The present disclosure relates generally to the fields of food chemistry and food preservation. More particular, the disclosure relates to a poly-albumen-based green polymer/film coating and its use to enhance shelf-life of perishable foods.
World hunger is a rising issue; however, a third of the food produced around the globe is wasted and never consumed (Nature Editorial, 2019). This is largely due to perishable foods expiring during or shortly after the time it takes to distribute foods from farms to retail stores. The issue is especially prominent in fresh produce such as fruits and vegetables where around 40-50% of crops produced in the field are wasted every year (Global Food Losses and Food Waster, 2011). A major challenge is that produce is easily perishable with a shelf life of only a few days once they reach retailors. The main factors affecting the quality and post-harvest life of such produces are attributed to water loss or dehydration, texture deterioration, respiration and senescence processes, and microbial growth. Development of a cost-effective and green solution to extend the shelf life of produce by controlling these factors is paramount in alleviating food wastage.
Various methods have been reported to improve the shelf life of perishable fruits by controlling some of the factors known to accelerate degradation: microbial growth, dehydration, temperature (Han et al., 2018; Jongen, 2002; Ahvenainen, 2003). In addition to lengthening the shelf life of foods, it is also imperative that any method is biocompatible, biodegradable, have antimicrobial properties, be capable of forming a uniform membrane, and be safe for human consumption. Commercially, several preservation technologies are used to increase the shelf life of fruits (Jongen, 2005). One common method is fruit waxing which extends shelf life by artificially coating the fruits in preservatives using weak organic acids and their derivatives. However, on entering the human body, the gut cells fragment the preservatives into ions to maintain the physiological balance, resulting in several adverse effects including ion accumulation, membrane disruption, essential metabolite inhibition, draining energy to restore the homeostasis, and reductions in body-weight gain (Bracy et al., 1998; Krebs et al., 1983). Other prevalent methods to increase shelf-life include refrigeration, modified atmospheric packaging (MAP) with increased concentrations of carbon dioxide, and paraffin-based active coatings. However, these methods are expensive, time-consuming, visually alter the appearance of fruits, and affect the flavor of fruits. Therefore, there is a critical need for alternative green strategies to increase the shelf-life of perishable foods without altering the biological, physicochemical, and physiological characteristics of the products.
Recently, natural materials such as polysaccharides, proteins, lipids, chitosan, and alginate have been increasingly used in post-harvest preservation of fruits and vegetables (Mkandawire and Aryee, 2018; Patel, 2020; Tao et al., 2012). However, none of the developed materials displays distinctive properties in multiple important requirements of fruit preservation including preservation effectiveness, material flexibility, edibility, washability, and appearance, indicating a need for a more multifunctional coating material. The animal egg is an especially promising product to preserve fruits because it exhibits high protein and lipid content and accounts for up to ˜2% of food waste in the USA (Rahman et al., 2014; Chang et al., 2011). Thus, if eggs that would potentially end up as waste could instead be used to preserve other food items, then this would represent a sustainable and economically efficient method to reduce the perishability of food.
Thus, in accordance with the present disclosure, there is provided a composition comprising poly-albumen, cellulose nanocrystals (CNCs), and an external plasticizer. The poly-albumen may be derived from whole egg or egg white proteins. The external plasticizer may be a polyol, such as a low molecule weight polyol, such as glycerol, ethyleneglycol (EG), diethylene glycol (DEG), triethylene glycol(TEG), tetraethylene glycol, propylene glycol (PG), and polyethylene glycol (PEG) or a sugar alcohol such as sorbitol, mannitol, maltitol, xylitol, erythritol or isomalt, or monosaccharides (glucose, mannose, fructose, sucrose). The composition may comprise an anti-microbial and/or antioxidant, such as curcumin, riboflavin or cinnamaldehyde. The curcumin, cinnamaldehyde, and/or riboflavin may be crosslinked with said poly-albumen.
The CNCs may be present at about 15-45 wt % of said poly-albumen. The external plasticizer may be present at about 10-40 wt % of said poly-albumen. The anti-microbial and/or antioxidant may be present at about 1-10 wt % of said poly-albumen. The composition may further comprise egg yolk protein, such as at about 10-20 wt % of said poly-albumen. The CNCs may have a length and/or diameter of less than 1 μm, or less than 500 nm, or less than 250 nm, such as a length of about 90-130 nm and a diameter of about 4-8 nm. The CNCs may have a crystallinity index of about 84%. The composition may have a basic pH, such as about pH 8.0-12, or about pH 10.0.
Also provided is a perishable food product coated with the composition as described in the present disclosure. The product may be a fruit or vegetable, such as a climacteric or non-climacteric fruit or vegetable. The climacteric fruit may be apple, avocado, banana, breadfruit, cherimoya, durian, feijoa, fig, guava, kiwifruit, mango, muskmelon, papaya, passion fruit, pears, persimmon, plantain, quince, sapodilla, sapote, soursop, tomato or stone fruit (apricots, nectarines, peaches, plums). The non-climacteric fruit may be strawberry, blueberry, blackberry, pineapple, grape, raspberry, cherry, orange, lime, lemon, or grapefruit. The climacteric vegetable may be cantaloupe or potato. The non-climacteric vegetable may be cucumber, eggplant, pepper, summer squash or watermelon. The product may be an egg or a nut, such as a shelled nut.
