Flour whiteness has an important influence on wheat cultivation, milling industry and flour food industry. Flour whiteness is one of the important indexes for evaluating the grade of flour, and the requirement for whiteness in China’s wheat flour grade standard is: more than 76% for Grade I, more than 75% for Grade II, and more than 72% for Grade III. Flour whiteness is also one of the important factors affecting consumer consumption, China’s noodle products are mainly boiled food, such as noodles, steamed bread, dumplings, etc., the whiteness of boiled food is one of the main requirements of consumers in terms of color and luster. Therefore, it is of great significance to study the factors affecting the whiteness of flour.

Factors affecting the whiteness of flour are wheat varieties, wheat growing environment, wheat quality and shape, wheat processing technology, flour quality, flour transportation and storage, flour additives and other factors. On the main factors affecting the whiteness of flour, this paper carries out an in-depth and comprehensive analysis and discussion, in order to improve the whiteness of flour to provide a scientific basis.

Wheat varieties on the impact of flour whiteness

Flour whiteness is a quantitative trait, with multiple gene control and high heritability. Flour mainly comes from the endosperm portion of wheat, and the pigments, polyphenol oxidase and proteins in the endosperm are the main intrinsic factors affecting flour whiteness.

Baik et al. showed that for the same wheat variety, the effect of protein on dough color stability during noodle making was greater than that of polyphenol oxidase, while for unused varieties, the effect of polyphenol oxidase on dough color stability was greater than that of protein, which suggests that polyphenol oxidase is mainly under genetic control. Zhang Chunqing et al. showed that the whiteness of wheat buns of HMW 5+10 subgrain varieties was significantly lower than that of 11MW 4+12, and proposed a pathway for the selection of high-quality bun varieties.

Bhatt et al. showed that low pigment content is partially dominant or super-dominant, flour color is controlled by 1 or 2 genes, polyphenol oxidase is controlled by 1-2 main genes, flour whiteness or color is mainly controlled by genetic control, and there are significant differences in flour whiteness among varieties, and flour color can be effectively improved by breeding, reducing polyphenol oxidase activity and improving flour whiteness. Liu et al. showed that most of the varieties with high flour whiteness (80%) are weak gluten flour wheat, and there is a lack of strong and medium gluten wheat varieties with high flour whiteness.

Therefore, pigments, polyphenol oxidase, and proteins associated with genetic factors are important factors affecting wheat flour whiteness, and wheat varieties can be improved through breeding and biotechnology to increase flour whiteness.

Wheat growing environment on the impact of flour whiteness

Wheat growing environment mainly includes climate, soil, cultivation technology, environmental factors have an important impact on flour whiteness. Liu Jianjun in four environmental conditions on 104 wheat varieties (lines) for flour whiteness research results show that climate factors, soil factors, cultivation conditions with wheat flour whiteness have a significant impact. Climate, soil, cultivation techniques and other environmental factors can lead to varying degrees of variation in wheat varieties, wheat growth ring Ling as an exogenous factor affecting the genetic factors of wheat as an endogenous factor, thus affecting the chemical composition and physical structure of wheat, and thus affecting flour whiteness.

The influence of wheat quality and shape on flour whiteness

Wheat quality and shape have both external and internal qualities. Among them, the appearance quality includes capacity, thousand grain weight, grain color, angularity, hardness. Intrinsic quality includes wet gluten content, sedimentation value, protein content, starch content, ash content, water absorption. The main quality traits of wheat have an important influence on flour whiteness.

3.1 The effect of wheat appearance quality on flour whiteness

Thousand kernel weight and capacity weight respond to the fullness of wheat, and the hardness of wheat is related to the angular quality. Lan Jing et al. showed that the thousand kernel weight and capacity of Long Spoke Wheat 12 were highly significant positively correlated with the L value of flour, while only the angularity of Long Wheat 26 was highly significant positively correlated with the a value of flour.

Hu Ruibo et al. showed that flour whiteness was highly significantly negatively correlated with seed hardness, which was due to the hard endosperm in which starch granules were tightly bound to proteins, but its endosperm was easy to be separated from the bran, with a high milling rate, and most of them were ruptured along the direction of the cell wall during milling, which resulted in the formation of larger flour particles with a more neat shape, which dispersed more to the light, and the whiteness declined, whereas the flour particles of the soft wheat were small and irregular, which dispersed less to the light, and the flour particles of the soft wheat were smaller and less irregular. less.

In addition, flour granularity and broken starch content were also significantly negatively correlated with flour whiteness, and durum wheat had large granularity and more broken starch grains during milling. This indicates that the external quality of wheat has a certain influence on the whiteness of flour.

3.2 The effect of wheat intrinsic quality on flour whiteness

Wet gluten content, sedimentation value, protein content, starch content and ash content mainly determine the intrinsic quality of flour quality.

Liu et al. showed that flour whiteness was significantly negatively correlated with flour water absorption, protein and gluten content, while flour whiteness had nothing to do with the quality of protein and gluten, and flour whiteness was not affected by the indexes that mainly reflected the quality of protein (gluten strength), such as the settling value, the dough stabilization time, the maximum boost and the tensile area. Li Zongzhi et al. confirmed a significant negative correlation between flour whiteness and both protein content.

