The Chemical and Rheological Properties of Corn Extrudates Enriched with Zn- and Se-Fortified Wheat Flour

Abstract

This paper analyzed the influence of the addition of Zn- and Se-fortified wheat flour to corn extrudates on viscosity, total starch content, starch damage, and bioavailability of zinc and selenium. Fortified wheat flour was added to corn grits in 90:10, 80:20, 70:30, and 60:40 ratios at three extrusion temperature profiles: 140/170/170 °C, 150/180/180 °C, and 160/190/190 °C. Viscosity values decreased significantly at different extrusion temperature profiles and at different proportions of wheat. The extrusion process increased the starch content, regardless of the extrusion temperature, and decreased it by adding different proportions of flour enriched with zinc and selenium. The starch damage increased with extrusion, without significant changes with extrusion temperature increment. The addition of different proportions of Zn- and Se-fortified wheat flour reduced starch damage values proportionally to the added content of enriched wheat. Increasing the temperature and the proportions, the total zinc content in the extrudates increased. Zinc bioavailability increased with increasing extrusion temperature. As for selenium, the total content increased by proportion increment but decreased with an increase in the extrusion temperature, though there were no significant differences in selenium bioavailability regardless of changes in extrusion temperature or the proportion of enriched wheat.

1. Introduction

It is estimated that one in three people worldwide suffers from “hidden hunger”, a term that describes the insufficient intake of minerals and vitamins through diet, negatively impacting health. This term describes a diet with enough calories but lacking in micronutrients. A significant portion of the global population’s diet is deficient in iron (Fe), zinc (Zn), calcium (Ca), magnesium (Mg), selenium (Se), and iodine (I), affecting both public health and the economy of a country. The highest deficiency occurs in areas where diets are not diverse, meaning they lack adequate amounts of fruits, vegetables, or animal-based foods that contain relatively high levels of micronutrients [1]. Mineral malnutrition can be addressed by increasing the intake of fish and animal-based foods, supplementing minerals, fortifying foods, or enhancing the bioavailability of mineral elements in raw materials. Among these strategies, biofortification of crops using appropriate mineral fertilizers and varieties with increased absorption capacity during growth is most advocated [2].

The bioavailability of an element refers to the portion of the ingested nutrient component that is soluble in the intestines and subsequently available for absorption through intestinal epithelial cells [3]. This fraction of the element is estimated in vitro by simulating digestion in two steps: gastric digestion with pepsin at pH 2, followed by intestinal digestion with amylase, pancreatin, and bile salts at neutral pH [4].

Cereal-based diets cannot provide sufficient amounts of zinc for absorption in the body due to its low bioavailability from cereal grains. The bioavailability of zinc in the human body is reduced due to high levels of phytic acid, which inhibits its bioavailability [5].

The bioavailability of selenium is influenced not only by its chemical form but also by other macro- and micronutrients in food products. The organic form of selenium as selenomethionine, found in cereal products, is considered more effective in preventing selenium deficiency, while the inorganic form of selenium as selenite is not recommended for long-term use due to potential toxicity. Selenium in wheat flour is largely present as selenomethionine [6], which is an effective form of selenium for increasing serum levels and other indicators of selenium status [7]. Since selenium is minimally lost from grains during milling and baking, biofortification is considered an effective process for increasing the intake of bioavailable selenium in populations with dietary deficiencies of this element [8].

Numerous previous studies show the effect of adding different raw materials and using different extrusion parameters to increase the nutritional value and improve the properties of extruded products [9,10,11,12]. Extrusion processing can change how starch is digested. The extent of this change depends on the composition of the raw materials and the specific extrusion parameters, like feed moisture, extrusion temperature, and screw speed [13]. Higher temperatures often lead to starch gelatinization and the breakdown of starch crystallites during extrusion, which can result in increased starch digestibility [14]. Viscosity is influenced by temperature, and lowering the die temperature can reduce the degree of starch gelatinization, which in turn increases viscosity. Conversely, raising the die temperature results in a decrease in viscosity [15,16,17].

Extrusion cooking is known to improve the bioavailability of minerals. This improvement is mainly due to changes in the mineral-binding properties of proteins, phytic acid, dietary fibers, and phenolic compounds caused by the extrusion process. During extrusion, minerals bound to other compounds are released through processes such as the dephosphorylation of phytic acids, the breakdown or polymerization of phenolic acids, the fragmentation of dietary fibers, and the thermal degradation of peptides and amino acids. Lower feed moisture and higher barrel temperatures help in breaking down these mineral-binding compounds [18]. The heat stability of minerals allows for fortification during pre-processing without the risk of damaging the minerals. Overall, extrusion cooking enhances mineral absorption by reducing the factors that inhibit it [19].