In yet another embodiment, there is provided a method of preparing a food preserving composition comprising (a) dissolving poly-albumen in an aqueous solution, optionally including adjusting the pH of the dissolved poly-albumen solution to be at or greater than pH 8.0 and a temperature of 50-80° C.; (b) adding an external plasticizer to the solution of step (a); (c) adding cellulose nanocrystals (CNCs) to the solution of step (b). The poly-albumen may be derived from whole egg or egg white proteins. The external plasticizer may be a polyol, such as a low molecule weight polyol, such as glycerol, ethyleneglycol (EG), diethylene glycol (DEG), triethylene glycol(TEG), tetraethylene glycol, propylene glycol (PG), and polyethylene glycol (PEG) or a sugar alcohol such as sorbitol, mannitol, maltitol, xylitol, erythritol or isomalt, or monosaccharides (glucose, mannose, fructose, sucrose). The food preserving composition may further comprise an anti-microbial and/or antioxidant. The anti-microbial may be cinnamaldehyde, riboflavin or curcumin. The curcumin, cinnamaldehyde, and/or riboflavin may be crosslinked with said poly-albumen. The CNCs may be present at about 15-45 wt % of said poly-albumen. The external plasticizer may be present at about 10-40 wt % of said poly-albumen. The anti-microbial and/or anti-oxidant may be present at about 1-20 wt % of said poly-albumen. The method may further comprise, after step (b) and before step (c), adding egg yolk protein, such as at about 10-20 wt % of said poly-albumen.
In yet another embodiment, there is provided a method of preparing a food preserving composition comprising (a) dissolving poly-albumen in an aqueous solution, optionally including adjusting the pH of the dissolved poly-albumen solution to be at or greater than pH 8.0 and a temperature of 50-80° C.; (b) adding an external plasticizer to the solution of step (a); (c) adding cellulose nanocrystals (CNCs) to the solution of step (b). The poly-albumen may be derived from whole egg or egg white proteins. The external plasticizer may be a polyol, such as a low molecule weight polyol, such as glycerol, trimethylolpropate or pentaerythritol or a sugar alcohol such as sorbitol, maltitol, xylitol, erythritol or isomalt. The food preserving composition may further comprise an anti-microbial and/or anti-oxidant. The anti-microbial may be cinnamaldehyde, riboflavin or curcumin. The cinnamaldehyde, riboflavin and/or curcumin may be crosslinked with said poly-albumen. The CNCs may be present at about 15-45 wt % of said poly-albumen. The external plasticizer may be present at about 15-45 wt % of said poly-albumen. The anti-microbial and/or anti-oxidant may be present at about 1-20 wt % of said poly-albumen. The method may further comprise, after step (b) and before step (c), adding egg yolk protein, such as at about 10-20 wt % of said poly-albumen.
The use of the word “a” or “an” when used in conjunction with the term “comprising” in the claims and/or the specification may mean “one,” but it is also consistent with the meaning of “one or more,” “at least one,” and “one or more than one.” The word “about” means plus or minus 5% of the stated number.
It is contemplated that any method or composition described herein can be implemented with respect to any other method or composition described herein. Other objects, features and advantages of the present disclosure will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating specific embodiments of the disclosure, are given by way of illustration only, since various changes and modifications within the spirit and scope of the disclosure will become apparent to those skilled in the art from this detailed description.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
The following drawings form part of the present specification and are included to further demonstrate certain aspects of the present disclosure. The disclosure may be better understood by reference to one or more of these drawings in combination with the detailed description of specific embodiments presented herein.
Fruits and vegetables are an integral part of our food, providing us with various nutrients for a complete diet. A major challenge faced by the food industry is that fruits and vegetables are easily perishable with a shelf life of only a few days. The main factors affecting the quality and post-harvest life of fruits and vegetables are water loss, enzymatic browning, texture deterioration, senescence processes and microbial growth, among others. Around 40-50% of harvested produce is lost before consumption every year. Moreover, the common practice of food waxing does not adequately address this significant need in the industry.
A potential sustainable bionanocomposite that has yet to be investigated is egg white or egg albumen and yolks together reinforced with cellulose nanomaterials in increasing the shelf life of perishable produces. It is noted that cellulose, the most abundant biopolymer on earth, has excellent mechanical and gas barrier properties at nanoscale and can be synthesized from waste sources as well (Moon et al., 2011; Kontturi et al., 2018; Almeida et al., 2018). In this study, the inventors developed an edible and washable nanocomposite based on egg white or poly(albumen) and cellulose nanocrystals which can be coated conformally onto different perishable fruits as a micron-thickness coating using different approaches such as dip and spray coating. The coating successfully reduces microbial growth, respiration, and dehydration of fruits all of which contribute to an increased shelf-life while being edible and washable. Strawberries, avocadoes, papayas, and bananas have been utilized to demonstrate the effectiveness of the coating. The coating preserves the cosmetic appearance and shelf life of the fruits for longer and is easily washed off so that the taste for consumers is not altered.
Thus, it is proposed here that an egg-based multifunctional coating is promising as an economical and sustainable method to reduce global food waste by preventing post-harvest losses of perishable foods. The following disclosure describes the development of a poly-albumen polymer based edible film for fruit and vegetable preservatives. The egg yolk serves as an internal plasticizer and the egg white or poly-albumen is crosslinked with cinnamaldehyde, riboflavin or curcumin. Glycerol is employed as an external plasticizer in the film enhancing the flexible nature of the coating. The edible coating possesses antimicrobial and antioxidant properties, which arises from the employment of cinnamaldehyde, riboflavin or curcumin crosslinkers.
These and other aspects of the disclosure are described in detail below.
A. Fruits
In botany, a fruit is the seed-bearing structure in flowering plants that is formed from the ovary after flowering. Fruits are the means by which flowering plants (also known as angiosperms) disseminate their seeds. Edible fruits in particular have long propagated using the movements of humans and animals in a symbiotic relationship that is the means for seed dispersal for the one group and nutrition for the other; in fact, humans and many animals have become dependent on fruits as a source of food. Consequently, fruits account for a substantial fraction of the world's agricultural output, and some (such as the apple and the pomegranate) have acquired extensive cultural and symbolic meanings.
In common language usage, “fruit” normally means the fleshy seed-associated structures (or produce) of plants that typically are sweet or sour and edible in the raw state, such as apples, bananas, grapes, lemons, oranges, and strawberries. In botanical usage, the term “fruit” also includes many structures that are not commonly called “fruits,” such as nuts, bean pods, corn kernels, tomatoes, and wheat grains.
