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by Marion Bonnefille, Communication Manager, Alpha M.O.S.

The Alpha M.O.S. FOX E-Nose analyzer.
Flavor and taste control evaluation can be performed by gas and liquid sensor array systems—the so-called electronic nose and tongue. Due to the speed and objectivity of these systems, they have become attractive for use within major food and drinks companies. The E-Nose and E-Tongue can be used for a wide range of applications including quality control, product matching, origin identification, spoilage detection and flavor quantification.

Odor and Taste Evaluation
These sensor-based analyzers have the particularity to run a global analysis of the total complex chemistry of the sample (chemical fingerprint). They were developed to perform qualitative/quantitative analyses of organoleptic and chemical properties of products.

The Electronic Tongue is designed for taste analysis of liquid matrices such as gels, syrups, solutions, emulsions or solids dissolved in liquid solution. The Electronic Nose is able to detect and analyze a wide range of odors and volatile organic compounds. E-nose and E-Tongue systems comprise several parts: a sampling system to ensure reproducibility of the injection, an array of sensors, electronic data acquisition, a control system and pattern recognition software.

Sensors
Principle of operation. Click to enlarge.
Molecules of the tested sample interact with the active part of the sensor (metal oxide for the gas sensors and chemical sensitive layer for the liquid sensors). These bindings and interactions modify the physical properties of the sensor and generate either resistance (E-Nose technology) or potential variations (E-Tongue technology). These electrical output signals are recorded over time. The raw data registered for each sensor (between six and 18 metal oxide sensors for the Electronic Nose and seven liquid sensors for the Electronic Tongue) are processed by mathematical models included in software.

The E-Nose chemical sensors (metal oxide sensors) are detectors that measure conductance changes upon contact with volatile chemical compounds.

The E-Tongue detectors are liquid cross-sensitive sensors made from silicon transistors with an organic coating that governs sensitivity and selectivity of each individual sensor. The measurement consists of a potentiometric difference between each individually coated sensor with the Ag/AgCl reference electrode (silver-silver chloride reference electrode).

E-Nose and E-Tongue sensors, like human ones, are cross-selective and can react to many substances responsible for odors or tastes. Each sensor is sensitive to a range of components that is different from the other. The information given by all sensors is complementary, and the combination of all responses result in a unique fingerprint.

Pattern Recognition Software
Quality Control Chart of Onion Samples. Click to enlarge.
Sophisticated software is required to handle the complex information yielded by the sensor responses. This software is based on powerful algorithms for pattern recognition and includes techniques such as principle component analysis (PCA), discrimination function analysis (DFA), soft independent model class analogy (SIMCA), statistical quality control (SQC) and partial least square (PLS). The different pattern recognition models are used to obtain a correlation between instrumental results and sensory tests. The choice of the mathematical model used depends on the objectives.

Analysis
Analysis with E-Nose and E-Tongue always follow the same scheme:

• Training phase: Known samples already qualified (by sensory panel, or chromatography, etc.) are analyzed with the instruments in order to build a mathematical model.

• Pattern recognition and validation: Samples declared as unknown for the instrument but known for the operator are analyzed. Based on the results obtained, the model is validated or not.

After these steps, the system can be used for routine tests, providing a rapid and objective response that can be easily interpreted.

Application for Food Ingredients Analysis
Principal Component Analysis of Fresh and Aged Onions. Click to enlarge.
In the food industry, quality control (QC) of ingredients or products is extremely important and is usually achieved by sensory assessment. Indeed, consumers demand regular and reproducible quality and flavor (taste and smell). Therefore QC processes must be monitored in real time on the production line with suitable tools to fulfill the constraints of delay, objectivity and reliability.

Two applications for food ingredients will illustrate some of the possible uses of the E-Nose and E-Tongue technologies with the aim to help food manufacturers insure the quality of their products and processes.

Aroma Quality Control of Onions with an Electronic Nose

This first study illustrates the ability to use an Electronic Nose for the quality control of onions, with the following objectives:

• Quality control monitoring of manufactured batches

• Aroma quality follow-up over time.

Samples/Method
In order to assess their aroma, seven batches of onions, used as an ingredient in salsa, were analyzed with a sensor-array-based electronic nose. All onion batches were previously tested by a sensory panel to discriminate good (three batches: G_S001 to G_S004) and bad ones (four batches: B_D01 to B_D10). Both good and bad samples allow us to set-up qualitative models with the FOX E-Nose. Finally, three blind samples (U_001 to U_003) will be analyzed with the E-Nose for quality determination.

Results
Principle Component Analysis (PCA)Click to enlarge.
For routine analysis purposes, a statistical quality control can be used. To establish the range of acceptable grade (blue band), all good samples were taken as the reference quality. All future analyzed samples that are plotted in this band will be recognized as good, and all samples outside will be rejected as out of specifications. Using this quality control card, the three blind samples were assessed as bad.

In order to follow-up onion aroma quality over time and monitor shelf-life, samples were tested at different times. Samples of “good” onions were analyzed when freshly collected (day 0 = D_00) and then at various intervals of time (day 1, 3, 4, 7, 8 & 9: D_01 to D_09) by the E-Nose and sensory panel to determine good and bad products.

To follow-up onion quality over time, a principal component analysis was built-up.

At day 4, the quality was still good whereas on day seven it turned bad. Intermediate analysis is required to precisely determine the deadline for safe consumption.

New Orange Juice Development: Taste Benchmarking with an Electronic Tongue
Today food industries face new market challenges: varied and rapidly changing consumer preferences and increasing demand for healthy and safety-oriented products. Consequently, R&D departments need to shorten product development for a faster product launch. Taste and aroma are crucial in the development process because they guarantee the product meets consumer expectations. An Electronic Tongue was used in order to benchmark competitive products during the development of new formulations of orange juice.

An existing product (orange with artificial flavors), a prototype formulation (orange with new flavors) and two target products (1, competitive orange juice and 2, natural orange juice) were tested. The objectives were to compare the taste of the existing product and the new formulation to two target products and then determine if a desired taste could be reached with the new formulation.

Instrumental measurement concluded that the prototype formulation was a better match with the taste of target product 1 (see PCA map) than the taste of target 2.

Conclusion
Electronic noses and tongues, which require minimal sample preparation, can objectively, safely and quickly characterize the global taste and odor profiles of ingredients or formulations. More widely, they can be used for:

• Development of new formulations

• Optimization of product quality and consumers’ acceptance

• Benchmarking and comparison of products

• Raw materials/finished products analyzed for consistency of quality/origin, shelf life and storing effect, contamination detection

• Process monitoring (mixing, drying, distillation, fermentation, cleaning in place, process efficiency)

• Packaging selection and interaction studies

• Fraud and counterfeiting detection

Useful as R&D and QC tools, the E-Nose and E-Tongue can speed up and optimize the food designer’s and manufacturer’s efforts.

For more information, contact Marion Bonnefille, communication manager, Alpha M.O.S. at communication@alpha-mos.com or by phone at +33 5 62 47 53 80.

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