Review: Utilization of cellulose in food products

Cellulose is one of the most abundant natural materials in the world. It has a long-chain carbohydrate polymer made up of repeating glucose units. Reviewed literature shows that the material has been widely explored as a functional ingredient in foods, including meat products, emulsions, beverages, dairy products, bread, confectionery, and fillings. This carbohydrate polymer has many promising applications in the functional food and nutraceutical industries. Cellulose can be isolated through chemical, mechanical, and biological means to produce a variety of functional materials in the form of cellulose crystals with varying shapes and sizes, including microcrystalline cellulose, micro fibrillated cellulose, nanocrystalline cellulose, nanofibrillated cellulose, and bacterial cellulose, based on its production techniques and sources. This review highlights the utilization and functions of cellulose as a material in food products.


Introduction
The food industry shows a very rapid development.The evolving lifestyle of the current society demands food production that not only meets the required quantity but also the desired quality by consumers.Therefore, the use of additives to improve product quality is increasingly necessary.
Cellulose is the main component that comprises the plant cell walls of high-level trees to primitive organisms such as algae, flagellates, and bacteria [1].Cellulose is a straight-chain carbohydrate with glucose as its monomer, where hydrogen bonds link the monomers.The properties of cellulose are not soluble in various solvents and resistant to treatment with various chemicals, except for strong acids, due to the hydrogen bonds between hydroxyl groups in the cellulose chain [2].The property of cellulose is insoluble in water.It readily absorbs water making it further processable as a food additive that functions as an anti-caking agent, emulsifier, thickener, and binder.This article aims to determine the utilization of cellulose as a safe food additive for food products.

Isolation of cellulose 2.1. Mechanical process
Cellulose Nano-Fibrils, also known as CNF, were first introduced by Turbak, Snyder, and Sandberg in 1983 [3].They produced gel-like materials from processed wood fibers using mechanical treatment with a High-Pressure Homogenizer and heat treatment.In this conventional method, no pre-treatment of the raw material is carried out.The presence of hydrogen bonds in the pulp fibers requires a high amount of energy (700-1400 MJ Kg -1 ) for the fibrillation process, which is carried out by passing the pulp suspension through the HPH machine multiple times.

High-pressure homogenizer
This method utilizes a high-pressure homogenizer (HPH) machine, which can be used in laboratory and industrial-scale applications.HPH machines emulsify food, dairy, and cosmetic factory mixtures.In principle, this machine passes the mixture solution through a narrow gap orifice at an extremely highpressure differential (100-2000 MPa).
The width of the gap can be adjusted depending on the viscosity of the suspension solution and the chosen pressure.The process of fibrillation on cellulose fiber suspension occurs due to the drastic pressure change that causes the formation of micro gas bubbles, the frictional force between fibers, a collision between fibers, and turbulent flow around the orifice.The degree of fiber fibrillation into nano cellulose depends on the suspension cycle passing through the HPH machine.This mechanical treatment can produce nano-fibrillated cellulose without pre-treatment [4]

Grinding
The grinding method's cellulose isolation process passes a 2% concentration cellulose suspension solution through a narrow gap between two discs made of wear-resistant stone, silicon carbide, or heattreated iron.Fibrillation occurs due to the contact between the discs and the fibers, which causes the fibers to receive frictional force and pressure.The degree of fibrillation can be adjusted by controlling the gap width between the discs and the suspension cycle passing through the grinding machine.One advantage of this process over the HPH machine is the lower chance of blockages as the gap width can be easily adjusted [5]).