Consistent with the three modes of fruit development plant scientists have classified fruits into three main groups: simple fruits, aggregate fruits, and multiple (or composite) fruits. The groupings reflect how the ovary and other flower organs are arranged and how the fruits develop, but they are not evolutionarily relevant as diverse plant taxa may be in the same group.
B. Vegetables
Vegetables are parts of plants that are consumed by humans or other animals as food. The original meaning is still commonly used and is applied to plants collectively to refer to all edible plant matter, including the flowers, fruits, stems, leaves, roots, and seeds. An alternate definition of the term is applied somewhat arbitrarily, often by culinary and cultural tradition. It may exclude foods derived from some plants that are fruits, flowers, nuts, and cereal grains, but include savoury fruits such as tomatoes and courgettes, flowers such as broccoli, and seeds such as pulses.
The exact definition of “vegetable” may vary simply because of the many parts of a plant consumed as food worldwide—roots, stems, leaves, flowers, fruits, and seeds. The broadest definition is the word's use adjectivally to mean “matter of plant origin.” More specifically, a vegetable may be defined as “any plant, part of which is used for food,” a secondary meaning then being “the edible part of such a plant.” A more precise definition is “any plant part consumed for food that is not a fruit or seed but including mature fruits that are eaten as part of a main meal.” Falling outside these definitions are edible fungi (such as edible mushrooms) and edible seaweed which, although not parts of plants, are often treated as vegetables.
In the latter-mentioned definition of vegetable, which is used in everyday language, the words fruit and vegetable are mutually exclusive. Fruit has a precise botanical meaning, being a part that developed from the ovary of a flowering plant. This is considerably different from the word's culinary meaning. While peaches, plums, and oranges are fruit in both senses, many items commonly called vegetables, such as eggplants, bell peppers, and tomatoes, are botanically fruits.
Vegetables can be eaten either raw or cooked and play an important role in human nutrition, being mostly low in fat and carbohydrates, but high in vitamins, minerals and dietary fiber. Many nutritionists encourage people to consume plenty of fruit and vegetables, five or more portions a day often being recommended.
A. Egg White and Yolk
Egg white is the clear liquid, also called the albumen, contained within an egg. In chickens it is formed from the layers of secretions of the anterior section of the hen's oviduct during the passage of the egg. It forms around fertilized or unfertilized egg yolks. The primary natural purpose of egg white is to protect the yolk and provide additional nutrition for the growth of the embryo (when fertilized). Egg white consists primarily of about 90% water into which about 10% proteins (including albumins, mucoproteins, and globulins) are dissolved. Unlike the yolk, which is high in lipids (fats), egg white contains almost no fat, and carbohydrate content is less than 1%. Egg whites contain about 56% of the protein in the egg. Egg white has many food applications as well as many other uses (e.g., in the preparation of vaccines such as those for influenza).
Egg white makes up around two-thirds of a chicken egg by weight. Water constitutes about 90% of this, with protein, trace minerals, fatty material, vitamins, and glucose contributing the remainder. A raw U.S. large egg contains around 33 grams of egg white with 3.6 grams of protein, 0.24 grams of carbohydrate and 55 milligrams of sodium. It contains no cholesterol, and the energy content is about 17 calories. Egg white is an alkaline solution and contains around 148 proteins.
Ovalbumin is the most abundant protein in albumen. Classed as phosphoglycoprotein, during storage, it converts into s-ovalbumin (5% at the time of laying) and can reach up to 80% after six months of cold storage. Ovalbumin in solution is heat-resistant. Denaturation temperature is around 84° C., but it can be easily denatured by physical stresses. Conalbumin/ovotransferrin is a glycoprotein which has the capacity to bind the bi- and trivalent metal cations into a complex and is more heat sensitive than ovalbumin. At its isoelectric pH (6.5), it can bind two cations and assume a red or yellow color. These metal complexes are more heat stable than the native state. Ovomucoid is the major allergen from egg white and is a heat-resistant glycoprotein found to be a trypsin inhibitor. Lysozyme is a holoprotein which can lyse the wall of certain Gram-positive bacteria and is found at high levels in the chalaziferous layer and the chalazae which anchor the yolk towards the middle of the egg. Ovomucin is a glycoprotein which may contribute to the gel-like structure of thick albumen. The amount of ovomucin in the thick albumen is four times greater than in the thin albumen.
Egg white can be crosslinked to create poly-albumen. Albumen and poly-albumen are generally considered the same material but the polymeric properties of poly-albumen make it very useful in a variety of chemical and manufacturing endeavors. Methods for preparing poly-albumen from albumen are set forth in the examples. The same material used to crosslink albumen, creating poly-albumen, may act as an anti-oxidant and/or an anti-microbial.
Among animals which produce eggs, the yolk (also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example because they are laid in situations where the food supply is sufficient (such as in the body of the host of a parasitoid) or because the embryo develops in the parent's body, which supplies the food, usually through a placenta. Reproductive systems in which the mother's body supplies the embryo directly are said to be matrotrophic; those in which the embryo is supplied by yolk are said to be lecithotrophic. In many species, such as all birds, and most reptiles and insects, the yolk takes the form of a special storage organ constructed in the reproductive tract of the mother. In many other animals, especially very small species such as some fish and invertebrates, the yolk material is not in a special organ, but inside the egg cell (ovum). Yolks are often rich in vitamins, minerals, lipids and proteins. The proteins function partly as food in their own right, and partly in regulating the storage and supply of the other nutrients. For example, in some species the amount of yolk in an egg cell affects the developmental processes that follow fertilization.
