Where Is the Nano in Our Foods? - American Chemical Society

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Where is the nano in our foods? Jose Miguel Aguilera J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/jf5016939 • Publication Date (Web): 22 Sep 2014 Downloaded from http://pubs.acs.org on September 29, 2014

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Journal of Agricultural and Food Chemistry

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Perspective

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to be the case in foods where potential adverse effects are resented by consumers.

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However, unknown to many people some of the most desirable properties of our daily

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foods reside in a microstructure where the nanolevel plays an important role in the

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form of macromolecular arrangements, aggregates, colloidal networks, interfaces and

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nanoparticles. This article unveils where the “nano” in our kitchens is.

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Where is the nano in our foods? José Miguel Aguilera* Department of Chemical and Bioprocess Engineering Pontificia Universidad Católica de Chile, Santiago, Chile (Phone: 562 2354-4256. Email: [email protected])

Although nanotechnology has opened opportunities in many fields this does not appear

Introduction Imagine nanoemulsions containing antimicrobials that impede the growth of

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microorganisms causing food spoilage and food infections. Or liposomes that protect

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nutrients and bioactive compounds so they are more efficiently delivered for absorption

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in our intestine. Think about vegetable protein nanofibers that when bound together

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resemble a chunk of meat at a fraction of the energy input to produce them. These are

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only some examples of how novel nanostructures may greatly assist in reducing food

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waste, improving our health and providing new sources of tasty foods.

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While nanomaterials have opened opportunities in many fields this does not appear to

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be the case in foods where potential adverse effects of added nanostructures are

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hindering their acceptance by consumers and regulatory agencies alike.1 Nevertheless, * Foreign member, U.S. National Academy of Engineering 1

ACS Paragon Plus Environment


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nanostructures smaller than 300 nm (e.g., macromolecules, molecular assemblies,

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nanostructures, tiny particles and interfaces) are ubiquitously present as part of our daily

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foods. We do not eat a homogeneous “soup” of nutrients but palatable structures in

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which molecules are either assembled in a hierarchical progression (e.g., meat), or

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become kinetically trapped during processing as fine networks (e.g., a gelatin dessert),

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microdomains (e.g., aromas in instant coffee) or composites (e.g., bread crumb).2 So

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far, none of these food nanostructures have proven to be harmful as they are easily

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digested or eliminated by our body.

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Food nanotechnology in Nature Milk in our refrigerators is perhaps the best example of a food “nano-fabricated” by

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Nature. Milk proteins (i.e., casein and the globular proteins β-lactoglobulin and α-

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lactalbumin) are synthesized in the cells of the cow’s udder and transported out into the

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plasma or aqueous phase. Casein is secreted as a micelle (300-400 nm in size)

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assembled from different casein subunits held together by colloidal calcium phosphate

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(Figure 1). Triacylglycerides are gathered together as small droplets which fuse as the

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growing lipid droplet (100 nm to 10 µm) moves toward the apical plasma membrane.

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Lipid droplets are released from inside the cell surrounded by the cellular membrane

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and become the fat globules in milk.3 In turn, the milk fat globule membrane is a

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complex lipid bilayer, four to 25 nm thick, containing several types of bioactive

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molecules within its structure (Figure 1).4 It is remarkable that the wide assortment of

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dairy products is based principally on the interactions between only two types of

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building blocks: milk proteins and the fat globules.

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Several nutrients that we derive from plants are neatly packed in organelles inside cells.

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Obliteration of these arrangements has culinary implications as experimented when 2


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crushing the garlic cloves and triggering the enzyme-substrate reaction responsible for

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the flavor and odor of fresh garlic. The attractive green color of some vegetables is

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preserved as long as chlorophylls - the porphyrin rings precisely doped with a

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magnesium atom at their center - remain in cylindrical membrane stacks around 300-

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600 nm in diameter (stroma) inside the chloroplasts. The cell wall in dry legumes, a

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source of dietary fiber, is actually a 300 to 1,000 nm thick composite material made up

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of cellulose microfibrils, hemicelluloses, pectins, lignin and proteins. Cooking dissolves

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the portion of the walls cementing the cells together giving grains a soft texture during

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mastication.