Hu Ruibo et al. showed that flour whiteness was highly significantly positively correlated with starch content, positively correlated with branched-chain starch content, the ratio of branched to straight-chain starch content, but not at a significant level, and significantly negatively correlated with flour protein content and ash content. Regarding the relationship between ash content and flour whiteness, the results of different studies are not consistent.

Some studies concluded that flour whiteness was negatively correlated with ash content, to Li Zongzhi et al. concluded that there was no correlation between flour whiteness and ash content.

Wang Zhijian et al. concluded that the paste powder layer is a colorless and transparent substance with high ash content, but its miller’s flour has little effect on flour whiteness. Ash, as an important index of flour grade, is related to the processing accuracy of flour.

Influence of wheat processing technology on flour whiteness

The influence of wheat processing technology on flour whiteness is relatively complex, for a given wheat raw material, every detail of its processing technology has an important influence on flour whiteness.

4.1 Effect of cleaning process on flour whiteness

The quality of wheat raw material is an important factor affecting the whiteness of flour. Wheat seeds contain diseased seeds, moldy seeds, broken seeds and deflated seeds, and wheat also contains mud, gravel, dust and other magazines, which will seriously affect the whiteness of flour. Therefore, before entering the mill, a reasonable and efficient cleaning process is needed to remove the quality fission particles and impurities that affect the whiteness of flour.

4.2 Impact of moistening process on flour whiteness

In the water moistening wheat, due to the wheat grain cortex, paste powder layer and endosperm water absorption and expansion of different abilities and sequential order, in the radial direction of the wheat grain cross-section will produce a small amount of displacement, thereby weakening the combination of the three, easy to peel and scrape, separation, moistening the wheat when the amount of water to be moderate.

Moisture content is too low, the wheat grain water absorption is insufficient, the wheat skin and endosperm is not easy to separate, grinding caused by the flour in the bran star more, powder color is poor; high moisture content can not be ground sun. Therefore, the endosperm and bran should have similar moisture, and there is an important parameter in the milling industry which is the optimal milling moisture of wheat.

4.3 Influence of grinding process on flour whiteness

Flour is mainly extracted by grinding and scraping wheat endosperm from the inside out by grinding rollers. There are various grinding and extraction processes, mainly the front way out of the flour method and the middle way out of the flour hair. The ash content is high, the bran breakage is more serious, more bran stars are mixed into the flour, and the whiteness of the flour is lower. Compared with the front way milling method, the middle way milling method has higher processing precision. The principle of the middle way milling method is to first extract a large number of coarse grains and coarse powder with good quality from the skin mill, and then process them through the clearing and residue milling system, while a large amount of flour is produced in the heart mill. This ensures bran integrity, thus reducing bran stars in the flour and increasing the whiteness of the flour.

4.4 The influence of flour mixing process on the whiteness of flour

Wheat processing technology in the flour stream more, each stream of flow, ash, protein, starch content varies, resulting in each stream of flour whiteness varies. According to the actual requirements for the quality of flour, according to the flow of each material flow, ash, egg degeneration and other components of the content, the preparation of flour to meet the requirements. Generally, the front and middle streams have lower ash content, which is suitable for preparing high precision flour with higher whiteness. And the back road due to grinding material particle size is small, containing more bran chips, not easy to sieve clean, resulting in more bran stars, high ash, flour whiteness is poor.

The influence of flour additives on flour whiteness

The main substances affecting the whiteness of flour are bran stars and color pigment. By improving the wheat processing technology to reduce the content of gluten star, by adding flour whitening agent to destroy the chemical structure of pigment, so as to increase the whiteness of flour.

Newly milled flour is physiologically active and light yellow, and continues to undergo biochemical reactions during storage, resulting in automatic whitening, but the time required is long and whiteness is limited. Flour whitening agents can quickly improve the color of flour and make it whiter. The pigments in flour are mainly yellow pigments and a small amount of brown pigments.

Yellow pigments are mainly carotenoids, including beta-carotene, lutein and flavonoids. Yellow pigments show a highly significant positive correlation with flour color grade, and brown pigments show a significant correlation with both flour whiteness and color. Pigments contain conjugated double bonds, through oxidation to destroy the double bonds to make it fade, which is the mechanism of flour whitening agent whitening.

Commonly used flour whitening agents are benzoyl peroxide and potassium bromate. Other flour whitening agents include azomethine, ascorbic acid, chlorine dioxide and so on.

Summarize

Wheat flour whiteness is one of the most important indicators of milling quality, reflecting the precision of wheat milling, and is an important criterion for classifying flour grades. From the consumer’s point of view, it is always believed that the whiter the flour, the better. However, there is a contradiction between wheat milling accuracy and flour nutrition, and the relationship between flour whiteness and nutrition should be viewed rationally.

There are many factors affecting the whiteness of flour, should take a scientific approach, both to raise the whiteness of flour, without destroying the nutrition of flour. The best way to improve the whiteness of flour has two aspects, on the one hand, from the small flour genetic breeding and gene technology to start in-depth research, cultivate low pigment content of wheat varieties; on the other hand, it is necessary to continue to study to improve the whiteness of wheat processing technology, reduce the gluten in the flour content of the star.