The incorporation of Zn- and Se-fortified wheat flour has been found to enhance the textural and sensory attributes of extruded snacks [20]. This article aims to investigate the effects of different extrusion parameters on the chemical and rheological characteristics of enriched extruded products with the addition of agrobiofortified wheat flour with selenium and zinc contributing to the development of new functional extruded products based on newly developed agrofortified raw materials.

2. Materials and Methods

2.1. Sample Preparation

The raw materials used for this experiment were as follows: corn grits (control), obtained from Žito Ltd. (Osijek, Croatia) and Zn- and Se-fortified wheat flours, obtained from the Faculty of Agrobiotechnical Sciences Osijek. The wheat samples were ground using an IKA MF 10 mill (Ika-Werke GmbH & Co., Staufen, Germany) with a 2.0 mm sieve. The corn grits were mixed with Zn- and Se-fortified wheat flours in the following proportions: 90:10, 80:20, 70:30, and 60:40.

After adjusting the total moisture content of the mixtures to 15%, the mixtures were placed in plastic bags and stored for 24 h at 4 °C, then brought to room temperature before extrusion. The extrusion was performed using a laboratory single-screw extruder (Brabender 19/20 DN, Duisburg, Germany) with the following parameters: temperature profiles of 140/170/170 °C, 150/180/180 °C, and 160/190/190 °C; a screw compression ratio of 4:1; a screw speed of 100 rpm; and a feed rate of 15 rpm. The extrudates were air-dried at ambient temperature and stored until analysis.

2.2. Viscosity Determination

Viscosity was determined using a Brabender Micro Visco-Amylograph (Brabender, Duisburg, Germany) according to the method by Jozinović et al. (2012) [21]. Samples were weighed into the Brabender Micro Visco-Amylograph container to obtain 115 g of aqueous suspension with 14% solids. The measurement involved the following temperature program: heating from 30 to 92 °C at 7.5 °C/min; isothermal hold at 92 °C for 5 min; cooling from 92 to 50 °C at 7.5 °C/min; and isothermal hold at 50 °C for 1 min. During measurement, the rotor speed was 250 rpm, and the following parameters were recorded:

-

Initial gelatinization temperature of starch [°C];

-

Peak viscosity: maximum viscosity during gelatinization [BU];

-

Viscosity at 92 °C [BU];

-

Viscosity after 5 min at 92 °C [BU];

-

Viscosity at 50 °C [BU];

-

Viscosity after 1 min at 50 °C [BU];

-

Breakdown: calculated by subtracting the viscosity after 5 min at 92 °C from the peak viscosity, indicating stability during mixing at high temperatures (92 °C) [BU];

-

“Setback”: calculated by subtracting the viscosity after 5 min at 92 °C from the viscosity at 50 °C, indicating the tendency of the starch paste to retrograde [BU].

2.3. Total Starch Content and Degree of Starch Damage

Total starch content was determined by the polarimetric method (ISO 6493:2000) [22] and starch damage (DS) was determined according to the AACC 76-31.01 method [23], using a K-SDAM kit (Megazyme Int., Wicklow, Ireland).

2.4. Determination of Bioavailability of Zinc and Selenium

The concentrations of zinc and selenium in the samples after in vitro digestion simulation, conducted according to the Kiers method (2000) [24], as well as the concentration of the total share of these elements, were determined using inductively coupled plasma–optical emission spectrometry (ICP-OES), manufactured by Perkin Elmer (Waltham, MA, USA) (model: Optima 2100 DV).

2.5. Experimental Design and Data Analysis

The statistical significance of the regression coefficients was determined by an analysis of variance (ANOVA), at a 95% level, using post hoc LSD at a 95% level.

3. Results and Discussion

3.1. Viscosity of Mixtures and Extrudates

The influence of the addition of different proportions of wheat, biofortified with zinc and selenium, on the rheological properties of the mixture for extrusion in relation to the control sample of corn grits, is shown in

Table 1. The effect of the addition of zinc and selenium-fortified wheat on the viscosity of the mixtures.

