F. Xavier Malcata

Microalgal reactors: A review of enclosed system designs and performances

One major challenge to industrial microalgal culturing is to devise and develop technical apparata, cultivation procedures and algal strains susceptible of undergoing substantial increases in efficiency of use of solar energy and carbon dioxide. Despite several research efforts developed to date, there is no such thing as “the best reactor system”– defined, in an absolute fashion, as the one able to achieve maximum productivity with minimum operation costs, irrespective of the biological and chemical system at stake. In fact, choice of the most suitable system is situation‐dependent, as both the species of alga available and the final purpose intended will play a role. The need of accurate control impairs use of open‐system configurations, so current investigation has focused mostly on closed systems. In this review, several types of closed bioreactors described in the technical literature as able to support production of microalgae are comprehensively presented and duly discussed, using transport phenomenon and process engineering methodological approaches. The text is subdivided into subsections on: reactor design, which includes tubular reactors, flat plate reactors and fermenter‐type reactors; and processing parameters, which include gaseous transfer, medium mixing and light requirements.

Bifidobacterium spp. and Lactobacillus acidophilus: biological, biochemical, technological and therapeutical properties relevant for use as probiotics

This review focuses on the biological properties and consequent technological roles of intestinal bacteria with potential health-promoting capacities, and provides selected examples available in the literature that are pertinent to the aforementioned concepts. A comprehensive overview pertaining to the taxonomy and ecology, as well as nutritional and health effects of Bifidobacterium spp. and Lactobacillus acidophilus, is provided; particular attention is paid to their incorporation, and growth and acidification features in fermented dairy products. The typical poor growth of these species in milk is highlighted, and the use of bifidogenic and growth factors, including their nature and function, is discussed. Efforts to establish optimum environmental conditions for their growth are critically reviewed, in addition to the effects of the food and storage conditions on microbial survival. Criteria for selection of effective microbial strains for their probiotic effect are listed, and modifications to improve fermentation efficiency and shelf-life of final dairy products are suggested; among these, the incorporation of Bifidobacterium spp. and L. acidophilus into a solid matrix, such as cheese, is emphasized.

Immobilized lipase reactors for modification of fats and oils—A review

This review focuses on the use of immobilized lipase technology to effect hydrolysis, ester synthesis, and interesterification reactions. The various immobilization procedures, reactor configurations, and process considerations are all reviewed and discussed.

Bovine whey proteins : Overview on their main biological properties

Abstract Whey, a liquid by-product, is widely accepted to contain many valuable constituents. These include especially proteins that possess important nutritional and biological properties – particularly with regard to promotion of health, as well as prevention of diseases and health conditions. Antimicrobial and antiviral actions, immune system stimulation, anticarcinogenic activity and other metabolic features have indeed been associated with such whey proteins, as α-lactalbumin, β-lactoglobulin, lactoferrin, lactoperoxidase, and bovine serum albumin. The most important advances reported to date pertaining to biological properties of whey proteins are reviewed in this communication.

Kinetics and mechanisms of reactions catalysed by immobilized lipases

This review focuses on the kinetics and mechanisms of reactions catalysed by immobilized lipases. The effects of pH, temperature, and various substances on the catalytic properties of immobilized lipases and on the processes by which they are deactivated are reviewed and discussed.

Light requirements in microalgal photobioreactors: an overview of biophotonic aspects

In order to enhance microalgal growth in photobioreactors (PBRs), light requirement is one of the most important parameters to be addressed; light should indeed be provided at the appropriate intensity, duration, and wavelength. Excessive intensity may lead to photo-oxidation and -inhibition, whereas low light levels will become growth-limiting. The constraint of light saturation may be overcome via either of two approaches: increasing photosynthetic efficiency by genetic engineering, aimed at changing the chlorophyll antenna size; or increasing flux tolerance, via tailoring the photonic spectrum, coupled with its intensity and temporal characteristics. These approaches will allow an increased control over the illumination features, leading to maximization of microalgal biomass and metabolite productivity. This minireview briefly introduces the nature of light, and describes its harvesting and transformation by microalgae, as well as its metabolic effects under excessively low or high supply. Optimization of the photosynthetic efficiency is discussed under the two approaches referred to above; the selection of light sources, coupled with recent improvements in light handling by PBRs, are chronologically reviewed and critically compared.

