Application of a microbial sensor for determination of short-chain fatty acids in raw milk samples

Microbial biosensors

A microbial biosensor consists of a transducer in conjunction with immobilised viable or non-viable microbial cells. Non-viable cells obtained after permeabilisation or whole cells containing periplasmic enzymes have mostly been used as an economical substitute for enzymes. Viable cells make use of the respiratory and metabolic functions of the cell, the analyte to be monitored being either a substrate or an inhibitor of these processes. Bioluminescence-based microbial biosensors have also been developed using genetically engineered microorganisms constructed by fusing the lux gene with an inducible gene promoter for toxicity and bioavailability testing. In this review, some of the recent trends in microbial biosensors with reference to the advantages and limitations are been discussed. Some of the recent applications of microbial biosensors in environmental monitoring and for use in food, fermentation and allied fields have been reviewed. Prospective future microbial biosensor designs have also been identified.

Sample preparation for the analysis of flavors and off-flavors in foods

Off-flavors in foods may originate from environmental pollutants, the growth of microorganisms, oxidation of lipids, or endogenous enzymatic decomposition in the foods. The chromatographic analysis of flavors and off-flavors in foods usually requires that the samples first be processed to remove as many interfering compounds as possible. For analysis of foods by gas chromatography (GC), sample preparation may include mincing, homogenation, centrifugation, distillation, simple solvent extraction, supercritical fluid extraction, pressurized-fluid extraction, microwave-assisted extraction, Soxhlet extraction, or methylation. For high-performance liquid chromatography of amines in fish, cheese, sausage and olive oil or aldehydes in fruit juice, sample preparation may include solvent extraction and derivatization. Headspace GC analysis of orange juice, fish, dehydrated potatoes, and milk requires almost no sample preparation. Purge-and-trap GC analysis of dairy products, seafoods, and garlic may require heating, microwave-mediated distillation, purging the sample with inert gases and trapping the analytes with Tenax or C18, thermal desorption, cryofocusing, or elution with ethyl acetate. Solid-phase microextraction GC analysis of spices, milk and fish can involve microwave-mediated distillation, and usually requires adsorption on poly(dimethyl)siloxane or electrodeposition on fibers followed by thermal desorption. For short-path thermal desorption GC analysis of spices, herbs, coffee, peanuts, candy, mushrooms, beverages, olive oil, honey, and milk, samples are placed in a glass-lined stainless steel thermal desorption tube, which is purged with helium and then heated gradually to desorb the volatiles for analysis. Few of the methods that are available for analysis of food flavors and off-flavors can be described simultaneously as cheap, easy and good.

Microbial biosensor for free fatty acids using an oxygen electrode based on thick film technology

A microbial biosensor based on thick film technology was developed. The microorganisms, Arthrobacter nicotianae, were immobilized in Ca-alginate directly on the electrode surface. For the stability of the calcium alginate gel the addition of 0.5 mM CaCl2 to the assay buffer was necessary. The respiratory activity of the microorganisms was monitored by oxygen consumption at -600 mV vs. Ag/AgCl reference electrode. The sensor was used in a batch system and was applied to the determination of free fatty acids in milk. Short-chain fatty acids (C4:0-C12:0) were the preferential substrates, with butyric acid being the main substrate. Consequently, the concentration of free short-chain fatty acids was represented as the butyric acid equivalent. The sensor showed linearity over the concentration range 9.5-165.5 microM (correlation coefficient, r = 0.99920). The response time of the sensor was approximately 3 min. No additional dialysis membrane was necessary, which led to a high sensitivity of the sensor and fast response times. Recovery rates of 98-113% were found for butyric acid in milk samples using the sensor without any additional membrane and a sample dilution of 200 by the assay. Two widespread disadvantages of microbial sensors, long response times and long times to return to the baseline signal after use, could be overcome.

Immobilized cells used for detection and analysis

Many kinds of biosensor have now been developed and utilized for various types of analysis including clinical, medical and environmental monitoring, industrial process control, as well as many other applications. The microbial biosensor has advantages, such as longer lifetime and lower cost, over other types of biosensor. Recently, photobacteria and recombinant bacteria have been employed in biosensors both for determining biochemical oxygen demand and for detecting heavy metals and other toxic compounds.