M.J. Navas

Review of chemiluminescent methods in food analysis

The purpose of the present review is to sketch out the scope of chemiluminescence in food analysis. Practical considerations are discussed. Specific applications to the determination of N-nitroso compounds, sugars, food oxidation, hormonal anabolics and metabolites, metals and other interesting compounds. There is also discussion on how alcohols, enzymes, etc. have been revised. Possibilities and limitations of the various reaction and detection systems are evaluated.

Analysis and Antioxidant Capacity of Anthocyanin Pigments. Part I: General Considerations Concerning Polyphenols and Flavonoids

Many epidemiological studies have shown the benefits of a diet rich in fruit and vegetables to human health and for the prevention of various diseases associated with oxidative stress, such as cancer and cardiovascular diseases. Anthocyanins, natural pigments belonging to the group of flavonoids, are common components of the human diet, as they are present in many foods, fruits, and vegetables, especially in berries. Their use as colorants has considerable interest because of their coloring properties. Moreover, they have an antioxidant activity. Various adverse effects on health have frequently been attributed to synthetic antioxidants. For these reasons, currently, there is a trend towards relying on antioxidants derived from natural products. The efficacy of anthocyanins as antioxidants depends, to a large extent, upon their chemical structure. They act as antioxidants both in the foodstuffs in which they are found and in the organism after intake of these foods. With this in mind, an introduction to polyphenols is made with emphasis on their role as secondary metabolites, classification, and health relevance. Flavonoid intake, biological activities, databases, classification and structure, distribution, and dietary sources are then considered. Aspects of anthocyanin concerning its early history and chemical structure, color, and intake are dealt with in the second part of the series. The extraction and analysis of anthocyanin pigments and their antioxidant power, paying special attention to the oxidation process, will be the subject of the third part. Bioavailability and metabolism of anthocyanic pigments, the methods used for measuring the antioxidant activity of anthocyanins, and the influence of anthocyanins on the antioxidant activity of wine will finally be covered in the fourth part. The present review intends to reflect the interdisciplinary nature of the research that is currently carried out in this prolific area. Key research articles and reviews are mainly referenced, and we apologize to those researchers whose work is not cited directly by us.

An Overview of the Kjeldahl Method of Nitrogen Determination. Part I. Early History, Chemistry of the Procedure, and Titrimetric Finish

In 1883 Kjeldahl devised a method for the determination of nitrogen, which has become a classical measurement in analytical chemistry and has been used extensively over the past 130 years. In the original method, sulfuric acid alone was used as a digestion medium. The use of a catalyst in Kjeldahl digestion accelerates oxidation and completes the digestion to allow the subsequent determination of nitrogen. Mercury (its use being in decline because of environmental concerns), selenium, and copper are the catalysts of choice, though for certain applications titanium has found some usage. Short digestion times in association with maximum nitrogen recovery may be achieved by using a methodology based on experimental design and response surfaces, with microwave digestion processes, and with the aid of the couple sulfuric acid-hydrogen peroxide without catalyst. The quantification of distilled ammonia is generally achieved by titration; the ammonia is absorbed in an excess of boric acid, followed by titration with standard acid in the presence of a suitable indicator. The Kjeldahl method can be done with limited resources; nitrogen determination with the Kjeldahl method does not require expensive devices nor specialized techniques and is precise and accurate. The Kjeldahl method is used for calibrating other protein assays; it is still the primary reference method for protein analysis today. The original method as presented by Kjeldahl has been continuously improved. Todays digestion systems offer safety both from a personal perspective and from an environmental point of view. The determination of nitrogen content is a frequently conducted analysis in industry and commerce, and numerous organizations have official methods. The use of instrumental finish in Kjeldhal applications will be the subject of the second part of this review.