Atomic force microscopy study of the antibacterial effects of chitosans on Escherichia coli and Staphylococcus aureus

Chitosan has been reported to be a non-toxic, biodegradable antibacterial agent. The aim of this work was to elucidate the relationship between the molecular weight of chitosan and its antimicrobial activity upon two model microorganisms, one Gram-positive (Staphylococcus aureus) and one Gram-negative (Escherichia coli). Atomic force microscopy (AFM) imaging was used to obtain high-resolution images of the effect of chitosans on the bacterial morphology. The AFM measurements were correlated with viable cell numbers, which show that the two species reacted differently to the high- and low-molecular-weight chitosan derivatives. The images obtained revealed not only the antibacterial effects, but also the response strategies used by the bacteria; cell wall collapse and morphological changes reflected cell death, whereas clustering of bacteria appeared to be associated with cell survival. In addition, nanoindentation experiments with the AFM revealed mechanical changes in the bacterial cell wall induced by the treatment. The nanoindentation results suggested that despite little modification observed in the Gram-positive bacteria in morphological studies, cell wall damage had indeed occurred, since cell wall stiffness was reduced after chitooligosaccharide treatment.

Microalgae as sources of high added-value compounds—a brief review of recent work

Microalgae have found commercial applications as natural sources of valuable macromolecules, including carotenoids, long‐chain polyunsaturated fatty acids, and phycocolloids. As photoautotrophs, their simple growth requirements make them attractive for bioprocesses aimed at producing high added‐value compounds that are in large demand by the pharmaceutical market. A few compounds synthesized by microalgae have indeed proven to possess anti‐inflammatory, antiviral, antimicrobial, and antitumoral features; astaxanthin, a known antioxidant produced by Haematococcus pluvialis, is an illustrative example with important anti‐inflammatory and antitumoral roles. From a chemical standpoint, several such compounds are polysaccharides or long chain fatty acids, where the latter can be either saturated or unsaturated. Additionally, their chemical structures are often atypical, whereas their concentrations can exceed those found in many other natural sources. The productivity and biochemical composition of microalgae depend strongly on the mode of cultivation, medium composition, and nutrient profile. Consequently, numerous efforts aimed at elucidating the practical impacts of the aforementioned parameters have been developed. This review accordingly covers the knowledge produced in the last two decades on the uses of microalgae to obtain physiologically active compounds, and on the optimization of the underlying production and purification processes. It also identifies major gaps and opportunities in this field that should be addressed or exploited in the near future.

Antimicrobial effects of chitosans and chitooligosaccharides, upon Staphylococcus aureus and Escherichia coli, in food model systems

The objective of this study was to elucidate the controversial relationship between the molecular weight (MW) of chitosans and their antibacterial activity (upon different inoculum levels, at several concentrations). The influence of food components on the activity was also ascertained, as well as acceptance by a sensory panel. All the compounds tested exhibited antibacterial activity against Staphylococcus aureus and Escherichia coli. This activity was shown to be closely dependent on the inoculum level, MW and concentration used. Within 4h at 10(3) cells/mL, all five compounds, at every concentration (0.5%, 0.25% and 0.1%, w/v), proved to be bactericidal; for higher inocula, 0.1% (w/v) was only bacteriostatic; at 10(7) or 10(5) cells/mL, and independently of the inoculum level, 0.25% (w/v) of any chitooligosaccharide (COS) mixture was sufficient to reduce the E. coli initial population by at least 3 log cycles; COS never exhibited bactericidal action over S. aureus, unlike high and medium MW chitosans-which, at 0.5% (w/v), presented a bactericidal effect even against 10(7) cells/mL. When incorporated in liquid food matrices, medium and high MW chitosans maintained their activity, for both matrices and bacteria, although a slower activity was noticeable in milk; however, COS lost their activity upon both bacteria in milk after 4-8h. Furthermore, addition of chitosans to apple juice led to several unpleasant off-flavors, such as astringency and after taste--which increased in magnitude with MW.

Effects of Various Sugars Added to Growth and Drying Media upon Thermotolerance and Survival throughout Storage of Freeze‐Dried lactobacillus delbrueckii ssp. bulgaricus

The aim of this research effort was to investigate the role of various sugar substrates in the growth medium upon thermotolerance and upon survival during storage after freeze‐drying of Lactobacillus bulgaricus. Addition of the sugars tested to the growth medium, and of these and sorbitol to the drying medium (skim milk) was investigated so as to determine whether a relationship exists between growth and drying media, in terms of protection of freeze‐dried cells throughout storage. The lowest decrease in viability of L. bulgaricuscells after freeze‐drying was obtained when that organism was grown in the presence of mannose. However, L. bulgaricusclearly survived better during storage when cells had been grown in the presence of fructose, lactose or mannose rather than glucose (the standard sugar in the growth medium). A similar effect could not be observed in terms of thermotolerance; in this case, the growth medium supplemented with lactose was found to yield cells bearing the highest heat resistance. Supplementation of the drying medium with glucose, fructose, lactose, mannose or sorbitol led in most cases to enhancement of protection during storage, to a degree that was growth medium‐dependent.