Ultrasonic
This mechanical process uses ultrasonic energy, converting sound energy into physical and chemical energy.This treatment differs from conventional mechanical treatments such as HPH and grinding systems.In the ultrasonic liquid, microbubbles are generated that will pound the surface of the fibers resulting in hot spots and a cavitation process, leading to the delamination of fiber layers.The energy formed by the cavitation process and the bursting of microbubbles can exceed the hydrogen bonds in fibers, ranging from 10-100 kJ mol -1 [6].The formation of hot spots can increase the temperature of the solution.Hence the cooling process is necessary.After pulp bleaching, poplar wood powder is further fibrillated using ultrasonication with power variations ranging from 400-1200 watts.Cellulose Nano Fibrils with a 5-20 nm diameter can be produced under optimum conditions and minimum power input [7] 2.2.Biological and enzymatic process One of the reasons for the limited commercialization of nanocellulose is the high total energy requirement for cellulose isolation using mechanical treatment, which is still around 70 MWh ton -1 .Treatment with enzymes or chemicals can aid the fiber fibrillation process and reduce the total energy requirement to around 2 MWh ton -1 [8].One such enzyme that can be used is the cellulase enzyme produced by fungi.Cellulase enzymes can be used as cellulase enzymes produced by fungi.Cellulase enzymes can be classified into three classes: endoglucanase, which effectively degrades the amorphous cellulose phase; exoglucanase, which progressively degrades the crystalline and amorphous cellulose phases into disaccharides; and β-glucosidase, which hydrolyzes disaccharides and tetrasaccharides into glucose [9].
Production of CNF using endoglucanase enzymes was introduced by [10].During the hydrolysis process with enzymes, the degree of polymerization of cellulose decreases, and the crystallinity of cellulose increases.After fibrillation with enzymes, mechanical treatment using a homogenizer can produce CNF.However, using all three enzyme types simultaneously did not yield better results than adding single-component endoglucanase in terms of a massive reduction in the degree of polymerization.

Chemical process
In contrast to mechanical and enzymatic treatments, which do not result in functional group modifications on cellulose, chemical treatments can produce cellulose surfaces with or without changes in functional groups.Changes in the surface properties of cellulose to anionic or cationic due to chemical treatment significantly affect the resulting Cellulose Nano-Fibrils (CNF) or Cellulose Nano-Crystals (CNC) characteristics.Chemical processes are very effective in reducing energy and increasing nano cellulose yield.Some examples of chemical processes that can facilitate the isolation of CNF or CNC are as follows:

Acid hydrolysis
Acid hydrolysis was first introduced by Nickerson and Habrle in 1974.They found that cellulose degradation under acidic conditions and high temperatures stopped only after the entire amorphous phase had been completely degraded, leaving only the crystalline phase of cellulose.The type of acid used produces a significant difference in the properties of the resulting CNC.Reaction with concentrated H2SO4 will result in a crystalline phase with negatively charged sulfate groups on the surface of cellulose through partial substitution of OH groups at C6.The sulfate content ranges from 0.81 to 0.51 mmol g -1 , whereas reaction with HCl produces uncharged CNC.The sulfate groups on the cellulose chain cause better CNC dispersion in water than in uncharged CNC.However, the heat resistance of cellulose will decrease [11].

Carboxymethylation
The carboxymethylation process imparts a negative charge to the cellulose.The carboxymethylation process followed by homogenization, ultrasonication, and centrifugation to separate the undegraded fibers can produce CNF with a 5-15 nm diameter and a length of up to 1 nm.Substituting hydroxyl groups with carboxymethyl is also done to prevent agglomeration during CNF drying, which aims to save storage and transportation costs.According to Aulin et al. (2009) [12], CNF produced from this process has smaller and more uniform dimensions than CNF obtained from enzymatic processes.

TEMPO oxidation
The oxidation process using TEMPO is the most widely used method for cellulose extraction.TEMPO Oxidation is a cellulose modification process that uses 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) compound as a catalyst and sodium hypochlorite as an oxidizing agent to convert the hydroxyl groups at the C6 position of anhydroglucose into carboxylic acid groups.This method also produces negatively charged CNF, where the zeta potential of TEMPO-CNF in water can reach -75 mV.This reaction is highly selective, modifying only the hydroxyl groups at C6, thus maintaining the dimensions and crystallinity of the resulting cellulose.However, depolymerization can still occur if the oxidant concentration is excessive and the oxidation process is prolonged.The common oxidation system used is TEMPO/NaOCl/NaBr at pH 10.5 and room temperature, where the pH is kept constant during the reaction by adding NaOH [13].

Periodate-chlorite oxidation
In contrast to the selective attack on C6-OH groups by the TEMPO oxidation process, periodate-chlorite oxidation modifies hydroxyl groups on secondary alcohols (C3, C2).During the modification process, the hydroxyl groups at C3 and C2 are oxidized by adding sodium periodate to form aldehyde groups and subsequently transformed into carboxyl groups by adding sodium chlorite.Tejado, Alam, Yang, and Van de Ven (2012) [14] reported that CNF could be obtained without mechanical treatment if the maximum carboxyl content is achieved.This is due to two carboxyl groups in one anhydro glucose unit, causing repulsive forces between the cellulose chains

