Nanocellulose and Its Application in the Food Industry ^†

Abstract

This work presents a review related to the obtainment of cellulose from different structures in agro-industrial residues, both for application in the food industry and for the reinforcement of other materials. Cellulose nanofibers are produced by the heart of palm (Bactris gasipaes) industry in Brazil and are used as a stabilizer in avocado oil emulsions; conversely, cellulose nanocrystals are produced in waste from the tequila industry (Agave tequilana Weber var. Azul) in Jalisco, Mexico, and are used for reinforcement applications.

1. Introduction

Cellulose is considered a natural polymer of great abundance, since it is possible to obtain it from very diverse sources such as animals; microorganisms; non-timber fibers such as resins, gums, and waxes; and others (fungi, seeds, leaves, nopal, stems, fruits, etc.) [1]. Cellulose can be obtained from plants in their virgin state and from the waste they themselves generate. In this way, a material that would otherwise be discarded, and that could cause environmental problems, is revalued [2].

The name nanocellulose refers to the nanometric-scale dimensions of this natural polymer. There are three types of nanocellulose, categorized depending on their production and extraction: crystal-shaped nanocellulose (NCC), nanocellulose fibers (NFC), and bacterial nanocellulose (NCB) [3].

The generation of new materials based on nanocellulose has become an increasingly attractive area of development, because these nanomaterials have the characteristics of sustainability, biodegradability, non-toxicity, and economic production [4].

Applications of nanomaterials include important industries such as paper, food, electronics, pharmaceutical, biomedical engineering, construction, packaging, etc. [5,6].

Despite the fact that all of these industries generate waste, the food industry is one of the sectors that generates the most environmental impact, due to its processes and the different products they generate [7]. Therefore, finding an application for waste generated in industries like this is a challenge, but a necessity for the achievement of a sustainable society.

2. Lignocellulosic Waste

Due to the large-scale production of tequila in the state of Jalisco in Mexico, agave bagasse is an abundant source of lignocellulosic biomass [8,9,10]. Bagasse is a solid by-product of a fibrous nature, obtained after the grinding of the agave pineapple and the extraction of fermentable sugars in the manufacture of tequila [8]. It is estimated that the fibrous biomass resulting from the grinding of agave pineapple is equivalent to 40% of the total wet weight [11]. Agave bagasse, due to its high availability, has traditionally always presented serious problems for the industry, because its final disposal comes at high management costs [12]. This has led to the problem of environmental contamination, because most of this waste ends up as waste in clandestine dumps due to a lack of environmental regulation. This causes negative effects on the fertility of farmland [13], leachate contamination, and phytosanitary risks due to the inadequate incorporation of this material into soil [12,14]. However, based on its chemical composition, it is known that agave bagasse contains 44.5% cellulose, 25.3% hemicellulose, and; 20.1% lignin [9,15], so researchers have tried to diversify its applications in different areas, such as the production of biopolymers, composting, animal feed, and the generation of biofuels [8,11] and reinforcement materials. It has recently been studied in relation to the production of nanofibers and cellulose nanocrystals [9,10,15,16].

In the case of the of peach palm heart production in Brazil, where the economy is most reliant on agriculture, the exploitation of different lignocellulosic residues and wastes for nanocellulose production offers a great chance to increase the income of small-scale companies and farmers, and to achieve the sustainable develop- ment of agriculture. The extraction of peach palm from Bactris gasipaes (aka pupunha) palm trees to produce peach palm heart (palmito) produces high amounts of residues, since just 10% of the tree is used for food production, and the other parts generally are used in farming activities or energy production. Brazil is the major producer and consumer of palmito, and the waste from their plantation in 2017 represented approximately 5 × 10⁵ tons of cellulose that was released into the field or incinerated, but could have been used for more justifiable and profitable purposes [17,18]. Figure 1 shows the scheme of a method for obtaining nanofibers from palm residues and nanocrystals from agave bagasse.

3. Characterization of Cellulose Nanostructures

Lignocellulosic materials are characterized by the presence of cell walls, mainly made up of a series of coaxial layers of cellulose microfibrils (skeleton) dispersed in an amorphous matrix of hemicelluloses and lignin, which together represent 80–90% of the total weight [19]. Among the structural components of the cell wall are lignin and polysaccharides, the most abundant being cellulose and hemicelluloses.

Nanomaterials or nanometric materials have attracted scientific interest in recent years because they have better properties, whether electrical, mechanical, thermal, etc., than materials with the same composition but a macrometric size. By definition, these must have at least one of their dimensions in the range of 0.1 to 100 nm, although some authors consider them up to 600 nm [20].

The extraction and production of nanocellulose from various sources has attracted increasing interest due to this material’s abundance, strength, rigidity, low weight, and biodegradability [21]. Different terms are used in the literature to designate these cellulose nanoparticles in the form of crystalline rods. They are mainly referred to as whiskers, nanowhiskers (NWC), nanofibers (NFC), cellulose nanocrystals (NCC), monocrystals, and microcrystals [22]. However, the dimensions of cellulosic nanoparticles depend on several factors, including the source of cellulose and the exact preparation conditions [23].