Sample	Non-Extruded Zn Samples
	Peak (BU)	Viscosity at 92 °C (BU)	Hot Viscosity (BU)	Cold Viscosity (BU)	Viscosity after Mixing at 50 °C (BU)	Breakdown (BU)	“Setback” (BU)
Corn grits	45.25 ± 20.01 a	496.50 ± 21.92 d	92.00 ± 0.00 a	23.50 ± 6.36 a	499.00 ± 18.38 d	925.50 ± 12.02 b	878.50 ± 17.65 c
Corn:Wheat Zn 90:10	35.40 ± 3.25 a	453.50 ± 7.78 c	91.30 ± 0.71 a	21.50 ± 9.19 a	449.50 ± 3.54 c	901.50 ± 10.61 b	898.00 ± 0.00 c,d
Corn:Wheat Zn 80:20	60.25 ± 33.30 a	503.50 ± 0.71 d	92.15 ± 0.21 a	81.50 ± 2.12 b	500.50 ± 6.36 d	937.50 ± 27.58 b	932.00 ± 14.14 d
Corn:Wheat Zn 70:30	65.25 ± 39.67 a	295.50 ± 17.68 a	92.20 ± 0.42 a	25.50 ± 36.06 a	301.00 ± 15.56 a	753.50 ± 10.61 a	706.50 ± 12.02 a
Corn:Wheat Zn 60:40	34.25 ± 0.78 a	344.50 ± 12.02 b	91.85 ± 0.35 a	581.00 ± 4.24 a,b	336.50 ± 12.02 b	775.00 ± 9.90 a	807.00 ± 19.80 b
Sample	Non-Extruded Se Samples
	Peak (BU)	Viscosity at 92 °C (BU)	Hot Viscosity (BU)	Cold Viscosity (BU)	Viscosity after Mixing at 50 °C (BU)	Breakdown (BU)	“Setback” (BU)
Corn grits	45.25 ± 20.01 a	496.50 ± 21.92 c	92.00 ± 0.00 a,b	23.50 ± 6.36 a	499.00 ± 18.38 c	925.50 ± 12.02 b,c	878.50 ± 17.65 b
Corn:Wheat Se 90:10	48.85 ± 8.27 a	481.00 ± 24.04 c	92.30 ± 0.14 b	37.00 ± 1.41 a	483.00 ± 22.63 c	998.00 ± 31.11 c	956.5 ± 34.65 c
Corn:Wheat Se 80:20	61.40 ± 36.06 a	392.00 ± 42.43 b	91.50 ± 0.28 a	33.50 ± 27.58 a	284.50 ± 33.23 b	902.00 ± 33.94 b	863.5 ± 28.99 b
Corn:Wheat Se 70:30	34.05 ± 1.20 a	328.00 ± 12.73 a,b	92.25 ± 0.49 b	50.50 ± 2.12 a	326.50 ± 13.44 a	783.00 ± 57.98 a	767.00 ± 21.21 a
Corn:Wheat Se 60:40	75.55 ± 22.13 a	283.50 ± 16.26 a	92.60 ± 0.00 b	34.50 ± 14.85 a	287.00 ± 14.14 a	701.50 ± 23.33 a	712.00 ± 8.49 a

Values with different letters in the same column and same group are significantly different (p < 0.05).

.

The results indicate that the peak viscosity, representing the maximum viscosity of starch gelatinization, decreased with 10% (minimum value 34.25 ± 0.78) and 40% zinc-biofortified wheat, while it increased with 20% and 30% (maximum value 65.25 ± 39.67) compared to corn grits (45.25 ± 20.01), though these differences were not statistically significant. For selenium-biofortified wheat, there were also no statistically significant differences in peak viscosity values, except that the value for 30% addition was lower compared to corn grits, while other values were higher. The highest peak viscosity value was found in the sample with 40% (75.55 ± 22.13) selenium-biofortified wheat. Other studies also indicate an increase in viscosity with the addition of dietary fiber [25]. The hot viscosity values (viscosity after 5 min of mixing at 92 °C) for mixes with added zinc- and selenium-biofortified wheat did not show statistically significant changes compared to corn grits. Cold viscosity (viscosity at 50 °C) increased in mixes with added zinc-biofortified wheat, reaching a maximum with a 40% addition, and in mixes with added selenium-biofortified wheat, the maximum was reached with a 30% addition. Values for “breakdown” (stability during mixing at high temperatures) and “setback” (tendency of starch paste to retrograde) decreased with the addition of higher proportions of both zinc- and selenium-biofortified wheat.

The addition of wheat biofortified with zinc and selenium reduces the viscosity values compared to non-extruded mixtures (

Table 2. The effect of the addition of zinc-fortified wheat on the viscosity of the extrudates.

