Dependence of fat acidity value on wheat grain storage conditions

Introduction

Reduction of the wheat grain losses during its storage period on a national scale potentially gives results similar to an increase in bulk yield. The task of ensuring the safety of wheat food grain can be accomplished not only on the basis of existing regulatory rules, but also by taking into account the results of new research.

The main method of storage in metal containers, according to the instructions [1, 2] and safety regulations is the storage of grain in a dry and purified state [3]. It is known that not only humidity and purity, but also the temperature of the grain put into storage affect the storage period [4]. The lower the temperature, the longer the storage time becomes.

It has been established that the decrease in the organoleptic characteristics of wheat grains, which determine the periods of safe storage and shelf life, is associated with changes in the lipid complex due to the activity of the lipase enzyme. Lipase during storage period splits grain triglycerides to glycerol and free fatty acids, which in cereals are mainly represented by three unsaturated fatty acids: oleic, linoleic and linolenic with one, two or three double bonds respectively. This process leads to a change in organoleptic characteristics, primarily to the deterioration of taste and smell [5]. It is established that these changes reliably reflect the indicator of the fat acidity value (FAV) [6].

Materials and method

An analytical study of the published materials showed:
• the FAV, which determines the period of safe storage period of food grains of wheat, has not been uniformly established and, according to various literary sources, ranges from 11 to 23 mg KOH per 1 g of fat;

• the time-period for ripening of wheat flour after grinding according to literary data is also not established and ranges from 10 days to two months;
• an indicator that determines the ripening of wheat flour is not standardized (acidity, FAV, quality of gluten, quality of bread).

Results

When storing grain products with the humidity specified in the target standards, the hydrolytic processes continue as well as the more intense they are, the higher the temperature and relative humidity of the air in the storage are observed. As a result there is an accumulation of hydrolysis products: free fatty acids by the hydrolysis of lipids, amino acids from partial decomposition of water-soluble proteins, acid phosphates from decomposition of phytin, etc.

To determine the change in moisture content during the ripening of wheat flour, this indicator was weekly determined. The results are presented in Table 1.

The analysis of the results in Table 1 shows that the moisture content of the food grain of wheat ranged from 7.1 to 12.5 %, and the moisture produced from the flour was from 13.0 to 14.8 %. It should be noted that in the process of flour ripening of each studied sample, the humidity changed within the limits of permissible variations of the control definitions (0.5 %) of this indicator; which eliminates its effect on the change of FAV in the flour. Table 2 shows the results of the determination of the FAV of the wheat food grains and of the flour produced from it during the ripening process.


Table 1. The change in moisture content of freshly ground wheat flour while ripening at storage period

№	Sample №	Moisture content, %
		grain	flour, storage period, week
			Start point	1	2	3	4	5	6	7
1	8	10.5	13.7	13.5	13.5	13.6	13.6	13.6	13.8	13.7
2	9	8.9	13.0	13.1	13.0	13.0	13.0	13.0	12.9	13.0
3	71	12.5	14.8	14.3	14.3	14.4	14.3	14.3	14.3	14.3
4	76	12.1	14.6	14.8	14.7	14.8	14.8	14.7	14.7	-
5	26	9.2	13.9	13.9	13.8	13.9	13.9	13.8	13.9	13.8
6	5	10.2	13.7	13.8	13.7	13.7	13.7	13.6	13.8	13.7
7	11	10.7	14.3	14.0	14.3	14.2	14.1	14.1	14.1	-
8	75	12.0	14.5	14.5	14.4	14.4	14.3	14.2	14.1	-
9	13	7.4	13.7	14.1	13.7	13.9	13.9	13.8	13.8	-
10	21	9.5	13.6	13.7	13.7	13.7	13.7	13.5	13.4	-
11	12	7.1	14.0	14.4	14.3	14.3	14.3	14.1	14.0	-
12	18	9.3	14.2	14.1	14.2	14.0	14.1	14.0	-	-

Table 2. FAV of the wheat food grains and of the flour produced from it during the ripening process

Order No.	Sample No.	FAV, mg КОН per 1 g of fat
		Grain	flour, storage period, week
			Start point	1	2	3	4	5	6	7
1	8	9.1	15.9	17.1	18.0	-	20.5	21.8	20.8	20.2
2	9	9.5	15.6	16.9	18.4	-	21.3	21.3	21.9	21.5
3	71	12.3	16.4	17.4	22.4	23.4	24.8	27.8	28.5	28.4
4	76	13.9	23.0	23.6	28.2	31.6	32.5	33.5	33.4	-
5	26	14.7	21.6	21.7	22.8	-	24.3	26.0	24.9	26.5
6	5	14.9	19.2	20.0	22.8	-	24.9	30.8	29.4	28.9
7	75	17.0	31.0	35.3	35.4	37.7	35.5	37.8	37.2	-
8	11	18.5	27.0	30.9	31.7	32.9	35.6	35.0	35.0	-
9	13	22.7	38.6	38.9	42.5	43.9	43.6	43.8	43.6	-
10	21	27.4	40.0	40.6	44.5	48	50.3	47.9	-	-
11	12	29.2	53.8	57.0	59.5	63.2	60	63.1	61.2	61.1
12	18	34.2	52.9	51.9	54.3	53.8	53.2	53.1	-	-

Analysis of the obtained data shows that as for all the studied samples FAV of freshly ground flour is significantly higher than the one of the grain from which it was produced [7]. This difference is close to the difference between the acidity of the grain and the acidity of freshly milled flour.

However, if during the flour ripening the acidity value was practically unchanged, then the FAV of the flour changed significantly during the entire ripening period of all the studied samples.