As food, the chicken egg yolk is a major source of vitamins and minerals. It contains all of the egg's fat and cholesterol, and nearly half of the protein. Egg yolk contains an antibody called antiglobulin (IgY). The antibody transfers from the laying hen to the egg yolk by passive immunity to protect both embryo and hatchling from microorganism invasion. The yolk makes up about 33% of the liquid weight of the egg; it contains about 60 kilocalories (250 kJ), three times the energy content of the egg white, mostly due to its fat content. All of the fat-soluble vitamins (A, D, E and K) are found in the egg yolk. Egg yolk is one of the few foods naturally containing vitamin D. The composition (by weight) of the most prevalent fatty acids in egg yolk typically is:
The different yolk proteins have distinct roles. Phosvitins are important in sequestering calcium, iron, and other cations for the developing embryo. Phosvitins are one of the most phosphorylated (10%) proteins in nature; the high concentration of phosphate groups provides efficient metal-binding sites in clusters. Lipovitellins are involved in lipid and metal storage and contain a heterogeneous mixture of about 16% (w/w) noncovalently bound lipid, most being phospholipid. Lipovitellin-1 contains two chains, LV1N and LV1C.
Yolks hold more than 90% of the calcium, iron, phosphorus, zinc, thiamine, vitamin B6, folate, vitamin B12, and pantothenic acid of the egg. In addition, yolks cover all of the fat-soluble vitamins: A, D, E, and K in the egg, as well as all of the essential fatty acids. A single yolk from a large egg contains roughly 22 mg of calcium, 66 mg of phosphorus, 9.5 micrograms of selenium, and 19 mg of potassium, according to the USDA.
B. Cellulose Nanocrystals
Nanocellulose is a term referring to nano-structured cellulose. This may be either cellulose nanocrystal (CNC or NCC), cellulose nanofibers (CNF) also called nanofibrillated cellulose (NFC), or bacterial nanocellulose, which refers to nano-structured cellulose produced by bacteria. Nanocellulose can be used as a low-calorie replacement for carbohydrate additives used as thickeners, flavor carriers, and suspension stabilizers in a wide variety of food products. It is useful for producing fillings, crushes, chips, wafers, soups, gravies, puddings, etc. The food applications arise from the rheological behavior of the nanocellulose gel.
CNF is a material composed of nanosized cellulose fibrils with a high aspect ratio (length to width ratio). Typical fibril widths are 5-20 nanometers with a wide range of lengths, typically several micrometers. It is pseudo-plastic and exhibits thixotropy, the property of certain gels or fluids that are thick (viscous) under normal conditions but become less viscous when shaken or agitated. When the shearing forces are removed the gel regains much of its original state. The fibrils are isolated from any cellulose containing source including wood-based fibers (pulp fibers) through high-pressure, high temperature and high velocity impact homogenization, grinding or microfluidization (see manufacture below).
Nanocellulose can also be obtained from native fibers by an acid hydrolysis, giving rise to highly crystalline and rigid nanoparticles which are shorter (100 s to 1000 nanometers) than the cellulose nanofibrils (CNF) obtained through homogenization, microfluiodization or grinding routes. The resulting material is known as cellulose nanocrystal (CNC).
The ultrastructure of nanocellulose derived from various sources has been extensively studied. Techniques such as transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), wide angle X-ray scattering (WAXS), small incidence angle X-ray diffraction and solid state 13C cross-polarization magic angle spinning (CP/MAS), nuclear magnetic resonance (NMR) and spectroscopy have been used to characterize typically dried nanocellulose morphology.
A combination of microscopic techniques with image analysis can provide information on fibril widths, it is more difficult to determine fibril lengths, because of entanglements and difficulties in identifying both ends of individual nanofibrils. Also, nanocellulose suspensions may not be homogeneous and can consist of various structural components, including cellulose nanofibrils and nanofibril bundles.
Crystalline cellulose has a stiffness about 140-220 GPa, comparable with that of Kevlar and better than that of glass fiber, both of which are used commercially to reinforce plastics. Films made from nanocellulose have high strength (over 200 MPa), high stiffness (around 20 GPa) but lack of high strain (12%). Its strength/weight ratio is 8 times that of stainless steel. Fibers made from nanocellulose have high strength (up to 1.57 GPa) and stiffness (up to 86 GPa)
In semi-crystalline polymers, the crystalline regions are considered to be gas impermeable. Due to relatively high crystallinity, in combination with the ability of the nanofibers to form a dense network held together by strong inter-fibrillar bonds (high cohesive energy density), it has been suggested that nanocellulose might act as a barrier material. Although the number of reported oxygen permeability values are limited, reports attribute high oxygen barrier properties to nanocellulose films. One study reported an oxygen permeability of 0.0006 (cm3 μm)/(m2 day kPa) for a ca. 5 μm thin nanocellulose film at 23° C. and 0% RH. In a related study, a more than 700-fold decrease in oxygen permeability of a polylactide (PLA) film when a nanocellulose layer was added to the PLA surface was reported.
Changing the surface functionality of the cellulose nanoparticle can affect the permeability of nanocellulose films. Films constituted of negatively charged cellulose nanowhiskers could effectively reduce permeation of negatively charged ions, while leaving neutral ions virtually unaffected. Positively charged ions were found to accumulate in the membrane.
C. Anti-Microbials and Anti-Oxidants
Other materials that can be used in the compositions of this disclosure may also include anti-microbials and/or anti-oxidants such as cinnamaldehyde, riboflavin and curcumin.
Cinnamaldehyde is an organic compound with the formula C6H5CH═CHCHO. Occurring naturally as predominantly the trans (E) isomer, it gives cinnamon its flavor and odor. It is a phenylpropanoid that is naturally synthesized by the shikimate pathway. This pale yellow, viscous liquid occurs in the bark of cinnamon trees and other species of the genus Cinnamomum. The essential oil of cinnamon bark is about 90% cinnamaldehyde.
Riboflavin, also known as vitamin B2, is a vitamin found in food and used as a dietary supplement. It is required by the body for cellular respiration. Food sources include eggs, green vegetables, milk and other dairy products, meat, mushrooms, and almonds. Some countries require its addition to grains. As a supplement, it is used to prevent and treat riboflavin deficiency. At amounts far in excess of what is needed to meet dietary needs as a nutrient, riboflavin may prevent migraines. Riboflavin may be given by mouth or injection. It is nearly always well tolerated. Normal doses are safe during pregnancy.