An Overview of the Kjeldahl Method of Nitrogen Determination. Part II. Sample Preparation, Working Scale, Instrumental Finish, and Quality Control

The Kjeldahl method was introduced in 1883 and consists of three main steps: sample digestion, distillation, and ammonia determination (titration being the primary method). The Kjeldahl method uses sulfuric acid, a variety of catalysts, and salts to convert organically bound nitrogen in samples to ammonium with its subsequent measurement (Sáez-Plaza et al., 2013). Today, this method is universally accepted and used in tens of thousands of laboratories throughout the world for nitrogen analysis in a wide variety of materials, such as foods, beverages, agricultural products, environmental samples, chemicals, biochemicals, and pharmaceuticals. However, successful analysis requires proper sampling and sample handling, which depend on the type of material. The Kjeldahl method has been validated and standardized for total (crude) protein estimation for a wide variety of food matrices, indirectly determined by their nitrogen content, and is the reference method adopted by many international organizations. The Kjeldahl procedure has several variants, based mainly on a sample size and apparatus required. A number of rapid and accurate instrumental methods have been gradually introduced that have some advantages compared to older techniques, if a large number of samples are to be run. Thus, extracted nitrogen from Kjeldahl can be determined by several other methods, i.e., spectrophotometric, potentiometric with ion selective electrode, FIA, ion chromatographic, and chemiluminescent methods. Quality control is essential for accurate and precise measurements of nitrogen by the Kjeldahl method. The importance of quality control in Kjeldahl analysis is stressed in this review. Despite some negative factors (i.e., it is hazardous, lengthy, and labor intensive), the Kjeldahl method and its variants with instrumental finish remain as accurate and reliable methods.

Air analysis: determination of nitrogen compounds by chemiluminescence

Abstract Chemiluminescent techniques applied to atmospheric pollutants such as nitrogen compounds are reviewed and some practical considerations are discussed. Determinations of NO, NO2, nitrate, nitrite, nitric acid, nitrous acid and alkyl nitrates are described. Possibilities and limitations of the various procedures are evaluated.

Air analysis: determination of hydrogen peroxide by chemiluminescence

Abstract The purpose of the present review is to examine the application of chemiluminescence for H2O2 in air analysis. The revised literature covers the papers of analytical interest which have appeared in the past few years. Practical considerations, requirements and methodologies are described in order to emphasize the scope and possibilities of chemiluminescence in this area.

Mathematical treatment of absorbance versus pH graphs of polybasic acids

Mathematical relationships determining the shape of absorbance vs. pH graphs of polybasic acids are derived, and new graphical and numerical techniques for evaluating the acidity constants of two-step overlapping equilibria are examined.

Analysis and Antioxidant Capacity of Anthocyanin Pigments. Part IV: Extraction of Anthocyanins

Anthocyanins are naturally occurring polyphenol compounds that impart orange, red, purple, and blue color to many fruits, vegetables, grains, flowers, and plants. In recent decades, interest in anthocyanin pigments has increased due to their possible utilization as natural food colorants and especially because their consumption has been linked to protection against many chronic diseases. It seems that anthocyanin posseses strong antioxidant properties leading to a variety of health benefits. Coupled to increasing consumer demand, food manufacturers have moved towards increased usage of approved natural colors. Despite the great potential in applications that anthocyanins represent for food, pharmaceutical, and cosmetic industries, their use has been limited because of their relative instability and low extraction percentage. Growing demands have been made on sample pretreatment, and over time some novel extraction techniques have been developed. Solid phase extraction, countercurrent chromatography, adsorption, pressurized liquid or fluid extraction, and microwave-assisted extraction are environmentally friendlier in terms of using smaller amount of solvents (often nontoxic) and reducing working time. The past few years have been characterized by wide interest in these techniques, and many contributions describing these methods have been published. The aim of this article is to review the literature available on the most important procedures proposed for the extraction of anthocyanins; the use of non-thermal technologies in the assisted extraction of anthocyanins will be covered in a separate report.

Thermal Lens Spectrometry as Analytical Tool

Thermal lens spectrometry as an analytical technique is summarized. Specific applications to the determination of metal ions, pesticides, fatty acids, carotenoids, and some interesting biomolecules as nucleotides or hepatitis B antigen are described. The possibilities and limitations of this TLS detection system is evaluated.

Spectrophotometric methods for the evaluation of acidity constants—II: Numerical methods for two-step overlapping equilibria

A brief review of the numerical methods employed in the evaluation of acidity constants of two step overlapping equilibria is given. A new alternative to the method of three equations developed by Thamer is also proposed in this paper. Taking into account the Gauss-elimination method, several variants to the original method of Thamer are devised.