The nanocellulose is the smallest structural part of the cellulosic biomass of various organisms. While NFC is generally produced by mechanical process, NCC is by concentrated acid hydrolysis. Hence, the NFC is more in amorphous nature when compared to NCC. These basic structural differences lead to different types of application for NFC and NCC.

The determination of stiffness by theoretical and experimental means has shown that native cellulose (cellulose I) has a Young’s modulus of 167.5 GPa and that of regenerated cellulose (cellulose II) is 162.1 GPa [22] . Using atomic force microscopy (AFM), various researchers evaluated the elastic modulus of both NCC and NFC. Self-standing TEMPO oxidized cellulose nanofibres are transparent and flexible, with high tensile strengths of 200 - 300 MPa and elastic moduli of 6 - 7 GPa [6] . The high strength of nanofibrillar cellulose combined with its potential economic advantages offers the opportunity to make lighter and strong materials with greater durability. Indeed, because of these properties nanofibres have attracted a lot of research effort in different disciplines and continues to be a subject of its utility in everyday materials such as paints, packaging, cosmetic bases, pigments, food modifiers, sensor applications, biomedical sciences and composites.

For nanocellulose production, the inter-fibrillar hydrogen bonding (of cellulose) energy has to be overcome. Since, more than one type of hydrogen bond is present, a range of values need to be considered to quantify the hydrogen bond strength; and, it ranges between 19 and 21 MJ/kg mol [23] . Traditionally, mechanical homogenization process is used in large-scale production of nanocellulose that leads to huge energy consumption to the tune of 20,000 to 30,000 kWh per tonne [8] ; and the required energy is 4 - 5 times that of the stored energy of that cellulosic biomass [24] . In one of the earliest patent [25] , NFC was produced by passing a liquid suspension of cellulose through small diameter orifice in which the suspension is subjected to a pressure drop of at least 3000 psi and a high velocity shearing action followed by a high velocity decelerating impact, with repeated passes (11 numbers) till formation stable suspension. The production of cellulose nanofibrils from a bleached eucalyptus pulp (2% consistency) using a commercial stone grinder (SuperMassColliderTM) required the energy in the order of 5 - 30 kWh per kg [26] . To reduce the energy consumption, various types of pretreatments (chemical and biological) are being evaluated that results in drastic reduction of energy consumption less than 1000 kWh per tonne.

Energy requirement by mechanical pretreatment to produce MFC films with maximum obtainable properties for each processing method, the total energy required was approximately 9180 kJ/kg for the microfluidizer with pretreatment, 9090 kJ/kg for the grinder with pretreatment, and 5580 kJ/kg for the grinder without pretreatment and 31,520 kJ/kg for the homogenizer with pretreatment. Here, the pretreatment was a mechanical beating process (processed in a valley beater) [27] .

The literature review on the effect of various chemical pretreatments on energy conservation or requirement for nanocellulose production is listed in the Table 1. Depending on the process and their analyses, the energy conservation/requirement are listed in the references. The energy calculated focused only on the actual energy required for mechanical comminution process without including the energy required during chemical pretreatments. Hence, a fair comparison of energy conservation is not possible.

Within the framework of EU-FP7 INNOBITE project, the Spanish applied research institute Tecnalia Research & Innovation and Ecopulp Finland Oy, a SME devoted to the production of shape moulded pulps out of waste paper, are developing a new method for converting waste paper into a new value-added material: newspaper-based nanocellulose [33] . The target is reducing the energy needed for the paper fibrillation process, for which a specialty pulp is generated via oxidation reactions. This chemical treatment converts the hydroxyl groups at the native cellulose fibres into carboxylate groups, thus creating anionic charges that will subsequently turn into repulsion forces within the internal structure of the fibres. Combining Ecopulp’s industrial facilities and Tecnalia’s know-how, such a specialty cellulose pulp has already been produced in mid-scale. So far, the energy requirement for complete paper fibrillation has been decreased to values of 2.1 kWh per kilo of dry matter that equals the range of other extensively applied industrial treatments such as pulp refining.

While chemical pretreatment results in substantial reduction of energy consumption, there erupts the problem of waste disposal and release of chemical pollutants into the environment. Also, depending on the nature of the chemical treatment, the surface chemistry of cellulose gets modified. To circumvent these problems, biological pretreatment shall be an ideal means of reducing energy consumption without creating chemical effluents.

The high hemicellulose content of the pulp decreases the cell wall cohesion of the fibers, making cell wall delamination easier. But, this alone was not sufficient to avoid blocking of the orifice in the homogenizer and to reduce energy consumption. Hence, small additions of the mono-component endoglucanases enzyme promoted cell wall delamination and prevented the blocking of homogenizer [34] .

The bio-mechanical process overcomes the high energy requirement to a certain extent; however, the use of cellulose hydrolyzing enzymes also has a negative impact on the molecular weight and the chain length of the isolated nanocellulose [35] . Hence, pretreatment with the fungus secreting hydrogen bond-specific enzymes was tested to produce nanocellulose with mechanical strength marginally higher to that of those isolated via a conventional mechanical process since the bio-pretreatment produced nanocellulose had higher aspect ratio.

In spite of various protocols claiming energy reduction due to enzyme/biological pretreatments, energy calculations are not available for comparison of actual energy saving. In our lab, we are working with the cellulase enzyme pretreatments of various cellulosic biomasses (cotton linter pulp, soft wood pulp, hard wood pulp and bagasse) before proceeding for mechanical treatments by refiner and microfluidization for production of nanocellulose (both NCC and NFC). In case of NCC production from cellulose powder, a minimum of 10 passes are required in the high pressure homogenizer OR microfluidizer to reach the desired size range below 100 nm. But, in case of enzyme (cellulase) pretreatment, with 5 passes itself, we could achieve the desired size range; and hence, a 50% reduction in energy consumption can be claimed. But, the problem faced was the time & process required for the removal of enzyme after pretreatment; and the same needs to be quantified in terms of energy requirement. In case of NFC production from cellulosic pulp, it is not possible to treat with high pressure homogenizer OR microfluidizer without enzyme pretreatment. If a mechanical pretreatment like beating/refining is imparted to make it amenable for homogenization/microfluidization, then the energy required in case of both mechanical pretreatment and enzyme pretreatment were same. The energy requirement in enzyme pretreatment (including heating and stirring) was high enough to make them equivalent to that of mechanical pretreatment.

Figure 3 shows the comparison of three different processes for manufacturing nanocellulose (mechanical, chemo-mechanical and bio-mechanical) in terms of productivity, energy efficiency and ease of handling in radar chart. All three protocols display their own strength and weaknesses. The mechanical process shows its strength in productivity, chemo-mechanical process in ease of handling and bio-mechanical process in energy efficiency. In weaknesses, only bio-mechanical process suffers both in terms of productivity and ease of handling while mechanical process suffers in energy efficiency. For selection among these three different protocols during commercial production, apart from the above said three parameters, quality of the nanocellulose needs to be considered based on the required area of application. We have formulated a “Nanocellulose Quality Index” to choose the nanocellulose for desired application (Figure 4). The two influential (on final product) quality para-

meters, aspect ratio and crystallinity are included in arriving at the index. While crystallinity improves strength and modifies rheological attributes, aspect ratio acts towards web formation in films, displays and improves strength. Based on their combined values, their suitability of applications can be determined as suggested in the Figure 4.