Curcumin is a bright yellow chemical produced by plants of the Curcuma longa species. It is the principal curcuminoid of turmeric (Curcuma longa), a member of the ginger family, Zingiberaceae. It is sold as an herbal supplement, cosmetics ingredient, food flavoring, and food coloring. Chemically, curcumin is a diarylheptanoid, belonging to the group of curcuminoids, which are natural phenols responsible for turmeric's yellow color. It is a Keto-enol tautomer, existing in enolic form in organic solvents and in keto form in water.
Laboratory and clinical research have not confirmed any medical use for curcumin. It is difficult to study because it is both unstable and poorly bioavailable. It is unlikely to produce useful leads for drug development.
D. Glyceryol
Glycerol (also called glycerine in British English or glycerin in American English) is a simple polyol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in those lipids known as glycerides. Due to having antimicrobial and antiviral properties it is widely used in FDA approved wound and burn treatments. It can be used as an effective marker to measure liver disease. It is also widely used as a sweetener in the food industry and as a humectant in pharmaceutical formulations. Owing to the presence of three hydroxyl groups, glycerol is miscible with water and is hygroscopic in nature. Although achiral, glycerol is prochiral with respect to reactions of one of the two primary alcohols. Thus, in substituted derivatives, the stereospecific numbering labels the molecule with a “sn-” prefix before the stem name of the molecule.
In food and beverages, glycerol serves as a humectant, solvent, and sweetener, and may help preserve foods. It is also used as filler in commercially prepared low-fat foods, and as a thickening agent in liqueurs. Glycerol and water are used to preserve certain types of plant leaves. As a sugar substitute, it has approximately 27 kilocalories per teaspoon (sugar has 20) and is 60% as sweet as sucrose. It does not feed the bacteria that form a dental plaque and cause dental cavities. As a food additive, glycerol is labeled as E number E422. It is added to icing (frosting) to prevent it from setting too hard.
As used in foods, glycerol is categorized by the U.S. Academy of Nutrition and Dietetics as a carbohydrate. The U.S. Food and Drug Administration (FDA) carbohydrate designation includes all caloric macronutrients excluding protein and fat. Glycerol has a caloric density similar to table sugar, but a lower glycemic index and different metabolic pathway within the body, so some dietary advocates accept glycerol as a sweetener compatible with low-carbohydrate diets. It is also recommended as an additive when using polyol sweeteners such as erythritol and xylitol which have a cooling effect, due to its heating effect in the mouth, if the cooling effect is not wanted.
The food coatings of this disclosure can be made as follows. First, one will dissolve poly-albumen in an aqueous solution and optionally adjust the pH of the dissolved poly-albumen solution to be at or greater than pH 8.0 at a temperature of 50-80° C. Next, an external plasticizer is added, followed by addition of cellulose nanocrystals (CNCs). The poly-albumen may in particular be derived from whole egg or egg white proteins. In some embodiments, an anti-microbial and/or anti-oxidant is added after the external plasticizer but for the CNCs. Additionally, or alternatively, egg yolk protein may be added in this same time as the anti-microbial and/or anti-oxidant.
Once the coating material has been prepared, it will be applied to the food product. Applying may involve dipping, spraying, rinsing, painting or any other form of contacting the surface of the food product. The application and/or subsequent drying of the coasting material can be performed at ambient/room temperature. The application generally will be performed quickly, such as over the course of a few second, or about 0.5 to about 5 seconds. However, there is no particular reason that longer applications times my not be employed, and in some cases may be advantageous. Also, the process may be repeated to produce multiple coats or thicker coatings, such as 2, 3, 4, 5, 6, 7, 8, 9, 10 or more applications.
The following examples are included to demonstrate preferred embodiments. It should be appreciated by those of skill in the art that the techniques disclosed in the examples that follow represent techniques discovered by the inventor to function well in the practice of embodiments, and thus can be considered to constitute preferred modes for its practice. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the spirit and scope of the disclosure.
Materials. Egg white powder was purchased from Judee's Gluten Frees. Egg yolk powder was purchased from Modernized Pantry. Analytical grade Sodium Hydroxide pellets were acquired from BDH, and glycerol (99.5%≥purity) was purchased from Sigma-Aldrich. Cellulose nanocrystals (CNC) (˜10.4 wt. %) were obtained from the University of Maine's Product Development Center. Organic curcumin powder with 95% Curcuminoids was purchased from Micro Ingredients.
Synthesis of cellulose nanocrystals. Cellulose nanocrystals (CNCs) are produced from wood pulp at U.S. Forest Service Cellulose Nano-Materials Pilot Plant in the Forest Products Laboratory (FPL) according to the procedure described by Beck-Candanedo et al. (2005). Briefly, CNCs were prepared using ˜64% sulfuric acid by hydrolysis of the amorphous regions in cellulose polymer, which yields the acid resistant crystals as a product. The crystals were purified by dilution and neutralization of the acid, followed by separation of the soluble components such as glucose and salt byproducts from the insoluble CNC using a vacuum filtration technique. Two stages of dilution and settling were conducted to remove most of the soluble components (˜90%). Once the ionic strength was low enough, the sulfonic acid groups on the CNCs provided a stable suspension and continuous dilution and filtration was used to remove the remaining soluble contents. The final stage was to remove water using the membrane filtration system, resulting in a 5-6.5% aqueous viscous slurry. It is noted that there was sufficiently low water content and high enough ionic strength from acid groups on the CNCs to limit bacterial or fungal growth in the stable slurry. The CNCs suspension was stored at ˜4° C. before use in the nanocomposites.
