simultaneously in the same sensor array. Potentiometric ion-selective electrode (ISE) sensors are more popular sensors in electronic tongue systems. Potentiometric measurement is carried out using a multichannel voltmeter with high input impedance. Values of the sensor potential is measured against a conventional Ag/AgCl reference electrode and stored as computer data files. Sample pretreatment prior to measurement is not necessary. Sensors made of chalcogenide glass and PVC-based polymer membranes with enhanced cross-sensitivity can be incorporated into a sensor array. A number of conventional ISEs, e.g., pH glass electrode, sodium- and chloride-selective electrodes, may also be included in the system. An array of sensors made of ultrathin films of polymers and composite films of several polymers are also used in the e-tongue system. Such films are deposited on top of a glass substrate that holds interdigitized microelectrodes. Sensors prepared from different materials produce different electric responses and their variation allows a “fingerprint” of the samples. Like e-nose, the e-tongue also consists of hardware and software components. The hardware is used for the capacitance measurements of sensorial units and the software controls the data acquisition, perform the calculations and analyze the electrical signals. The main hardware components are: signal generator; signal amplifier; multiplexer; data acquisition board and a lap-top computer. The software component deals with electrical signals and provides the capacitance values, which are stored into files using a pre-defined format. The software interface allows the definition of parameters to control the data acquisition. The data analysis is similar to that in case of the e-nose.
· Continuous control on product quality
· Detection of pollutants in water (environmental applications)
· Detection of analytes in low concentration solutions– difficult to be distinguished by human being or even impossible.
· Quantitative analysis and recognition (identification, classification) of a very wide range of liquids
· Quality control and identification of the conformity to standards for different food stuffs-juices, coffee, beer, wine, spirits, etc.
· Taste quality of a food to be monitored continuously from the raw material stage right through to final product.
17.4 Robotics
Robotics is a concept designed at creating a machine equivalent to humans. It was the famous Czech writer Karel Capek who coined the word robot. The word robot comes from the word robota, meaning "drudgery" or "hard work" in modern. Robot Institute of America (1979) defined a robot as a reprogrammable, multifunctional manipulator designed to move material, parts, tools or specialized devices through variable programmed motions for the performance of a variety of tasks. Robotics is a system that comprises of sensors, control systems, manipulators, power supplies and software all working together to perform a task. Designing, building, programming and testing a robot involves combining frontier areas of different sciences viz., physics, mechanical engineering, electrical engineering, structural engineering, mathematics and computing. In some cases biology, medicine, chemistry might also get involved.
A robot has the following essential characteristics: A robot is to be first able to sense its surroundings. It could accomplish this task by first using its sensors: light sensors (eyes), touch and pressure sensors (hands), chemical sensors (nose), hearing and sonar sensors (ears), and taste sensors (tongue). A robot then needs to be able to move around its environment. It could achieve this by rolling on wheels, walking on legs or propelling by thrusters. The robot can either move whole of itself or just parts of it (arm, neck etc). A robot can be powered through either solar energy or electrical energy. A robot also needs intelligence which can be supported by appropriate software programs.
17.4.1 Application of robots in dairy and food processing operations
Robots are finding applications in many food processing activities because of their versatile features, some of which are being listed below. In high speed repetitive operations in modern day food processing and packaging where human operators begin to show onset of fatigue thereby increasing risk of mental errors causing quality or hygiene problems. Furthermore, in food processing operations, where long durations of performing repetitive tasks may lead to error or injury thereby leading to loss of quality and work.
17.4.2 Features of robots
|
Features |
Benefits |
|
Better process control |
Easy to clean robot, minimum retention areas, connection protection |
|
High reliability, high speed |
Increased productivity |
|
High dexterity, several mounting positions |
Compact cell, less room required, simpler mechanical solution |
|
Cleanliness |
Better hygiene |
|
Flexibility |
Marketing innovative products and packaging |
|
Vision and conveyor tracking |
Product picked and controlled in process, in any position |
17.4.3 Some of the areas of robot applications are
· Precision curd slicing and ultrasonic cheese slicing
· Food packaging and stacking operations
· The sophisticated control system with a built-in palletizing function makes it possible to load and unload the objects with precision and accuracy.
· Robotics are used to reduce the chances of contamination and consequently eliminate many safety concerns associated with human contact..