[Object] The present invention provides a dry solid containing fine cellulose fibers having excellent redispersibility in a solvent such as water, and a simple production method for producing a dry solid containing fine cellulose fibers. Another object of the present invention is to provide a fine cellulose fiber redispersion liquid having excellent transparency and viscosity property. [Method for Achieving the Object] The dry solid containing fine cellulose fibers according to one embodiment of the present invention contains fine cellulose fibers having a sulfur introduction amount attributable to sulfo groups being 0.5 mmol/g or more. The production method of the dry solid containing fine cellulose fibers of the present invention comprises a drying step in which drying is performed at a relatively low temperature under a low pressure. The fine cellulose fiber redispersion liquid of the present invention contains the fine cellulose fiber redispersion liquid and water and has desirable redispersibility in terms of transparency, viscosity property, and precipitation property.

Objects of the present invention are to provide a phosphoric acid-esterified fine cellulose fiber of which slurry shows superior transparency, and to provide a method for producing a phosphorylated fine cellulose fiber showing superior transparency with good efficiency and high yield. According to the present invention, there is provided a phosphoric acid-esterified fine cellulose fiber, of which 0.2 mass % aqueous dispersion shows a solution haze of 15% or lower.

Objects of the present invention are to provide a phosphoric acid-esterified fine cellulose fiber of which slurry shows superior transparency, and to provide a method for producing a phosphorylated fine cellulose fiber showing superior transparency with good efficiency and high yield. According to the present invention, there is provided a phosphoric acid-esterified fine cellulose fiber, of which 0.2 mass % aqueous dispersion shows a solution haze of 15% or lower.

It is an object of the present invention to provide a method for producing ultrafine fibrous cellulose, which is capable of efficiently obtaining ultrafine fibrous cellulose having phosphoric acid groups with a high yield. The present invention relates to a method for producing fibrous cellulose having a fiber width of 1000 nm or less, comprising: a (A) of introducing phosphoric acid groups into cellulose fibers to form crosslinked structures via the phosphoric acid groups, thereby obtaining crosslinked phosphorylated cellulose fibers, a (B) of breaking some or all of the crosslinked structures to obtain crosslink-broken phosphorylated cellulose fibers, and a (C) of performing a mechanical treatment on the crosslink-broken phosphorylated cellulose fibers to obtain fibrous cellulose having a fiber width of 1000 nm or less, wherein, in the (A), crosslinked structures in an amount of 0.05 mmol/g or more and 2.0 mmol/g or less are formed, and the (B) is a step of performing the hydrolysis of the crosslinked structures in an aqueous solvent with pH 3 or more.

It is an object of the present invention to provide a method for producing ultrafine fibrous cellulose, which is capable of efficiently obtaining ultrafine fibrous cellulose having phosphoric acid groups with a high yield. The present invention relates to a method for producing fibrous cellulose having a fiber width of 1000 nm or less, comprising: a (A) of introducing phosphoric acid groups into cellulose fibers to form crosslinked structures via the phosphoric acid groups, thereby obtaining crosslinked phosphorylated cellulose fibers, a (B) of breaking some or all of the crosslinked structures to obtain crosslink-broken phosphorylated cellulose fibers, and a (C) of performing a mechanical treatment on the crosslink-broken phosphorylated cellulose fibers to obtain fibrous cellulose having a fiber width of 1000 nm or less, wherein, in the (A), crosslinked structures in an amount of 0.05 mmol/g or more and 2.0 mmol/g or less are formed, and the (B) is a step of performing the hydrolysis of the crosslinked structures in an aqueous solvent with pH 3 or more.

Objects of the present invention are to provide a phosphoric acid-esterified fine cellulose fiber of which slurry shows superior transparency, and to provide a method for producing a phosphorylated fine cellulose fiber showing superior transparency with good efficiency and high yield. According to the present invention, there is provided a phosphoric acid-esterified fine cellulose fiber, of which 0.2 mass % aqueous dispersion shows a solution haze of 15% or lower.

Objects of the present invention are to provide a phosphoric acid-esterified fine cellulose fiber of which slurry shows superior transparency, and to provide a method for producing a phosphorylated fine cellulose fiber showing superior transparency with good efficiency and high yield. According to the present invention, there is provided a phosphoric acid-esterified fine cellulose fiber, of which 0.2 mass % aqueous dispersion shows a solution haze of 15% or lower.

It is an object of the present invention to provide a composition comprising cellulose fibers having a fiber width of 1000 nm or less, the properties of which are maintained even after long-term storage. The present invention relates to a cellulose fiber-containing composition comprising cellulose fibers having a fiber width of 1000 nm or less and an antiseptic containing at least one type selected from a nitrogen atom, a sulfur atom and a halogen atom, wherein the content of the antiseptic is 10 parts by mass or less with respect to 100 parts by mass of the cellulose fibers

An object of the invention is to provide a material comprising ultrafine cellulose fibers, having good dispersibility of fine particles in an aqueous medium containing ultrafine cellulose fibers. According to the invention, there is provided a material comprising ultrafine cellulose fibers, wherein the content of the ultrafine cellulose fibers is 80% by mass or more, and the haze value of a sample, which has been prepared by adding the material comprising ultrafine cellulose fibers to pure water to a solid concentration of 0.4% by mass, and stirring the mixture with a disperser under conditions of 1500 rpm and 5 minutes, is 20% or less.

An object of the invention is to provide a material comprising ultrafine cellulose fibers, having good dispersibility of fine particles in an aqueous medium containing ultrafine cellulose fibers. According to the invention, there is provided a material comprising ultrafine cellulose fibers, wherein the content of the ultrafine cellulose fibers is 80% by mass or more, and the haze value of a sample, which has been prepared by adding the material comprising ultrafine cellulose fibers to pure water to a solid concentration of 0.4% by mass, and stirring the mixture with a disperser under conditions of 1500 rpm and 5 minutes, is 20% or less.