Synthesis of coating solution. The nanocomposite fruit coatings were prepared by dip-coating fruit into the nanocomposite solution. The solution was synthesized by first, egg white powder was dispersed into DI water 1:15 (w/v) by a magnetic stirrer for 15 min. After the mixture was homogenized, the pH of the solution was adjusted to 10 by adding NaOH pellets. Then, glycerol (30 wt. % of egg white powder) was added, and the solution was stirred for 15 min at 80° C. Egg yolk powder (15 wt. % of egg white powder) pre-dispersed in DI water was added into the solution and stirred for 5 min at the same temperature. Subsequently, curcumin powder (5 wt. % of egg white powder) dispersed into DI water was incorporated then stirred for another 5 min. Lastly, cellulose nanocrystals (30 wt. % egg white powder) was dispersed in the solution for about 5 min. The mixture was cooled at room temperature for 30 min. To produce the nanocomposite coating film, the solution was poured onto the Teflon sheet and air-dried at RT for 2˜3 consecutive days.
Preparation of Fruit Coating. After cooling, the nanocomposite solution is capable of dip or spray-coating onto fruits. In this study, the inventors chose the dip-coating method to coat fruits. Four different fruits (strawberries, bananas, papayas, and avocados) were soaked completely into the solution by holding the stem. After drying for 1 min through hanging on a rack with the stem tied by a string, the fruit was coated with a second layer then dried in the same way at room temperature.
Transmission electron microscopy (TEM). TEM micrographs were obtained using transmission electron microscope (Philips CM-100) with an acceleration voltage of 100 kV to analyze the size and shape of cellulose nanocrystals (CNCs). A drop of diluted dispersion of CNCs was deposited on carbon-coated grid and dried prior to observations. Image J software was used to measure the length and diameter of CNCs.
To understand the dispersion of CNCs in the nanocomposite coating, the nanocomposite film was treated overnight in 3% ethanolic glutaraldehyde, rinsed with fresh ethanol, then sections on 100 nm thickness were cut using an ultramicrotome. Sections were stained with 1% osmium tetroxide, post-stained with 3% uranyl acetate and 3% lead citrate, then rinsed and dried. TEM images were collected with a JEOL JEM-1230 at 80 kV.
Atomic force microscopy (AFM). Atomic force microscopy (AFM) images of CNCs and nanocomposite films were obtained in tapping mode using a Park NX-10 microscope. The measurements were carried out in air using silicon nitride cantilevers at room temperature. The procedure used for imaging the CNCs were similar to the procedures reported by Brinkmann et al. (2016). Briefly, freshly cleaved mica (1 in.×1 in.) was incubated in ˜200 μL of a ˜0.01 wt. % poly-L-lysine solution for ˜30 min and then the mica was rinsed five times with DI water and dried in a nitrogen stream. About 80 μL of a de-ionized (DI) water diluted suspension of CNCs (1:10000) was deposited onto the substrate and incubated for ˜1.5 min. The samples were then washed with DI water and dried once again with the nitrogen stream.
AFM images of nanocomposite films were recorded with a scan size of 25 μm×25 μm. The root-mean-square roughness (Rq) was evaluated over the scanning area as the standard deviation of the topography (M pixels N pixels).
Zeta (ζ) potential. Zeta (ζ) potentials of cellulose nanocrystal aqueous dispersions were measured at ambient temperature using Malvern Zetasizer Nano ZS90 according to the Smoluchowski's equation to understand the dispersion stability (Sze et al, 2003). Suspension was prepared by dispersing CNC in DI water using ultrasonication for 30 s at 25% intensity.
X-ray diffraction (XRD). XRD patterns of CNC films were recorded on an X-ray diffractometer (Panalytical Empyrean powder X-ray diffractometer) equipped with Cu Kα1 radiation (λ=0.154 nm) at 45 kV and 40 mA. Data were collected from 5° to 40° Bragg angles (2θ) at a scan rate of 2 deg/min and a step interval of 0.02°. The CNC film was prepared by using 24 g (˜5 wt. %) CNC suspension was placed into a polystyrene petri dish and sample was dried at room temperature for 72 h. The Peaks from XRD were identified and deconvoluted using Origin Pro software to calculate crystallinity index. The crystallinity index was calculated from the (200) plane of cellulose Iβ, using Segal equation:
where I is the intensity of the deconvoluted peak at the plane's characteristic angle.
Fourier-transform infrared spectroscopy (FTIR). The chemical structures of CNCs and different films were investigated by ATR-FTIR (Spectrum 100, PerkinElmer, Waltham, Mass., USA). The scan range was 4000 to 600 cm−1 with resolution of 4 cm−1.
Rheology and Contact Angle Measurement. Viscosity data was collected using an ARES G2 rheometer with a 25 mm cone and plate geometry with a 0.1-radian cone angle. The nanocomposite's viscosity was measured at ambient conditions using a flow experiment at shear rates ranging from 10−2 to 102 sec−1. A force tensiometer (K100, Kruss Instruments) was used to obtain the surface tension of the nanocomposite solution. Contact angle measurements of water on the dried nanocomposite film and the nanocomposite solution on papaya, avocado, and banana were preformed using a Drop Shape Analyzer (DSA 100, Kruss Instruments) at ambient conditions.
Confocal Microscopy. Coated and bare bananas were diced using fruit dicer. The diced peels were imaged at two different orientations, (i) to acquire images of the outer skin surface (top), the diced peel sample was placed with the inner skin (adjoining the banana pulp) facing down on the glass slide, and (ii) to acquire images of both the outer surface and inner face of the peel, i.e., along the depth of the peel, the diced samples were laid on their side (inverted) on the glass slide. Images were simultaneously acquired at three different excitation wavelengths 405, 488, and 594 nm using the FluoView-1000 confocal microscope (Olympus America, NY). Images were processed using background subtraction. A total of 30 thickness measurements were made using six single optical images (5 measurements per image) and OLYMPUS FLUOVIEW Ver.4.2.b software. Thickness measurements were confirmed via YZ projections, using Image J.
Mechanical properties. The tensile properties of the nanocomposite films were measured by dynamic mechanical analysis (Q800, TA Instrument, USA). The test was conducted at the ambient conditions with 5 N/min in controlled force rate mode. The length of the films was 30 mm with a gauge length of 10 mm. The thickness and width of the film is around 0.2 mm and 5 mm; hence the cross-sectional area is about 1 mm2. At least 5 samples were tested to ensure the consistency of the data.