Sample	Extruded Zn170 °C
	Peak (BU)	Viscosity at 92 °C (BU)	Hot Viscosity (BU)	Cold Viscosity (BU)	Viscosity after Mixing at 50 °C (BU)	Breakdown (BU)	“Setback” (BU)
Corn grits	30.65 ± 0.07 a	132.00 ± 4.24 c	52.65 ± 0.92 c	57.00 ± 1.41 b	46.00 ± 4.24 b	170.50 ± 28.99 c	178.00 ± 28.28 c
Corn:Wheat Zn 90:10 170 °C	33.10 ± 2.40 a	89.00 ± 15.56 b	48.80 ± 0.42 b	0.00 ± 0.00 a	0.00 ± 0.00 a	107.00 ± 2.83 b	111.00 ± 2.83 b
Corn:Wheat Zn 80:20 170 °C	49.70 ± 0.00 a,b	196.00 ± 1.41 d	93.30 ± 0.00 d	183.50 ± 0.71 c	182.00 ± 0.00 c	348.00 ± 2.83 d	349.00 ± 4.24 d
Corn:Wheat Zn 70:30 170 °C	34.75 ± 0.07 a	64.50 ± 4.95 a,b	46.15 ± 0.78 a	0.00 ± 0.00 a	0.00 ± 0.00 a	57.00 ± 1.41 a	61.00 ± 2.83 a
Corn:Wheat Zn 60:40 170 °C	56.65 ± 18.03 b	57.50 ± 17.68 a	92.75 ± 0.35 d	52.00 ± 15.56 b	50.50 ± 19.09 b	186.50 ± 26.16 c	180.50 ± 21.92 c
Sample	Extruded Zn180 °C
	Peak (BU)	Viscosity at 92 °C (BU)	Hot Viscosity (BU)	Cold Viscosity (BU)	Viscosity after Mixing at 50 °C (BU)	Breakdown (BU)	“Setback” (BU)
Corn grits	31.40 ± 0.71 a	123.00 ± 12.73 b	49.15 ± 6.86 a	25.00 ± 8.49 b	11.50 ± 7.78 b	133.00 ± 31.11 b	138.00 ± 33.84 b
Corn:Wheat Zn 90:10 180 °C	32.55 ± 2.19 a	106.50 ± 21.92 b	49.10 ± 2.69 a	30.00 ± 11.31 b	15.50 ± 12.02 b	139.50 ± 13.44 b	146.50 ± 10.61 b
Corn:Wheat Zn 80:20 180 °C	31.35 ± 0.78 a	111.00 ± 5.66 b	44.45 ± 3.75 a	22.00 ± 8.49 a,b	10.00 ± 8.49 a,b	99.00 ± 7.07 a,b	105.50 ± 6.36 a,b
Corn:Wheat Zn 70:30 180 °C	38.00 ± 0.85 b	39.00 ± 15.56 a	44.45 ± 0.07 a	0.00 ± 0.00 a	0.00 ± 0.00 a	75.00 ± 15.56 a	78.00 ± 18.38 a
Corn:Wheat Zn 60:40 180 °C	32.60 ± 0.28 a	101.50 ± 14.85 b	55.10 ± 6.93 a	35.00 ± 9.90 b	31.00 ± 4.24 b	131.50 ± 2.12 b	134.00 ± 5.66 b
Sample	Extruded Zn 190 °C
	Peak (BU)	Viscosity at 92 °C (BU)	Hot Viscosity (BU)	Cold Viscosity (BU)	Viscosity after Mixing at 50 °C (BU)	Breakdown (BU)	“Setback” (BU)
Corn grits	30.75 ± 0.07 a	70.50 ± 2.12 a,b	42.65 ± 2.76 a	4.00 ± 5.66 a,b	8.00 ± 11.31 a	140.50 ± 14.85 b,c	146.00 ± 12.73 b,c
Corn:Wheat Zn 90:10 190 °C	31.00 ± 0.28 a	139.50 ± 45.69 c	40.45 ± 1.48 a	50.50 ± 10.61 c	33.00 ± 4.24 b	151.00 ± 9.90 c	155.50 ± 7.78 c
Corn:Wheat Zn 80:20 190 °C	31.20 ± 0.00 a	116.50 ± 6.36 b, c	45.35 ± 9.26 a	25.00 ± 4.24 b	11.50 ± 9.19 a	108.50 ± 0.71 b,c	110.00 ± 2.83 a,b,c
Corn:Wheat Zn 70:30 190 °C	35.15 ± 6.43 a	6.00 ± 7.07 a	40.75 ± 1.48 a	0.00 ± 0.00 a	0.00 ± 0.00 a	60.00 ± 19.80 a	62.00 ± 26.87 a
Corn:Wheat Zn 60:40 190 °C	36.75 ± 6.86 a	59.00 ± 28.28 a,b	47.80 ± 1.84 a	10.50 ± 14.85 a,b	45.00 ± 6.36 a	101.50 ± 27.58 a,b	102.00 ± 31.11 a,b

Values with different letters in the same column and same group are significantly different (p < 0.05).

and

Table 3. The effect of the addition of selenium-fortified wheat on the viscosity of the extrudates.

