Nick Tucker and Behrooz Ghorani of the University of Lincoln describe a novel electrospinning delivery method for nano-tech food ingredients.
Introduction
Electrospinning is a method of producing fine fibres with sizes ranging from microns down to a few nanometres in diameter. The process was first developed in the late eighteen hundreds with attempts being made to commercialise it from the beginning of the last century. The process came into its own in the late 1930s when Soviet scientists realised the potential for ultrafine fibres as non-woven filtration media; to date this has been the most popular commercial activity using this process. In the past couple of decades, the research community has adopted electrospinning as a ready method for the construction of nano-scale materials. Electrospinning uses the power of electrostatic repulsion and attraction to draw fibres from a liquid solution or melt. If a liquid droplet is charged up by connecting it to a high voltage (typically greater than 10,000 volts) a surplus of charge will build up inside the droplet. Like charges repel and eventually these repulsive forces will overcome the surface tension that is holding the drop in its characteristic shape. The droplet surface ruptures and a stream of liquid erupts towards the nearest lower potential surface, ideally the designated product collecting surface. If the liquid has sufficient internal cohesion, then this jet will solidify into a fibre and if not then the jet will break up into a stream of droplets; this latter process is known as electrospraying. The mechanism of molecular cohesion may be physical entanglement of long chain molecules or hydrogen bonding for carbohydrates, such as glucose or cyclodextrins.
As a method of manufacture of food ingredients, the process has much to commend it. Fibre or droplet formation takes place at low temperatures – the lower limit being that the feedstock must not freeze. Water is a suitable solvent for a wide range of synthetic and natural origin materials. No coagulation chemistry is required – both fibre and droplet formation are fundamentally the physical processes of solvent evaporation or melt freezing. Thus we are in a good position to produce products containing environmentally sensitive ingredients without causing biochemical damage to them. The disadvantage of the process is that, particularly in the laboratory, the production rate can be slow – milligrams per hour. On the industrial scale this problem has been addressed by using a number of technological innovations that replace the lab scale single spinneret (often a blunt hypodermic syringe needle) with say a rotating cylinder or sawtooth disc. It is also possible to produce multi-layered structures by using a coaxial spinneret. The possibilities inherent in this technology for the delivery of biochemically sensitive materials to the digestive system are the subject of this article.
Applications
This technology has potential application in the production of non-commodity food stuffs for the emerging functional foods market with (potentially) positive effects on health beyond those of basic nutrition. Examples of functional foods are: cholesterol lowering spreads and cream cheeses, omega-3 fatty acid enriched margarines and milk to reduce the risk of cardiovascular disease, the addition of vitamin D to fruit juices to combat the onset of osteoporosis. One of the advantages of nanoscale provision of this type of ingredient is that useful quantities of the active or payload ingredient can be added to the food base without affecting the perceived sensory properties. The efficiency of absorption can also be strongly related to the size of the delivery vehicle. In some cases, it is necessary for the carrier particle to be submicron in size for it to be absorbed at all. Small particles are more prone to stick to surfaces, the effects of static charge attraction and van der Waals’ forces being significant at that scale. This leads to a prolonged gastrointestinal tract transit time. If we are lucky, then the payload material will be of itself spinnable, although this is not the case for the majority of these ingredients. To produce a spinnable raw material, the payload material can simply be mixed in with the spinnable base material to produce a fibre or particulate product that is a solid solution of carrier and payload.
Alternatively, the payload ingredient can be encapsulated within an external, resistant coating during the process of fibre or droplet formation. Encapsulation improves the stability, preservation, bioavailability and controlled release properties of the subject biomolecule; it can also mask unwanted odours or the taste of the delivered compound. The linear nature of the human digestive tract can be problematic for absorption of functional food ingredients. Foodstuffs can only be comfortably introduced by the mouth and the processing of the food into a digestible form occurs in strict sequence thereafter. As we chew, the mouthful is attacked by salivary amylase, starting the breakdown of starches into sugars and rendering the food into a bolus that can be squeezed down the gullet into the stomach, where digestion starts in earnest by churning the material in acid solution. The absorption of the digested material and the recovery of the aqueous solvent then occurs in sequence as the material progresses through the small and large intestines. Encapsulating the material to be delivered in a resistant outer coating protects the payload material against the digestive action of the stomach contents so that it can be passed on through to the intestinal tract for absorption into the body.