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Other times Nature builds hierarchical structures from the macromolecular to the tissue

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level. The gastronomic condition of a steak depends on the degree of denaturation of

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two extended proteins (myosin and actin, around 130 Å and 70 Å in diameter,

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respectively) residing inside myofibrils, the structural units of meat fibers, as well as on

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the breakdown of the sheath made of collagen fibers (ca. 200 nm in diameter) holding

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the fibers together. A high number of remaining crosslinks per unit volume of collagen

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correlates well with the toughness of cooked meat.5

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The build-up of food structures Major macromolecular components of foods – proteins and polysaccharides – have

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nanodimensions and during processing undergo transformations and interactions that

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span several length scales and are crucial in shaping the microstructure of many foods

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(Figure 1).6 Globular proteins in milk unfold when heated and may self-assemble into

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fibrils and even form aggregates that in the case of β-lactoglobulin (ca. 3.6 nm in the

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native state) range in size from 40 to 100 nm depending on the pH (Figure 2a).7

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Denatured globular proteins in milk form co-aggregates with casein resulting in stiff 3



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gels (e.g., yoghurt). Incidentally, α-lactalbumin has the ability to form nanotubules, but

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their presence in dairy products has not been reported.8 Wheat flour has two unique

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proteins: the macropolymer glutenin (approximate length of 50 nm and diameter of 1.8

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nm) and the monomeric units of gliadin. During kneading the long glutenin chains

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associate via physical entanglements into a large network and further interact with

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space-filling gliadins to form the thin films that give the unique viscoelastic properties

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and gas-holding capacity to wheat dough.9 Polysaccharide gels trap abundant water

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inside a polymer or colloidal network turning liquid foods into a semi-solid material.

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Electron micrographs show that some polysaccharide gels, as those of κ-carrageenan

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that may be present in flans and puddings, have fine and long filaments a few

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nanometers in diameter (Figure 2b).10 Conversely, casein gels the basic structures in

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cheeses and yoghurt, have thick strands formed by casein micelles attached as a string-

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of-beads (Figure 2c).

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All starches in cereals (and roots as well) are present in the form densely packed

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granules 5 to 50 µm in size. Inside the granule, the branched polysaccharide

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amylopectin is ordered as neat semi-crystalline layers 10 nm in thickness holding

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amylose molecules in inner spaces (Figure 1). Cooking in the presence of abundant

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water hydrates and swells the starch granules obliterating their native structure and

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releasing both polymers. Thus, a cake mix may now set in the oven into a firm matrix

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formed by gluten proteins interspersed with swollen starch granules, fat and sugar that

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harbors many gas cells. However, most cereal foods are metastable amorphous

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materials and the released starch biopolymers may recrystallize on cooling into domains

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10-20 nm in size, as those found in stale bakery products or cooled

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potatoes.11Conversely, expanded snacks are produced by heating and shearing a low-

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moisture starchy flour inside an extruder-cooker, physically fragmenting the granules to

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sub-micron sizes and breaking down amylose and amylopectin into dextrins. Upon

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exiting the extruder's die the melt is blown up by the released steam resulting in a

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spongy and soft material characteristic of extruded snacks.12

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Lipids in oilseeds are stored in discrete subcellular organelles called lipid bodies, less

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than 1 µm in size, surrounded by proteins which act as natural emulsifiers. Most lipids

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that we consume are triacylglycerol molecules (triglycerides) that may crystallize from

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the molten state as nanoplatelets 30-40 nm thick and extending for around 370 nm

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(Figure 2d).13 In turn, stacks of nanoplatelets form crystallites that cluster together into

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aggregates eventually developing a fat network that occludes portions of liquid fat

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resulting in a plastic structure typical of butter and margarine (Figure 1).