Sample	Extruded Se170 °C
	Peak (BU)	Viscosity at 92 °C (BU)	Hot Viscosity (BU)	Cold Viscosity (BU)	Viscosity after Mixing at 50 °C (BU)	Breakdown (BU)	“Setback” (BU)
Corn grits	30.65 ± 0.07 a	132.00 ± 4.24 b	52.65 ± 0.92 b	57.00 ± 1.41 b	46.00 ± 4.24 b	170.50 ± 28.99 b	178.00 ± 28.28 b
Corn:Wheat Se 90:10 170 °C	33.40 ± 0.00 a	97.50 ± 4.95 b	49.75 ± 2.47 a,b	19.00 ± 1.41 a	3.00 ± 0.00 a	146.00 ± 2.83 b	145.50 ± 0.71 b
Corn:Wheat Se 80:20 170 °C	31.65 ± 1.48 a	122.00 ± 26.87 b	50.85 ± 0.92 a,b	50.50 ± 21.92 b	35.00 ± 22.63 b	164.50 ± 23.33 b	171.00 ± 21.21 b
Corn:Wheat Se 70:30 170 °C	37.50 ± 0.28 b	47.50 ± 10.61 a	46.60 ± 1.98 a	0.00 ± 0.00 a	0.00 ± 0.00 a	77.50 ± 0.71 a	85.00 ± 0.00 a
Corn:Wheat Se 60:40 170 °C	40.45 ± 3.18 b	41.50 ± 3.54 a	50.30 ± 2.12 a,b	0.00 ± 0.00 a	0.00 ± 0.00 a	90.00 ± 4.24 a	92.00 ± 1.41 a
Sample	Extruded Se180 °C
	Peak (BU)	Viscosity at 92 °C (BU)	Hot Viscosity (BU)	Cold Viscosity (BU)	Viscosity after Mixing at 50 °C (BU)	Breakdown (BU)	“Setback” (BU)
Corn grits	31.40 ± 0.71 a	123.00 ± 12.73 b	49.15 ± 6.86 a	25.00 ± 8.49 b	11.50 ± 7.78 b	133.00 ± 31.11 c	138.00 ± 33.84 c
Corn:Wheat Se 90:10 180 °C	31.35 ± 0.07 a	107.00 ± 2.73 b,c	44.55 ± 2.90 a	5.00 ± 7.07 a	0.00 ± 0.00 a	87.50 ± 9.19 a,b,c	94.00 ± 9.90 b,c
Corn:Wheat Se 80:20 180 °C	35.00 ± 5.80 a	63.50 ± 28.99 b	39.45 ± 1.20 a	0.00 ± 0.00 a	0.00 ± 0.00 a	63.00 ± 16.97 a,b	70.00 ± 16.97 a,b
Corn:Wheat Se 70:30 180 °C	32.70 ± 2.12 a	81.50 ± 13.44 b,c	46.10 ± 0.00 a	8.00 ± 11.31 a,b	1.50 ± 2.12 a	109.00 ± 12.73 b,c	120.00 ± 7.07 b,c
Corn:Wheat Se 60:40 180 °C	34.75 ± 6.29 a	13.00 ± 18.38 a	36.30 ± 8.63 a	0.00 ± 0.00 a	0.00 ± 0.00 a	33.50 ± 28.99 a	35.50 ± 27.58 a
Sample	Extruded Se190 °C
	Peak (BU)	Viscosity at 92 °C (BU)	Hot Viscosity (BU)	Cold Viscosity (BU)	Viscosity after Mixing at 50 °C (BU)	Breakdown (BU)	“Setback” (BU)
Corn grits	30.75 ± 0.07 a	70.50 ± 2.12 a,b	42.65 ± 2.76 a	4.00 ± 5.66 a,b	8.00 ± 11.31 a	140.50 ± 14.85 b,c	146.00 ± 12.73 b,c
Corn:Wheat Se 90:10 190 °C	31.00 ± 0.28 a	139.50 ± 45.69 c	40.45 ± 1.48 a	50.50 ± 10.61 c	33.00 ± 4.24 b	151.00 ± 9.90 c	155.50 ± 7.78 c
Corn:Wheat Se 80:20 190 °C	31.20 ± 0.00 a	116.50 ± 6.36 b	45.35 ± 9.26 a	25.00 ± 4.24 b	11.50 ± 9.19 a	108.50 ± 0.71 b,c	110.00 ± 2.83 a,b,c
Corn:Wheat Se 70:30 190 °C	35.15 ± 6.43 a	6.00 ± 7.07 a	40.75 ± 1.48 a	0.00 ± 0.00 a	0.00 ± 0.00 a	60.00 ± 19.80 a	62.00 ± 26.87 a
Corn:Wheat Se 60:40 190 °C	36.75 ± 6.86 a	59.00 ± 28.28 a,b	47.8 ± 1.84 a	10.50 ± 14.85 a,b	45.00 ± 6.36 a	101.50 ± 27.58 a,b	102.00 ± 31.11 a,b

Values with different letters in the same column and same group are significantly different (p < 0.05).

). Studies have shown that increasing the proportion of buckwheat flour in the mixtures reduces viscosity, which they explain through lower starch gelatinization [26,27]. Liu et al. [28]. reported reductions in the viscosity of rice starch samples supplemented with soy protein isolate and further reductions compared to control rice starch samples after extrusion. The extrusion temperature is critical in determining the physicochemical properties of cereal flour and the corresponding multi-scale structure of starch. After high-temperature extrusion treatment, the degree of starch gelatinization increases, which is accompanied by significant damage to ordered structures and the degradation of molecular chains. This results in reduced viscosity and increased hydration capacity [29].

