The Influence of the Used Bleaching Earth on the Content of Natural Dyes in Hemp (Cannabis sativa L.) Oils

Abstract

Cold-pressed hemp oils are characterized by an intense color, which is undesirable when used directly. Therefore, research was undertaken on removing chlorophyll and carotenoids effectively. This publication presents the results of tests that verified the adsorption properties of seven bleaching earths (BE1–BE7) in two doses (2.5% and 5.0%) in the low-temperature bleaching process of hemp oils. These oils were obtained by cold and hot pressing of the seeds of three varieties of hemp (Cannabis sativa L.): Finola, Earlina 8FC, and Secuieni Jubileu. The color change and the content of carotenoid and chlorophyll pigments in the bleached oils were verified using the colorimetric method (CIE-Lab). The BEs used had different abilities to reduce the content of natural dyes connected with oil decolorization. The conducted research allowed us to characterize the influence of BEs on the organoleptic properties of the tested oils. Hemp oil obtained from the Secuieni Jubileu CP and HP hemp variety should be bleached with unmodified magnesian bentonite at 2.5%. Unmodified attapulgite clay is not recommended for this variety, as it strongly adsorbs carotenoids from the oil.

1. Introduction

Vegetable oils are the basis of a balanced human diet [1,2]. They provide essential fatty acids that carry fat-soluble vitamins, act as precursors of steroid hormones, and play a significant role in health protection and disease prevention [3]. Vegetable oils are also used in other industries, where they constitute a raw material producing liquid biofuels [4,5] or degradable polymer materials [6,7]. In Europe, the following oils are produced on an industrial scale: rapeseed, sunflower, and soybean [8,9]. In recent years, niche oils (i.e., those made from seeds or fruit pulps of plants not usually used in the oil industry) have become increasingly popular among consumers [10,11,12,13]. A high content of nutrients characterizes these oils. They are a rich source of monounsaturated (MUFA) and polyunsaturated (PUFA) fatty acids and other biologically active substances, such as sterols, tocopherols, phenolic compounds, and carotenoids [14,15]. Niche oils are derived from plants grown on a small scale compared to other oil plants (e.g., rapeseed). Moreover, their seeds are also characterized by lower oil content. They have better health-promoting properties than traditional oils, so they are becoming increasingly popular among customers. As demonstrated by Czwartkowski et al., essential factors influencing consumers’ willingness to purchase niche oils are high content of nutrients, production free from GMOs and harmful chemicals, and price [10]. Organoleptic characteristics (color, clarity, aroma, palatability) and physicochemical properties are also important for consumers. Niche oils should be characterized by high clarity, expressive color, and an aroma that is characteristic of a given plant (e.g., raspberry aroma for raspberry seed oils) [16]. Niche oils are obtained on an industrial scale by mechanical pressing and solvent extraction from plant raw materials. The European Union has no separate regulations containing quality standards for niche oils [17].

The color of vegetable oils is determined by the carotenoid and chlorophyll pigments [18]. Excess chlorophyll pigments are responsible for faster oil oxidation, bitter taste, and dark green color [19]. Therefore, removing chlorophyll pigments by bleaching will positively affect the sensory properties and extend the shelf life of vegetable oils. This process involves the adsorption of undesirable substances by bleaching earth [20], silica gel [21] and activated carbon [22]. Bleaching earths are mineral clays of aluminum silicates, such as bentonite, attapulgite, or sepiolite [23,24,25]. Bleaching earths are activated chemically or physically to increase their sorption surface. Bentonite bleaching earths have good adsorption properties for dyes and other undesirable substances, so they are often used for purifying vegetable oils [26].

An example of a niche oil is oil obtained by pressing hemp seeds (Cannabis sativa L.). Unrefined hemp oil is a rich source of vitamins, mineral salts, and phytosterols. This oil contains approximately 80% of polyunsaturated fatty acids, especially α-linolenic acid (18:3; n-3) and linoleic acid (18:2; n-6). The ratio of n-6 to n-3 acids is 3:1, positively affecting health-promoting properties [27].

Niche oils are produced on a small scale and are not refined. Their organoleptic parameters (i.e., color and taste) are intense, which limits their use. Of the four oil refining processes, bleaching is the process that is easy to apply in small-scale oil production. Therefore, verification of the effectiveness of the patented method (PL 232781 [28]) was conducted for this study. The type of absorbent is crucial; therefore, attapulgite clay, magnesium bentonite, and kerolite were used for the study because these BEs are dedicated to oil refining. The types of bleaching earths were selected based on consultation with their manufacturers. The test materials consisted of three vegetable oils pressed from selective hempseed varieties (Finola, Earlina 8FC, Secuieni Jubileu). The literature continues to present research results on hemp oils from the selected varieties on an industrial scale. Therefore, this study aimed to check the effect of the low-temperature bleaching process on the content of natural pigments in hemp oils. It was achieved by verifying the following research hypothesis: Among the bleaching earth variants selected for the study, it is possible to find an adsorbent that will remove a significant amount of chlorophylls but leave carotenoids in the oil’.