It was noted that the ripening period of each wheat flour sample differed from all the rest - the higher the FAV of the food grain, i.e. the “older” the grain is, the lower the ripening period is under the same storage conditions and close values of flour moisture (Table 2).

To perform a research on long-term laboratory storage, two charges of the 3 ^rd class wheat food grain of 2018 and 2015 harvests were selected. The main indicators of the quality of these charges are given in Table 3.
Analysis of the indicators given in Table 3 shows that the food grain of the 3rd class while laboratory storage, meets the requirements of GOST 9353-2016:

• grain moisture content - 13.1 and 13.2%

(according to GOST 13586.5 - 2015 - not more than 14 %); FN - 284, 392 (according to GOST 30498-97 - not less than 150 sec.);

• the amount of gluten - 23.8 and 22.8 % (according to GOST 27839-2013 - not less than 22 %); FDM - 60 units for both charges (according to GOST 27839-2013 18 - 102 units); quality group I good.

The grain of each charge in the amount of 25 - 30 kg was placed: in the refrigerator at T = + 10 ± 2 0С; in thermostats at T = +20 and +30 С and relative air humidity of 65 %.

To establish the expiration date of wheat grain during long-term storage, the indicator of the fat acidity value (FAV) in mg KOH was considered. The following method was applied: grain samples were placed in desiccators over salt solutions, which created a certain relative air humidity above their surface, Table 4. Samples of wheat grain were stored in desiccators from April to November 2018 at ambient temperature (20^25 °C), 10 ° C and 30 ° C - in thermostats. Table 4 presents the saturated salt solutions to set the values of relative air humidity.



Table 3. The main quality indicators of the of the 3^rd class wheat food grain of 2018 and 2015 at a long-term laboratory storage

Year	Moisture, %	Acidity, degrees	FAV, mg КОН per 1 g of fat	Gluten			Falling­number, sec.
				Number, %	Quality, units FDM	Quality Group
2018	13.1	0.9	17.0	23.8	60	I good	284
2015	13.2	1.5	24.4	22.8	60	I good	392

Salt solutions were poured into the desiccators to create a given relative humidity of air in the internal volume of the desiccator. The values of the relative humidity of the air above the salt solutions are presented in the table above [8]. Before putting in desiccators for storage, the samples of wheat grain were cleared of impurities on an air-sieve laboratory separator [9].

Two samples of wheat grains of the 3 ^d and 4^th classes each of about 500 g were placed in the desiccators; the layer of grain in the desiccators was 70-80 mm. To control the temperature and relative humidity, sensors "Logger 100 TV" were placed inside the grain samples. The interval for automatic recording of sensor readings was 2 hours. Periodically samples of grain were taken from each desiccator for FAV determination and for humidity changes monitoring [10].

Table 5 presents the actual values of the equilibrium moisture content of the grain and the relative air humidity in the desiccators. Figure 1 shows changes of the relative air humidity in the desiccators during the storage of grain samples. According to Figure 2, the moisture content of grain samples in a desiccator with a relative air humidity of 75 % reached an equilibrium value of 14-15% by the 2^nd month of storage while its initial moisture value was 11%. In the desiccators with lower relative air humidity values, the equilibrium state occurred after one month of storage and after then it practically did not change.

Some fluctuations in the relative humidity values during further storage, as can be seen from Fig. 1, are associated with temperature changes at the storage site (laboratory room), as well as with the opening times of the desiccator for taking grain samples to determine humidity and FAV. In all grain samples (desiccators), the values of relative air humidity (Fig. 1) and temperature (Fig. 3) can be considered constant within the measurement accuracy of the sensor, taking into account



Table 4. The saturated salt solutions to set the values of relative air humidity
 

№. Desiccator	Equilibrium grain moisture content during sample storage, %	Relative humidity set in a desiccator according to the table of salts, %	Name of the saturated salt solution	Average expected storage temperature, °C
1	12.4	55	Sodium bichromate Na2CnO7-2H2O	20
2	10.5	35	Magnesium chlorine MgCl2	20
3	13.7	65	Ammonium nitrate NH4CNO3)	20
4	11.2	45	Potassium carbonate K2CO3.2H2O	20
5	12.8	60	Ammonium nitrate NH4(NO3)	30
6	11.2	45	Potassium carbonate K2CO3.2H2O	10
7	15.1	75	Sodium chloride NaCl	20

Table 5. Average actual values of relative air humidity and temperature in the desiccators during the April - October period

№ of desiccator	Average value of the equilibrium moisture content of the grain kept as the sample in a desiccator	Actual value of the relative air humidity	Salt name	Average actual storage temperature in desiccators
4	11.0	44-46	Potassium carbonate K2CO3.2H2O	23.8-24.3
1	11.8	55-57	Sodium bichromate Na2Cr2O7.2H2O
3	12.9	65-66	Ammonium nitrate NH4(NO3)
7	14.3	75-76	Sodium chlorideNaCl
6	11.5	45-49	Potassium carbonate K2CO3.2H2O	9.5
2	10.1	33-36	Magnesium chlorine MgCl2	25
5	12.8	63	Ammonium nitrate NH4(NO3)	29.5

Fig. 1. Changes of the relative air humidity in the desiccators during the storage of grain samples (D1.. ,D7 - numbers of desiccators according to Table 1)



Fig. 2. Change in moisture content of grain samples in desiccators during storage period (D1 .D7 - numbers of desiccators according to Table 1).



Sample storage period (April-October)
Fig. 3. Temperature change in desiccators during storage (D1 ... D7 numbers of desiccators according to Table 1)