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Monoglycerides and phospholipids are used as emulsifiers due to their amphiphilic

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nature. In an aqueous phase they may self-associate into a multitude of nanosized

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structures, for example, micelles and vesicles, which may be found in emulsions such as

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salad dressings (Figure 1).14

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Two-phase systems in foods such as emulsions and liquid foams owe their existence to

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interfacial liquid films stabilized by amphiphilic lipids and/or proteins. Adsorption at

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O/W interfaces in protein-stabilized food emulsions occurs at dimensions of 10 nm.15

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The stability of beer foam (head) depends on the interaction of proteins and

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polypeptides extracted from malt and iso-α-acids from hops to form an elastic film

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around 10 nm thick at the liquid/air interface. Although the channels separating adjacent

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bubbles (lamellae) may have initially an average thickness of around 1 µm, liquid

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continuously drains from them until thinning leads to the eventual collapse of the

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foam.16 5



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Solid particles and droplets produced by mechanical size reduction are present in

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several foods and ingredients. From a sensorial viewpoint soft foods containing solid

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particles larger than 40 µm are described as having a “sandy texture”, thus the need for

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fine milling. Grinding and homogenization involve a kinetics of size reduction that

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yields a particle or droplet size distribution usually characterized by a mean value that in

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O/W emulsions (e.g., dressings and mayonnaise) produced by high pressure

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homogenization may be around 300 nm.17 Most particles in a commercial chocolate

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enrobing mass, for instance, are between 8 and 40 µm, but ca. 0.5% have sizes smaller

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than 1 µm.18 This may seem a low volume fraction but it represents a large number of

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particles due to their tiny size. The shelves in our kitchens store food powders that tend

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to agglomerate, so amorphous silica particles are a permitted additive to avoid caking.

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Some commercial samples of food-grade SiO2 contain aggregates in the range of 50-

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200 nm with a mode of 70-120 nm.19 Titanium dioxide nanoparticles, used to enhance

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the whiteness of a wide variety of foods (e.g., candy and chewing gum), are less than

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300 nm in at least one dimension and 30% of them smaller than 100 nm.20 They are

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eliminated in the urine and feces. However, some inorganic nanoparticles have been

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demonstrated to have biological activity at the cellular and subcellular levels and

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interfere with the are justified when they are neither soluble in water or oil nor degraded

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in the gastrointestinal tract or readily excreted.

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Several food components crystallize during processing or storage. The size of crystals

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increases continuously with time after nucleation, while the reverse occurs as they

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become dissolved in saliva or in the gastrointestinal fluids. Heat and mass transfer

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limitations during crystallization result in a distribution of sizes whose tail representing

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small crystals may well intrude into the nanorange. A domestic ice cream machine

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produces ice crystals 30-40 µm in size that may be perceived as sandiness in the tongue.

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However, pouring liquid nitrogen (-196 ºC) directly into the cream mix under vigorous

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agitation causes the formation of tiny ice crystals smaller than 1 µm giving ice cream a

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smooth texture that fascinates modern chefs.21On the contrary, it has been known for

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more than a century that the grittiness we appreciate in some cheeses corresponds to

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crystals of different amino acids that develop during ageing (e.g., Roquefort cheese and

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tyrosine crystals).22 Crystals (mainly of calcium oxalate) have also been found in many

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edible plants but they are probably dissolved during cooking or digestion.23

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Nano-processing of foods does not end in the kitchen but continues inside our bodies.

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Different to most other materials of our daily life the “utility” of food structures lies in

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the rate and extent in which they are broken down and disintegrated. It is quite likely

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that as nutrients become liberated from the food matrix during digestion they will

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participate in interactions among them and with other molecules, thus forming new

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nanosized

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nanostructures26, a phenomenon that we are just starting to understand.

particles24,

complex

aggregates25

and

self-assembled

colloidal

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Nanoparticles may unintentionally find their way into our foods, for example, after

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being released from food packaging, coatings of processing equipment or from products

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of plant or animal origin. Some nanoparticles, particularly those of inorganic sources,

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have been shown to transverse the gastro-intestinal mucosa, get into some organs and

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reach the cellular level.27 In vivo studies on the cytotoxicity or genotoxicity of these

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nanomaterials, generally conducted in mice and rats, are scarce and most of them fail at