Comparing the results for the total starch (TS) content in the mixtures to the control corn grits sample (

Table 4. Starch content in corn grits, mixtures, and extrudates with the addition of wheat biofortified with selenium and zinc.

Sample	Starch Content (% d.m.)
	Non-Extruded	Extruded
		170 °C	180 °C	190 °C
Corn grits	72.95 ± 0.32 d	74.29 ± 0.31 d	74.30 ± 0.08 e	74.80 ± 0.07 e
Corn: Wheat Zn 90:10	70.70 ± 0.53 c	72.70 ± 0.68 c	72.95 ± 0.06 d	72.74 ± 0.07 d
Corn: Wheat Zn 80:20	69.66 ± 0.04 b	71.50 ± 0.02 b	71.65 ± 0.19 c	70.95± 0.58 c
Corn: Wheat Zn 70:30	65.43 ± 0.08 a	70.90 ± 0.02 b	70.70 ± 0.34 b	69.52 ± 0.03 b
Corn: Wheat Zn 60:40	65.74 ± 0.21 a	67.29 ± 0.33 a	67.48 ± 0.06 a	67.27 ± 0.08 a
Corn: Wheat Se 90:10	72.93 ± 0.52 d	73.68 ± 0.16 d	73.66 ± 0.26 d	73.98 ± 0.01 d
Corn: Wheat Se 80:20	71.25 ± 0.28 c	72.86 ± 0.45 c	71.15 ± 0.25 c	71.62 ± 0.08 c
C.grits: Wheat Se 70:30	68.71 ± 0.44 b	71.14 ± 0.33 b	69.81 ± 0.11 b	70.43 ± 0.17 b
Corn: Wheat Se 60:40	67.44 ± 0.01 a	68.32 ± 0.94 a	67.85 ± 0.32 a	68.68 ± 0.01 a

Values with different letters in the same column and same group (Zn or Se) are significantly different (p < 0.05).

) and the degree of starch damage in extruded samples compared to the mixtures (Figure 1 and Figure 2) indicates that lower peak, hot, and cold viscosity values are associated with lower TS content and higher degrees of starch damage after extrusion. Additionally, this trend applies to different extrusion temperatures, which is consistent with other research [26,28,30].

3.2. Total Starch Content

The total starch content in the raw materials varied, ranging from a minimum of 58.78 ± 0.08 for selenium-biofortified wheat to 60.83 ± 0.37 for zinc-biofortified wheat, up to a maximum of 72.95 ± 0.32 for corn grits. The starch content in grits, blends, and extrudates with added selenium- and zinc-biofortified wheat is presented in Table 4.

Analyzing the results across different extrusion temperatures, it is evident that the total starch content (TS) increased through extrusion compared to blends, with no significant differences noted with increasing extrusion temperature. The highest value was obtained with pure corn grits extruded at 190 °C, measuring 74.80 ± 0.07. These findings align with previous studies [21,31,32]. The results of total starch content relative to varying proportions of added selenium- and zinc-biofortified wheat show a proportional decrease in starch content with higher proportions of added selenium- and zinc-biofortified wheat, with the lowest value obtained at 40% inclusion in blends with corn grits, measuring 67.44 ± 0.01. Reductions in starch content in extrudates with added whole-wheat flour is also found in studies involving rice-based extrudates [32,33].

3.3. Starch Damage

The impact of adding zinc-biofortified wheat and the extrusion process on starch damage (DS) in corn blends and extrudates is illustrated in Figure 1 and Figure 2.

The addition of varying proportions of zinc-biofortified and selenium-biofortified wheat did not significantly affect starch damage (DS), with the lowest value observed in blends containing 20% zinc-biofortified and selenium-biofortified wheat.

The extrusion process significantly increased DS in all samples, with values largely exceeding 50%, which is consistent to other research [34,35]. However, DS values did not significantly change with increasing extrusion temperature. Greater starch damage in extruded corn products is attributed to fragmentation within starch granules caused by intense shear within the extruder [36]. Mechanical shear induces starch gelatinization and protein denaturation [37]. The temperature inside the extruder is one of the most crucial factors determining the degree of starch gelatinization and protein denaturation [38].

The addition of different proportions of zinc-biofortified and selenium-biofortified wheat proportionally reduced DS values with higher proportions of biofortified wheat in both blends and at various temperatures. The sample with 40% added zinc-biofortified and selenium-biofortified wheat had the lowest DS value. These results are consistent with research conducted by Kabir et al. [39].

3.4. Bioavailabity of Zinc and Selenium

The results for the total zinc and selenium content and zinc and selenium bioavailability in mixtures and extrudates are presented in

Table 5. Total zinc content and bioavailability of zinc in mixtures and extrudates.