Cellulose source
Cellulose is a homopolymer consisting of β-D-glucopyranose units bound together by (1,4)-glycosidic bonds, where n is the degree of cellulose polymerization [15].Typically, cellulose contains about ~40-50% of the dry weight of lignocellulosic material.The type of biomass, growth age, location, position within the plant stem, and environmental factors influence this varying cellulose content.Cellulose can be grouped based on its sources, such as cellulose from wood, non-wood, marine fauna, and bacteria 1230 (2023) 012039 IOP Publishing doi:10.1088/1755-1315/1230/1/0120394 [16] The differences in cellulose content based on their sources are presented in Table 1.The source of cellulose will determine the morphological structure, dimensions, and methods of cellulose isolation.The primary cellulose source is wood, divided into two significant wood types: broad and needle-leaved.The two differ in their constituent cells [17].For non-wood cellulose sources it is usually produced from the plantation product group, such as cotton, kapok, empty palm fruit bunches, skin fibers such as ramie and murbai, grasses such as bamboo and alang-alang, agricultural waste such as rice and wheat straw, and bagasse such as sugarcane and sorghum [18].When compared to cellulose from wood, non-wood cellulose has several advantages, including most non-wood cellulose has a lower lignin content, a shorter harvesting time, is more environmentally friendly in terms of water requirements for irrigation, can be replenished in a relatively shorter time, and requires less energy for cellulose isolation [19] Besides wood and non-wood sources, high-purity cellulose can be obtained from a marine animal called tunicin.Tunicin has a different cellulose structure than the structure found in plant cell walls.In plant cells, cellulose is structured with fiber orientation, forming a specific microfibril angle, as seen in figure 1, and forming a helical structure [20].Meanwhile, the cellulose structure in tunicin is randomly structured or well-organized, forming braids [21].Besides tunicin, the only animal cell that can produce cellulose from the marine ecosystem, cellulose can also be obtained from types of marine flora or Posidonia oceanica grass [22].Cellulose can also be obtained from bacteria that synthesize and secrete their bodies to form cellulose fiber strings and form a cellulose membrane.Bacteria produce a cellulose membrane to keep aerobic bacteria on the surface of the media (oxygen source) and protect themselves from ultraviolet rays [23].According to research by Jonoobi et al. [24], cellulose produced from Acetobacter sp.bacteria has many advantages compared to lignocellulosic cellulose, including a high degree of crystallinity of up to 85% because it lacks non-cellulose components such as lignin and hemicellulose.In addition, bacterial cellulose has a lower density and elastic stiffness of up to 114 GPa [25].Bacterial cellulose also has a very high water absorbency of up to 99%.It is biocompatible and has a crystal structure that bacteria can form during the polymerization process in situ [26].Another alternative to obtaining cellulose is from algae organisms.According to research by Mihranyan, Edsman, and Stromme [27], two types of algae have potential as a cellulose source based on their cellulose content and degree of crystallinity, including the Cladophorales (Cladophora, Chaetomorpha, Rhizoclonium, and Microdyction) type and a small amount from the Siphonocladales class (Valonia, Dictyosphaeria, Siphonocladus, and Boergesenia).