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adequately characterizing the materials utilized, thus, no generalizations could be made

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of their biological activity.28 Despite efforts from food regulatory agencies and

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institutions (e.g,, FAO/WHO) gaps continue to exist as to the methodologies to be used

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in evaluating the safety of these nanomaterials and the relevance of the data generated.29

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Conclusions There are many identifiable nanostructures in the foods we eat. These edible

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nanostructures are assembled by nature, created during processing or cooking,

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intentionally added as ingredients, and perhaps incorporated inadvertently from the

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surroundings (e.g., nanoparticles in the air beaten into food foams!). So far they have

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proven to be either soluble, degradable in the gut or eliminated from our bodies, and

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nontoxic, conditions expected to be satisfied also by fabricated nanostructures added to

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foods. Increasing knowledge of the nanostructures present in our foods may provide a

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basis for the rational design of structures at the nanoscale to impart desired taste/texture

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attributes, thus replacing the trial-and-error approaches presently used. Conceivably,

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food scientists may be inspired by how these “natural” nanostructures come into being

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to develop novel and functional products “bottom-up”.

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provided by the nanosciences is not only assisting in our understanding of the nanoscale

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phenomena of the materials we eat and digest but also to assess the potential

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implications of new food nanotechnologies.30

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Meanwhile, the toolbox

References (1) Institute of Medicine. Safety and efficacy of nanomaterials in food products. In

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Nanotechnology in Food Products. National Academy Press, Washington, DC,

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2009, pp.55-83.

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(2) Aguilera, J.M. Food materials and structures. In Edible Structures: the basic

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science of what we eat. CRC Press/Taylor and Francis, Boca Raton, FL, 2012,

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pp. 53-105.

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(3) Heid, H.W.; Keenan, T.W. Intracellular origin and secretion of fat globules. Eur. J. Cell Biol. 2005, 84, 245–258. (4) Michalski, M.-C.; Januel, C. Does homogenization affect the human health properties of cow's milk? Trends Food Sci. Tech. 2006, 17, 423-437. (5) Lepetit, J. Collagen contribution to meat toughness: theoretical aspects. Meat Sci. 2008, 80, 960–967.

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(6) Donald, A. Food for thought. Nature Mater 2004, 3, 579 – 581.

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(7) Zuñiga, R.N.; Tolkach, A.; Kulozik, U.; Aguilera, J.M. Kinetics of formation

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and physicochemical characterization of thermally-induced β-lactoglobulin

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aggregates. J. Food Sci. 2010, 75, E261-E268.

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(8) Graveland-Bikker, J.F.; de Kruif, C.G. Unique milk protein based nanotubes: food and nanotechnology meet. Trends Food Sci. Tech. 2006, 17, 196–203.

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(9) Veraverbeke, W.S.; Delcour, J.A, Wheat protein composition and properties of

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wheat glutenin in relation to breadmaking functionality. Crit. Rev. Food Sci.

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2002, 42, 179-208.

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(10) Walther, B.; Lorén, N.; Nydén, M.; Hermansson, A.-M. Influence of kappa-

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carrageenan gel structures on diffusion of probe molecules determined by

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transmission electron microscopy and NMR diffusometry. Langmuir 2006, 22,

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8221-8228.

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(11) Chung, H.-J.; Liu, Q. Impact of molecular structure of amylase and

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amylopectin on amylase association during cooling. Carbohyd. Polym. 2009, 77,

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(12) Barron, C.; Bouchet, B.; Della Valle, G.; Gallant, D.J.; Planchot, V.

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Microscopical study of the destructuring of waxy maize and smooth pea starches

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by shear and heat at low hydration. J. Cereal Sci. 2001, 33, 289-300.