Sample	Total Content of Zn [mg/kg]	Bioavailable Content of Zn [mg/kg]
	Non-Extruded
Corn grits	3.07 ± 0.18 a	1.34 ± 0.72 a
Corn: Wheat Zn 90:10	7.03 ± 0.27 b	2.43 ± 0.90 a,b
Corn: Wheat Zn 80:20	13.60 ± 0.99 c	2.26 ± 0.22 a,b
Corn: Wheat Zn 70:30	14.93 ± 0.38 c	3.36 ± 0.83 b,c
Corn: Wheat Zn 60:40	21.01 ± 2.26 d	4.02 ± 0.38 c
	Extruded 170 °C
Corn grits	3.36 ± 0.12 a	1.11 ± 0.24 a
Corn: Wheat Zn 90:10	9.05 ± 0.29 b	1.77 ± 0.18 a
Corn: Wheat Zn 80:20	15.15 ± 0.06 c	1.92 ± 0.58 a
Corn: Wheat Zn 70:30	20.40 ± 0.30 d	1.40 ± 0.41 a
Corn: Wheat Zn 60:40	22.43 ± 0.29 e	1.37 ± 0.82 a
	Extruded 180 °C
Corn grits	3.45 ± 0.12 a	1.28 ± 0.27 a
Corn: Wheat Zn 90:10	7.59 ± 0.47 b	1.43 ± 0.98 a
Corn: Wheat Zn 80:20	12.97 ± 0.57 c	1.91 ± 0.49 a
Corn: Wheat Zn 70:30	18.05 ± 0.27 d	1.40 ± 0.33 a
Corn: Wheat Zn 60:40	22.24 ± 1.18 e	1.69 ± 0.29 a
	Extruded 190 °C
Corn grits	3.51 ± 0.09 a	1.43 ± 0.30 a,b
Corn: Wheat Zn 90:10	8.61 ± 0.37 b	2.08 ± 0.46 b
Corn: Wheat Zn 80:20	15.04 ± 0.77 c	1.72 ± 0.24 a,b
Corn: Wheat Zn 70:30	18.49 ± 0.22 d	1.70 ± 0.36 a,b
Corn: Wheat Zn 60:40	23.01 ± 0.37 e	1.40 ± 0.27 a

Values with different letters in the same column and same group are significantly different (p < 0.05).

and

Table 6. Total selenium content and bioavailability of selenium in mixtures and extrudates.

Sample	Total Content of Se [mg/kg]	Bioavailable Content of Se [mg/kg]
	Non-Extruded
Corn grits	21.89 ± 0.00 a	0.04 ± 0.01 a
Corn: Wheat Se 90:10	84.74 ± 0.00 b	0.27 ± 0.14 b
Corn: Wheat Se 80:20	111.86 ± 0.01 c	0.27 ± 0.17 b
Corn: Wheat Se 70:30	192.01 ± 0.01 d	0.23 ± 0.13 a,b
Corn: Wheat Se 60:40	254.59 ± 0.02 e	0.25 ± 0.12 a,b
	Extruded 170 °C
Corn grits	30.55 ± 0.01 a	0.55 ± 0.12 a
Corn: Wheat Se 90:10	92.81 ± 0.00 b	0.37 ± 0.27 a
Corn: Wheat Se 80:20	160.51 ± 0.01 c	0.41 ± 0.12 a
Corn: Wheat Se 70:30	222.27 ± 0.01 d	0.51 ± 0.10 a
Corn: Wheat Se 60:40	304.48 ± 0.03 e	0.53 ± 0.07 a
	Extruded 180 °C
Corn grits	28.00 ± 0.00 a	0.54 ± 0.15 a
Corn: Wheat Se 90:10	86.08 ± 0.00 b	0.37 ± 0.25 a
Corn: Wheat Se 80:20	131.46 ± 0.01 c	0.41 ± 0.14 a
Corn: Wheat Se 70:30	180.41 ± 0.01 d	0.50 ± 0.10 a
Corn: Wheat Se 60:40	267.08 ± 0.01 e	0.51 ± 0.12 a
	Extruded 190 °C
Corn grits	22.90 ± 0.00 a	0.55 ± 0.06 a
Corn: Wheat Se 90:10	96.33 ± 0.01 b	0.29 ± 0.20 a
Corn: Wheat Se 80:20	140.69 ± 0.04 c	0.39 ± 0.15 a
Corn: Wheat Se 70:30	211.69 ± 0.01 d	0.53 ± 0.13 a
Corn: Wheat Se 60:40	289.37 ± 0.02 e	0.47 ± 0.17 a

Values with different letters in the same column and same group are significantly different (p < 0.05).

.

The results indicate that both total zinc content and bioavailable zinc in blends increased proportionally with added zinc-biofortified wheat. The highest total zinc content in blends was 21.01 ± 2.26 mg/kg. After in vitro digestion, the bioavailability of zinc decreased in blends, with the highest bioavailability observed in the sample containing 40% zinc-biofortified wheat at 4.02 ± 0.38 mg/kg, while the lowest bioavailable zinc value was 1.34 ± 0.72 mg/kg in the control sample of pure corn grits.