Application of cellulose in food products
Cellulose is one of the most abundant natural material on earth.To obtain cellulose from lignocellulosic materials, isolation methods are used.For commercial cellulose, several processes are used to pulverize lignocellulosic biomass.Commercial cellulose nano-form preparations are divided into two main types: Cellulose Nano-Fibrils and Cellulose Nano-Crystals.The difference between the two types of cellulose can be seen in Figure 2. Typically, cellulose nanofibrils are produced by mechanical processes, while cellulose nanocrystals are produced by acid hydrolysis [8] Cellulose isolation can be done chemically, mechanically, and biologically to produce functional materials in the form of cellulose crystals with different shapes and sizes, including microcrystalline cellulose, microfibrillated cellulose, nanocrystalline cellulose, nano fibrillated cellulose, bacterial cellulose, based on the preparation technique and source [43].According to Jia et al. [44], despite significant advances in commercializing various forms of cellulose crystals, only microcrystalline cellulose has been successfully commercialized and produced in the form of hydrophilic powders or dispersions to form colloids.
The application of cellulose is very broad, affecting many sectors such as food, pharmaceuticals, cosmetics, cement, and the plastic industry.In 2015, Transparency Market Research Analysis reported that the global market for microcrystalline cellulose would reach USD 1.08 billion by 2020, with the pharmaceutical and food industries being the largest beneficiaries [45].Different types of microcrystalline cellulose can be applied to various food processing systems, affecting texture, flavor, and other organoleptic properties and influencing the choice and acceptance of food products as presented in Table 2.In general, both micro and nanoscale cellulose crystals can stabilize emulsions due to the presence of free hydroxyl groups on the surface of the material that acts as hydrophilic points, while the crystalline part can function as a hydrophobic edge that provides an overall amphiphilic property [46].
Microcrystalline cellulose is a good candidate for interface stabilization, especially in food, due to its non-toxic, sustainable, biodegradable, and renewable properties [47].Research conducted by Kalashnikova et al. (2011) [46] showed that without any dispersant, microcrystalline cellulose could effectively stabilize oil/water emulsions for several months through the Pickering mechanism of stabilization, as long as the particles are well dispersed.Similarly, nano-scale cellulose crystals obtained from asparagus through sulfuric acid hydrolysis successfully formed stable Pickering emulsions in palm oil/water for several weeks with a 30/70 (v/v) solution model.[ [48][49][50] Meat products (sausage) Acts as a fat replacer to improve texture, moisture, and mouthfeel and reduce brightness and softness) [51][52][53] Beverages (e.g., coconut drink) Improving suspension stability, creaminess, and particle suspension.[54,55] Dairy products (e.g., ice cream, frozen dessert, cheese) Thickener agent, crystallizing ice in ice cream, stabilizing foam.[56][57][58] Dressings, sauce, and spreads Reducing stickiness in powdered sauce, preventing caking in powdered sauce [54,59] Edible films and coatings Improving the mechanical properties of films and coatings [60] Probiotics and food encapsulation Encapsulating bacteria within a microcrystalline cellulose matrix ensures delivery to the large intestine.[61] Soy protein hydrolisate Improving the micro-rheological properties of the product.[62] In a study by Gibis, Schuh, and Weiss [52] on the effect of microcrystalline cellulose as a replacement for fat on the microstructure and sensory properties of patties, it was shown that microcrystalline cellulose could effectively replace up to 50% of the fat compared to standard products.The sensory evaluation also showed that patties with added microcrystalline cellulose were more accepted by the panelists where there was a mouthfeel sensation, and they were juicier compared to the control product.
Aside from being in the form of microcrystals, cellulose is also usually processed further into Carboxy Methyl Cellulose (CMC).A study by Prasetyo, Purwadi, and Djalal [63] showed that the organoleptic test of adding CMC to red guava fruit juice honey drink could affect panelist liking if added with the appropriate and proper concentration.CMC's properties can retain the guava seeds' aroma because CMC is a hydrocolloid that can function as a binding agent, so CMC binds the guava seed's distinctive aroma.Carboxy Methyl Cellulose is also often a drink component that acts as a thickening agent.The effect of CMC concentration on drink viscosity shows an increase.The presence of CMC in the solution can form cross-links in the polymer molecule, causing the solvent molecule to be trapped within, resulting in the immobilization of the solvent molecule that can form the molecular structure to become rigid and resistant to pressure.The higher the CMC concentration, the greater the formation of cross-links and the higher the immobilization of solvent molecules, increasing viscosity [64].
For using CMC as a gelling agent, a study was conducted by Linggawati, Adrianus, and Indah [65] on kiwi jam.In the syneresis value of kiwi jam with added CMC, a decrease occurred as the CMC concentration increased.This happened because CMC forms a stronger gel.The stronger gel is due to an increasing number of hydrogen bonds and causes the formed gel to maintain gel stability and release less water [66].Meanwhile, the viscosity value of kiwi jam increased as the CMC concentration increased.This is due to the hydrophilic nature of CMC, which will bind water and cause the gel to swell, along with an increase in viscosity.Gel formation occurs with hydrogen bridges between the hydroxyl groups of water and the methyl and carboxyl groups on CMC.This gel formation is influenced by pH.CMC will precipitate at a pH of less than 3 [67].

Conclusion
Cellulose has been applied in food products and has improved the quality and properties of the product.Variations of cellulose processing can be used as a fat replacement in meat products such as sausages, as a leavening agent in baked goods, as an emulsion in oil in water products, as a suspending agent in drinks, and as a thickener for emulsions and suspensions.The addition of cellulose in food products enhances the final product's texture and sensory and organoleptic characteristics and provides additional nutritional value as a source of dietary fiber.

Figure 1 .
Figure 1.Hierarchical structure of plant cell walls showing the position of cellulose and Its dimension [68]

Figure 2 .
Figure 2. The difference between nanofibril cellulose and nanocrystal cellulose