Through different mechanical, chemical, enzymatic or biological processes, it is possible to obtain nanofibers (NFC) and cellulose nanocrystals (NCC), which are the most basic structural forms of this polysaccharide. They have crystalline domains, which have excellent mechanical properties, and an elastic modulus of the order of 150 GPa, which is greater than the elastic modulus of glass fibers (85 GPa) and that of aramid (65 GPa) [24]. Therefore, these nanomaterials can provide considerable improvements to the mechanical properties of the matrices to which they are added. NFCs an elongated cylindrical shape with a high aspect ratio; they are very long in relation to their diameter. NCCs take the shape of an elongated grain of rice. These nanomaterials have high potential for use in multiple ways, particularly as reinforcing materials for the development of nanocomposites, due to the fact that they have a large specific surface area, that is, the area per unit mass wherein they can interact directly with the matrix [20]. Many studies have been performed for the isolation and characterization of NFC and NCC from various sources. This is why the elaboration of nanocomposites with replacement capacity, to act as regenerative agents and as structural supports for various materials, is a viable option [25]. In this sense, it is also hoped that these materials will be environmentally friendly, that they will allow production on an industrial scale [26], and that they will be of biological origin. All of the above factors will give them a comparative advantage over conventional materials, such as ceramics, metals, and polymers [27].

4. Nanocellulose in Food

Nanomaterials are used in the food industry to improve the quality of food products. They can prevent microbial degradation of packaged foods, improve their color, flavor or texture, and increase the bioavailability of vitamins and minerals [28,29].

Nanomaterials used in food can be classified into three different groups [30]:

Organic nanomaterials include lipids, proteins, and polysaccharides, which are used to encapsulate vitamins, antioxidants, dyes, flavorings and preservatives, and form micelles, liposomes, and nanospheres, etc. They allow for higher intake, absorption, bioavailability, and stability in the body.

Organic/inorganic combined nanomaterials are also called surface functionalized nanomaterials, generally added to a matrix for their specific functionality (antimicrobials, antioxidants, and permeability and rigidity regulators).

Inorganic nanomaterials are metals and metal oxides of Ag, Fe, Se, TiO₂, used as additives, food supplements, or in packaging [31].

Avocado is one of the most abundant fruits in Mexico. In fact, Mexico is the biggest exporter of avocado in the world. There are many studies that show that consumption of avocado provides optimal fats human needs [32]. Avocado oil, due to its fatty acid composition, meets nutritional recommendations that focus on reducing the amount of saturated fat in the diet [32,33,34]. Diets rich in avocado oil have been shown to be effective in reducing total cholesterol, LDL (low-density lipoprotein) cholesterol, and plasma triglycerides, as is the case with diets containing corn, soybean, or sunflower oil [32].

Avocado oil-based emulsions are widely used as a food supplement or dressing because they are healthier than alternatives. More and more methods of extending the expiration and stability of these emulsions are sought [35,36,37].

The use of cellulose nanofibers (NFC) to improve some properties in food can be carried out in three different ways: via food additives (food supplements), in packaging, or in emulsifiers [38]. In this case, their effects can be compared with those produced by other emulsifiers commonly used in this sector, such as sorbitan monostearate or Span 60 [39]. This behavior was reported by Talita et al. in a study of the stabilization of avocado oil emulsions with cellulose nanofibers obtained from the palpito (Bactris gasipaes) industry waste in Brazil [18]. These nanometric structures allowed the elimination of the phenomenon of coalescence in emulsions, and the formation of cream on the surface of them [18]. In addition, it was found that the stability of the emulsions created increased, as was the same as that of emulsions that were refrigerated. The particle size of the micelles formed decreased as the percentage of considered cellulose nanofibers increased, which allowed the researchers to establish the influence of these nanostructures on the final stability of the system.

5. Nanocellulose as Reinforcement

The use of cellulose as a reinforcement material for synthetic polymers and recycled plastic materials has generated great interest. There are many studies related to the analysis of the mechanical properties of biocomposites obtained via mixing both materials [40,41]. It has been possible to prove that the addition of nanocellulose to synthetic polymers significantly favors some of the properties of these polymers, such as tensile strength and thermal conductivity; however, this depends on the initial nature of the cellulose [42,43,44].

In studies about the reinforcement of polylactic acid (PLA) with nanocellulose, it has been verified that this combination of materials has the potential to be competitive, since the properties in general were similar to those of petroleum-derived polymers. Both the polymeric matrix and the reinforcement additives made it possible to achieve biodegradable materials, which were approved for contact with food [45]. Pech et al. reported that in using cellulose nanocrystals from tequila industry waste (residues of Agave tequilana Weber var. Blue), it is possible to observe this behavior [46].

6. Conclusions

In this paper, we present two direct applications of nanocellulose in the form of nanofibers and nanocrystals, both in the food industry and as a reinforcement for other materials.

This is a small sample of what it is possible to do with this biopolymer in pursuit of global sustainability. The use of the lignocellulosic waste that each region of the planet produces on a regular basis can be made into a source of income for the most disadvantaged populations. It is possible to improve food properties with the gradual incorporation of cellulose nanostructures, although we must be cautious about the side effects they can cause.