Biaxial tensile testing was performed using a high-throughput mechanical characterization (HTMECH) instrument (Sormana & Chattopadhyay, 2005) (
Uniaxial compressive tests on the bare and coated fruits were done at room temperature with a standard universal testing machine (Instron 4505, USA) equipped by 100 kN load cell. Samples were placed between two crossheads and checked to avoid misalignment or detachment, then compressed with a constant rate of 2 mm-s−1. The load was measured by load cell while the displacement of crosshead was recorded. The load-displacement data was scanned and recorded on the computer. At least 5 samples were tested to ensure the consistency of the data.
Weight Loss Measurement. Weight loss data were calculated from the measured initially purchased weight (Wo) and the weight at subsequent days after purchase (Wt). The weight loss value as a function of time was calculated using the equation:
Water vapor permeance. The steady-state water vapor permeability for a free-standing was measured according to ASTM-D1653 by a conventional permeability cup (purchased from Gardco, Fla.). Circular 10 mm diameter samples were precisely cut with a laser cutter and used as test specimens. The permeability cup was filled with anhydrous CaCl2 and sealed, leaving a small air gap between the specimen and desiccant. The whole arrangement was kept in a desiccator with water and maintained at 23±1° C. with very high (97±1%) relative humidity for this experiment. The increase in cup weight was measured by a microbalance for several days, and the WVTR values were calculated from the slope of the weight change versus time.
Oxygen Permeability. The oxygen permeability (OP) of films was obtained by using a MOCON OXTRAN 1/50 instrument at 50% RH and 23° C. The films were humidified for 1 min using an Electrotech ultrasonic humidification system (Glenside, Pa.) operating at 100% RH for easier handling. The films were cut into squares of approximately 10 cm×10 cm. The test cell for the MOCON measures over a 50 cm2 area of the film. The criteria for steady state was that the contiguous readings must differ by less than 1% or 0.05 cc/m2/day (convergence by cycles mode).
Cellular toxicity. Panc02 cells were cocultured with the coating solution, and viability was calculated after 24 h. The cells were cultured in DMEM solution with 10% fetal bovine serum and 1% penicillin-streptomycin and maintained in a 37° C., humidified atmosphere with 5% CO2. Cell viability was measured using a Promega MTS assay kit. 2500 cells were seeded in 96 well plates and incubated for 24 h before adding fresh solution mixed with different concentrations of coating solutions (0, 0.01, 0.1, 1.0 μg/mL). After 48 h, the MTS assay was performed. The cell solution was removed from each well, and 100 μL of fresh cell solution was added to each well followed by 20 μl of MTS solution. Cells were incubated at 37° C. for 4 h before reading the optical absorbance at 490 nm. Similar study has been conducted on with different concentrations of CNCs (0, 0.001, 0.1, 1, 10, 20 mg/ml) to investigate the biocompatibility and toxicity of CNCs.
Antimicrobial activity. Polymer specimens were tested for antimicrobial activity using a modified version of protocol ISO 22196. Soybean casein digest broth with lecithin and polyoxyethylene sorbitan monooleate (SCDLP broth) and Mueller-Hinton broth were prepared as previously described. Bacterial pre-culture was prepared from Escherichia coli (E. coli) strain BL21 cells. Polymer specimens were prepared by cutting portions measuring 26 mm×50 mm. As specimens reacted when exposed to solvents, they were sterilized before treatment by exposure to ultraviolet light while within a sterile petri dish. Samples were exposed to a Sterilamp G36T6L lamp with a power of 41 W for 30 min; samples were then flipped using sterile tweezers, and the other side of the sample was exposed for an additional 30 min. Samples were similarly prepared from Parafilm, an inert material, to serve as a control. Tested samples were treated with 200 μL aliquots of diluted bacterial culture and covered with a sterile coverslip measuring 24 mm×40 mm.
Sol-gel Analysis. Pre-weighed amounts of film specimens were immersed in a glass bottle and placed on a platform shaker until it dissolved. The contents of the bottle were filtered using a microfiber-based fabric filter and the gel residue was collected.
The inventors developed the cellulose nanocrystal reinforced poly(albumen) coating with various biocompatible modifiers to extend both the shelf-life and cosmetic appearance of fruits (Table S1, Example 1). To synthesize the nanocomposite coating, the inventors start with egg whites which are comprised mostly of albumen protein (˜54%) and enable the dried formation of strong edible films with a moderate gas barrier property. However, poly(albumen) is very brittle owing to its random organization of denatured proteins. The addition of plasticizers, such as glycerol, reduces intermolecular forces in the protein chain and increases mobility in the protein-polymer chains and its flexibility (Mekonnen et al., 2013; Rahman & Netravali, 2014). Therefore, an egg white plasticized with glycerol is capable of coating irregularly shaped objects like fruit without cracking. However, glycerol is hydrophilic and swells in humid environments. To prevent unwanted swelling, the inventors incorporated a small fraction of egg yolk, which is hydrophobic and rich in fatty acids and can alleviate the susceptibility to moisture. Next, they added curcumin which is an edible extract from turmeric that possesses antibacterial, antifungal and antibiofilm properties (Liu & Guo, 2018; Wu et al., 2018). These properties reduce microbial growth on the fruit surface while also decreasing 02 and increasing CO2 in the microenvironment which helps to maintain the fruit's freshness. Lastly, the inventors incorporate cellulose nanocrystals (CNCs) to decrease the water and gas permeance of the coating and to add mechanical reinforcement. CNCs is one of the main components in the preparation of the nanocomposites and synthesized via acid hydrolysis process using wood pulp (detail is provided in supplementary information). The inventors have reported in-depth characterization of the CNCs using transmission electron microscopy (TEM), atomic force microscopy (AFM), zeta potential, x-ray diffraction (XRD), and fourier-transform infrared spectroscopy (FTIR) studies (
First, the inventors measured the viscosity of the nanocomposite solution at room temperature as a function of shear rate to investigate the solution's processibility as a conformal coating. The coating solution exhibits shear-thinning behavior and the measured viscosity at a low shear rate (resting state) is around 200 Pa-s (
The inventors measured the affinity of the nanocomposite solution to fruits through contact angle measurement. It is important to understand the wetting of the coating solution on the fruit and its hydrophilicity to determine how the solution forms a uniform surface coating. First, they compared the contact angle of the freshly made coating solution on banana, papaya, and avocado peel. The contact angle on the avocado surface immediately after wetting with a drop was ˜45° and then decreased to ˜25° within 8 min (
To investigate the thickness and topology of the coating on fresh fruits, the inventors applied laser scanning fluorescence confocal microscopy. Banana fruit was used as a representative since its thicker peel allowed for ease of sample preparation for imaging. Multiple excitation wavelengths (405 nm, 488 nm, and 594 nm) were initially utilized to determine autofluorescence characteristics of uncoated and coated fruit skin.