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(13) Acevedo, N.C.; Marangoni, A.G. Characterization of the manoscale in triacylglycerol crystal networks. Cryst. Growth Des. 2010, 10, 3327–3333. (14) Mezzenga, R.; Schurtenberger, P.; Burbidge, A.; Michel, M. Understanding foods as soft materials. Nature Mater. 2005, 4, 729-740. (15) Dagleish, D.G. Food emulsions – their structures and structure-forming properties. Food Hydrocoll. 2006, 20, 415-422. (16) Mileva, E.; Tchoukov, P.; Exerowa, E. Amphiphilic nanostructures in thin liquid films. Adv. Colloid Interfac. Sci. 2005, 114–115, 47– 52. (17) Schubert, H., Ax, K. & Behrend, O. Product engineering of dispersed systems. Trends Food Sci. Tech. 2003, 14, 9-16. (18) Servais, C.; Jones, R.; Roberts, I. The influence of particle size distribution on the processing of foods. J. Food Eng. 2002, 51, 201-208.

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(19) Contardo, C.; Ravani, L.; Passarella, M. Size characterization by sedimentation

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field flow fractionation of silica particles used as food additives. Anal. Chim.

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Acta 2013, 788, 183-192.

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(20) Weir, A.; Westerhoff, P.; Fabricius, L.; Hristovski, K.; von Goetz, N. Titanium

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dioxide nanoparticles in food and personal care products. Environ. Sci. Technol.

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2012, 46, 2242-2250.

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(21) http://blog.khymos.org/2012/09/09/gastrophysics-symposium-in-copenhagen/ (accessed Dec 9, 2013). (22) Dox, A.W. The occurrence of tyrosine crystals in Roquefort cheese. J. Am. Chem. Soc. 1911, 33, 423-425.

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(23) Nakata, P.A. Advances in our understanding of calcium oxalate crystal formation and function in plants. Plant Sci. 2003, 164, 901-909.

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(24) Monro, J. A.; Mishra, S.; Hardacre, A. Glycemic impact regulation based on

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progressive geometric changes in starch-based food particles during digestion.

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Food Digestion 2011, 2, 1–12.

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(25) Mackie, A.; Macierzanka, A. Colloidal aspects of protein digestion. Curr. Opin. Colloid Interface Sci. 2010, 15, 102–108.

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(26) Porter, J. H.; Trevaskis, N. L.; Charman, W.N. Lipids and lipid-based

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formulations: optimizing the oral delivery of lipophylic drugs. Nat. Rev. Drug

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Discov. 2007, 6, 231-248.

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(27) Casals, E; Vázquez-Campos, S; Bastús, N.G; Puntes, V. Distribution and

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potential

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nanostructures in biological systems. TrAC 2008, 27, 672-683.

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(28) Magnuson, B.A,; Jonaitis, T.S.; Card, J.W. A brief review of the occurrence,

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use, and safety of food-related nanomaterials. J. Food Sci. 2011, 76, R126-33.

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(29) Takeuchi, M.; Kojima, M; Luetzow, M. State of the art on the initiatives and

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(30) Maynard, A.; Bowman, D.; Hodge, G. The problem of regulating sophisticated materials. Nature Mater. 2011, 10, 554 – 557.

Figures

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Figure 1

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Figure 2

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Figure legends Figure 1.

Proteins, starch and lipids undergo major transformations at several length

scales during processing and cooking. Casein micelles and fat globules in milk enter the kitchen already as micron or submicron structures to later give rise to a myriad of dairy products. Starch granules must be cooked so that amylose and amylopectin are released from their natural semicrystalline arrangement of nanosize dimensions. Lipid molecules may form nanostructures (monoglycerides) or become hierarchically assembled into fat crystal networks (triacylglycerols). [Approximate scales]. Figure 2. Images of some food structures at the nanoscale. a) Protein aggregates (arrows) produced by heating native β-lactoglobulin at 80ºC and pH 6.0.7 These type of aggregates may also be formed when heating milk; b) TEM image of a 1% κcarrageenan gel10; c) Yoghurt-type gel showing two milk fat globules (arrows) and casein micelles arranged as a string-of-beads.2 Note anchoring of the strands to the globules' surface; and, d) Cross-section of nanoplatelets (circle) in a fat crystal network.13 All images were acquired by transmission electron microscopy.

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Where Is the Nano in Our Foods? - American Chemical Society

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