Protein based encapsulation
In biochemical terms, proteins are the obvious choice for encapsulation as they are intrinsic to the normal balanced diet, are a major component of the human body and are often themselves valuable dietary supplements and functional food enhancers. However, despite being polymeric in nature proteins can be infamously difficult to electrospin, mainly because of their complex secondary and tertiary structures. To be spinnable, the proteins should be well dissolved in a random coil configuration. Globular proteins have too little internal interaction to entangle during the spinning process. It is possible to spin ‘unspinnable’ materials by the addition of a spinnable material to the mix. These adjunct materials are usually synthetic polymers and although they may be recognised as suitable for food use, they are likely to have an adverse effect on the marketability of the product to a sophisticated if perhaps illinformed target market. Whey protein isolate and concentrate, co-products from cheese manufacture and available at commodity price levels, have been shown to be electrospinnable without using a synthetic adjunct, as have marine and mammalian collagens and gelatin.
If a vegetable origin carrier is required, soy protein isolate has similar properties. The probiotic bacterium Bifidobacterium, has been spun and encapsulated without loss of viability using whey protein concentrate and the carbohydrate pullulan, although the whey protein concentrate proved to be more effective as an encapsulation material. It prolonged the survival of the bacterial cells even at high relative humidities. Whey protein isolate and concentrate have also been used to encapsulate the anti-oxidants β-carotene and β-lactoglobulin. Other candidate vegetable origin proteins are amaranth protein isolate and zein. The latter is of particular interest as it is extracted from maize using ethanol as a solvent and is thus not soluble in water. Zein has been used to encapsulate β-carotene, curcumin, (-)-epigallocatechingallate (EGCG), α-tocopherol, ferulic acid, and tannin, whilst still maintaining bioavailability of the payload material.
Carbohydrate based encapsulation
Natural and modified polysaccharides are also good candidate carriers for nanoencapsulation of functional food ingredients. Clearly these materials are both biocompatible and biodegradable and in addition can be chemically modified to achieve optimal spinnability. The ability to modify carbohydrates by the addition of functional groups means that they can interact with a wide range of bioactive compounds giving them a high degree of utility as carriers to bind and entrap a variety of both hydrophilic and hydrophobic functional food ingredients. In addition, the molecular structure of carbohydrates means that they are stable at higher temperatures compared to lipid or protein carrier materials, which would require an additional processing stage of being malted or denatured. Suitable carbohydrates including mono-, oligo- or polysaccharides are available from higher plant (e.g. starch, cellulose, pectin and guar gum), algal (alginate or carrageenan) or animal (chitin) origins.
Studies using cellulose acetate to immobilise vitamins A and E demonstrated that compared to a conventional cast film, the release rate was slow and even, rather than the sudden burst emitted by the film. The perillaldehyde aroma compound was immobilised by direct incorporation into an edible polysaccharide nanofibre matrix made from pullulancyclodextrin. Electrospinning from a base emulsion has been used to encapsulate the highly volatile limonene fragrance.
Conclusions
Encapsulation of bioactive compounds and probiotic bacteria within prebiotic material to protect the payload molecules during their passage through the digestive system is an emerging area of interest for the food industry. The construction of nanoscale encapsulations by electrospinning presents a method that does not cause damage to either sensitive biochemicals or probiotic bacteria. The presentation of payload ingredients at the nanoscale improves targetability to specific areas of the digestive tract and gives improved control of release rate. Adoption of these electrospinning technologies will allow the industry to develop a wide range of novel high added value functional foods.
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