The inventors evaluated the effectiveness of the coating in preserving fruit freshness in four readily available and representative fruits, including three climacteric fruits (banana, avocado, and papaya) and one non-climacteric fruit (strawberry). After 8-11 days post purchase, the uncoated climateric fruits all showed enzymatic browning and decaying on the exterior while the coated fruits retained the appearance for over a week (
The inventors next compared the firmness and compressibility between coated and uncoated fruits 7-9 days after the fruits are received and treated. As fruits over-ripen or perish, they become softer; therefore, these tests serve to provide further evidence that the coated fruits maintain their freshness longer than bare fruits. The fruit deformation in response to a compression force was recorded and graphed for a banana, avocado, and papaya (
To understand the mechanisms of the nanocomposite coating in preserving fruit freshness, the inventors prepared a free-standing nanocomposite film to perform tests on. Films with ˜70 μm thickness were obtained by solution casting into a Teflon sheet and solvent evaporation (
The film is extremely flexible as it can be repeatably bent and folded without breaking. Good mechanical properties are among the basic requirements for the film to be used as fruit coating to resist premature failure or cracking during handling and storage. To measure the mechanical performances of the film, the inventors conducted uniaxial and biaxial tensile testing (
Next, AFM images were used to investigate the topography of the dried film (
To understand the coatings' ability to prevent decay and dehumidification, the inventors measured the water vapor transmission rate through a 100-μm thick dried nanocomposite film at ambient conditions. The coating has a water vapor transmission rate of ˜15 g-mm/mg-day, which is low when compared to other common biopolymers for packaging, including chitosan, polylactic acid (PLA), pectin, protein isolate, and starch-based composites (
Another important parameter that affects fruit perishability is oxidation as it involves fruit respiration. The inventors measured the O2 gas barrier properties of the nanocomposite film to understand if the coating's effectiveness against ripening is in part due to its effect on oxidation. They found that the films have a low oxygen permeability (OP) compared to other packaging materials like PLA, carnauba wax, protein isolate, and starch (
The inventors evaluated the antimicrobial properties of the coating using an E. coli strain as microbial growth is known to contribute to fruit perishability. While instant exposure to the coating film does not decrease bacteria titers, overnight incubation on the film resulted in zero bacteria titers (
Next, the inventors evaluated the toxicity of the coating using in vitro studies with a human pancreatic cancer cell line (Panc02) to evaluate the edibility. Biocompatibility is crucial for the coating to be applied to fruit as consumers could intentionally or accidentally ingest the coating during the consumption of the fruit. After 24 h incubation with 0.1 μg/ml to 1 μg/ml coating, there is no significant change in Panc02 cell (
Further qualitative and quantitative comparison studies on two model fruits (avocado and banana) has been conducted using some common edible coating materials to understand the effectiveness of the inventors' nanocomposites (Example 1). The model fruits were avocado and banana. The inventors have selected wax and chitosan based edible coatings to compare with their nanocomposite coating. In terms of visual appearance (
Finally, the inventors conducted a sol-gel test to measure the solubility of the coating in water, as consumers might prefer the taste of the fruit without the coating. To demonstrate the washability of the film, a slightly thicker (100 μm) film compared to the actual coating thickness (23˜33 μm) was put into a vial with room temperature DI water. Upon shaking for ˜2 min at room temperature, the film completely disintegrated and dispersed into the DI water (
Food waste represents a huge impediment towards eliminating hunger around the globe and extending the shelf-life of perishable food offers a direct solution to this problem. Although various methods have been used to preserve fresh food, they are often limited by manufacturing cost and food safety concerns. An egg protein-based nanocomposite coating was developed for effective freshness preservation. The coating is mechanically robust, easy to produce, possesses extraordinary gas barrier properties, and could be sourced from waste biomaterials as well. Through the demonstration of four fruit models, the inventors showed that the coating can retain freshness, appearance, and aroma for at least one week longer than uncoated samples, validating the universal effectiveness of the coating in preventing fruit rotting. Its edibility (biocompatibility) and washability (solubility) in water also reduce food safety concerns. The inventors believe this work presents an innovative approach to addressing the global food waste problem as an environmentally friendly, highly scalable and low-cost freshness preserver.
All of the compositions and methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the compositions and methods of this disclosure have been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations may be applied to the compositions and methods and in the steps or in the sequence of steps of the method described herein without departing from the concept, spirit and scope of the disclosure. More specifically, it will be apparent that certain agents which are both chemically and physiologically related may be substituted for the agents described herein while the same or similar results would be achieved. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit, scope and concept of the disclosure as defined by the appended claims.
The following references, to the extent that they provide exemplary procedural or other details supplementary to those set forth herein, are specifically incorporated herein by reference.
This application claims benefit of priority to U.S. Provisional Application Ser. No. 63/050,987, filed Jul. 13, 2020, the entire contents of which are hereby incorporated by reference.
Number | Date | Country | |
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63050987 | Jul 2020 | US |