The influence of different temperature regimes and varying proportions of added zinc-biofortified wheat on the total zinc content in extrudates showed a positive correlation, as higher temperatures and higher proportions of added wheat increased the total zinc content. The highest value was 23.01 ± 0.37 mg/kg for the sample with 40% inclusion, whereas the control sample of pure corn grits at 170 °C had a total zinc content of 3.36 ± 0.12 mg/kg. The values for bioavailable zinc also increased with higher extrusion temperatures compared to the control sample. However, there was no statistically significant difference in bioavailable zinc values with varying proportions of biofortified wheat at 170 °C and 180 °C. At an extrusion temperature of 190 °C, the bioavailable zinc content decreased with higher proportions of biofortified wheat, showing a statistically significant decrease at 40% inclusion. The increase in bioavailable zinc is likely associated with a reduction in phytic acid content, a major inhibitor of zinc absorption in the body. The binding of phytic acid with various minerals, especially zinc, results in insoluble salts that reduce mineral bioavailability due to the formation of insoluble salts [40]. Heydysz et al. [41] demonstrated that the extrusion process with different temperature profiles reduced phytic acid levels in bean samples. The extrusion process significantly reduced TIA, RS, and phytate-P content by approximately 62%, as observed in the study by Zaworska-Zakrzewska et al. [42]. Augustin and Cole [43] also demonstrated a decrease in phytic acid content due to extrusion processing. Elevated temperatures activate phytase enzymes, contributing to phytate hydrolysis. Additionally, high temperatures promote the thermal degradation of phytates [44].

The total selenium content and selenium bioavailability in blends increased proportionally with added selenium-biofortified wheat. The highest total selenium content in blends was 254.59 ± 0.02 mg/kg. After in vitro digestion, the bioavailability of selenium decreased in blends without statistically significant differences across varying proportions of biofortified wheat. The highest bioavailability of selenium was 0.27 ± 0.17 mg/kg, while the lowest was 0.04 ± 0.01 mg/kg in the control sample of pure corn grits.

The impact of extrusion on selenium content was positive, as both total selenium and bioavailable selenium increased compared to non-extruded blends. The addition of varying proportions of selenium-biofortified wheat positively influenced the total selenium content in extrudates, with a statistically significant increase with higher proportions of added wheat. The highest value was 304.48 ± 0.03 mg/kg for the sample with 40% inclusion at an extrusion temperature of 170 °C, whereas the control sample of pure corn grits at 190 °C had a total selenium content of 22.90 ± 0.00 mg/kg. The total selenium content decreased with increasing extrusion temperature. The bioavailable selenium values did not significantly change with increasing extrusion temperature, and there was no statistically significant difference in values across varying proportions of biofortified wheat at all extrusion temperatures.

Several factors influence the variation in bioavailable selenium content. One of the most critical factors is the chemical form of selenium, which was not specified in this study. Selenium in the form of selenates or selenites is well absorbed but is less retained in the body compared to organic forms such as selenomethionine and selenocysteine [45]. Selenium absorption is also influenced by fiber content [46] and food processing methods [47]. Selenium absorption from whole wheat versus refined wheat showed that the presence of wheat bran reduced selenium absorption compared to the control sample, likely due to indigestible fibers in wheat bran. These fibers “encapsulate” proteins to which selenium is bound, making them inaccessible to enzymes in the upper digestive tract, resulting in undigested proteins and selenium being transported to the large intestine where they undergo bacterial fermentation. Bacteria incorporate selenium into their biomass, reducing its absorption even in the colon [48].

Selenium content in enriched extruded samples was significantly higher than in control samples [49]. Li et al. [50] demonstrated higher selenium bioavailability in extruded soy compared to fried soy.

4. Conclusions

This study presents the results of the addition of wheat biofortified with zinc and selenium on chemical and rheological properties. The viscosity values were reduced compared to non-extruded mixtures with the addition of wheat biofortified with zinc and selenium (6–45%). Also, a trend of viscosity reduction was recorded when the extrusion temperature increased (6–37%). As the temperature increased, the starch content in the products also increased (2–3%). However, when different amounts of wheat fortified with zinc and selenium were added, the starch content decreased (7%). During the extrusion process, there was a notable increase in the degree of starch damage, which refers to the breakdown of starch granules (1.15 to 60.91%). Interestingly, this degree of starch damage decreased when varying amounts of zinc- and selenium-fortified wheat were incorporated (60.91 to 48.07%). Extrusion had a positive effect on the bioavailability of zinc and selenium from the fortified wheat (14.93 ± 0.38 to 20.40 ± 0.30). Notably, there were no significant differences in bioavailability observed when different proportions of zinc- and selenium-fortified wheat were used.