2. Materials and Methods

2.1. Hemp Oils Pressing

Hemp oils were obtained by pressing three varieties of hemp seeds (Cannabis sativa L.): Finola, Earlina 8FC, and Secuieni Jubileu. All seeds came from certified cultivation in Kuyavian–Pomeranian voivodeship (Poland). Three BEs were based on attapulgite clay (BE1–BE3), three were based on magnesium bentonite (BE4–BE6), and one was based on kerolite-hydrated magnesium silicate (BE7). Basic physical parameters (oil content, raw material size, average moisture, and average bulk density) were determined for each seed variety. Pressing took place on a single-shaft screw press (Farmet^®, Česká Skalice, Czech Republic) under two temperature conditions (low- and high-temperature). Low-temperature pressing (CP) was carried out at a temperature not exceeding 40 °C. For high-temperature pressing (HP), the seeds were heated for 60 min in a SUP 100 dryer (Steinberg Systems, Hamburg, Germany) at 70 °C. Hot seeds were transferred to a screw press and pressed at a temperature >60 °C.

2.2. Low-Temperature Bleaching

The bleaching methodology was developed based on Marcinkowski et al. [19] and Kwasnica et al. [9] and patent method PL 232781. The bleaching process was performed in two stages. The first stage was the homogenization of the bleaching earth with oils. First, 5 dm³ of oil and 125 g or 250 g of bleaching earth (BE1–BE7) were introduced into the open tank. All of the ingredients were stirred intensively for 5 min at a temperature of 50–60 °C. The bleaching soils came from three suppliers and were dedicated to the refining of vegetable oil. Differences in the efficacy of attapulgite clay, magnesium bentonite, and kerolite, chemically or physically modified, were studied. Selected physicochemical parameters of bleaching earth are presented in our previous work [9]. In the second stage of the bleaching process, the mixture was poured into the FDN 200 × 200 plate filter (Farmet^®, Česká Skalice, Czech Republic) and repeatedly pumped by it at a pressure of 2.5–3 bar until a clear liquid was obtained. After filtering, the oil was recycled and pumped through a layer of soil. The method was verified in terms of applying industry standards.

2.3. Measurement Methodology

2.3.1. Measurements of Oil Color Using the CIE-Lab Method

Measurements were made in triplicate for each sample using a Minolta CR 400 chromameter (Chongqing, China) calibrated on a white standard. Approximately 5 mL of each sample analyzed was placed in a clean and thoroughly degreased Petri dish. After each measurement, the head of the Minolta CR 400 chromameter was thoroughly cleaned to obtain reliable results. The total color change was calculated using formula (1) based on the method described by Nkhata [29].

$$∆\mathrm{E}=\sqrt{{\left(∆{\mathrm{L}}^{*}\right)}^2+{\left({∆\mathrm{a}}^{\mathrm{*}}\right)}^2+{\left({∆\mathrm{b}}^{\mathrm{*}}\right)}^2}$$

(1)

where

• ∆L*—change of photometric brightness
• ∆a*—change of parameter (greenness for negative or redness for positive values)
• ∆b*—change of parameter (blueness for negative or yellowness for positive values)

2.3.2. Determination of Carotenoids and Chlorophylls in Hemp Oils

Chlorophyll and carotenoid dye contents were measured using the method described by Wellburn and PN-A-86934:1995 [30,31]. Approximately 1.5 g of the tested oils was dissolved in 10 mL of anhydrous methanol (>99%, Sigma-Aldrich, Poznan, Poland). Next, 2.5 mL of the obtained solution was taken and then transferred to a quartz spectrophotometric cuvette. UV-Vis absorbance measurements were performed using a Metertech SP-830 PLUS spectrophotometer (Taipei, Taiwan) in triplicate at wavelengths λ = 470.0 nm, λ = 652.4 nm, and λ = 665.2 nm.

The obtained absorbance values were used to calculate the content of chlorophyll pigments and carotenoids in the tested oils. The following formulas were used for this purpose (2)–(4):

chlorophyll a = 16.72 × B − 9.16 × A

(2)

chlorophyll b = 34.09 × A − 15.28 × B

(3)

$$\mathrm{carotenoids}=\frac{1000\times \mathrm{C}-1.63\times \left(1\right)-104.96\times \left(2\right)}{221}$$

(4)

where

• A—absorbance at λ = 652.4 nm
• B—absorbance at λ = 665.2 nm
• C—absorbance at λ = 470.0 nm

2.4. Statistical Analysis

Statistical analysis was conducted based on the results obtained after CIE Lab color measurements and spectrophotometric determination of chlorophylls and carotenoids in the tested oils. The analytical tools were Microsoft Excel and Principal Component Analysis in Statistica 13.3 (StatSoft, Krakow, Poland). At first, the increments/decrements of the studied parameters were determined, and then the PCA was analyzed in graphs.

3. Results and Discussion

The hemp seeds selected for research differed in their physical parameters. Particularly, significant differences could be seen in the seeds’ oil content and size. The Earlina 8FC variety was characterized by the highest seed oil content (>31%), the smallest size (1.096 mm), and therefore the highest bulk density (521.00 kg/m³). Moreover, this variety achieved the best efficiency in terms of the pressing process both at low temperatures (>69%) and at high temperatures (>79%). Islam et al. [32] previously indicated that the oil yield increases with an increase in the processing temperature, which was confirmed by the results obtained in this study. Moreover, pressing the Secuieni Jubileu variety, which has the largest seeds, showed the lowest efficiency when pressed at low temperatures. In the case of pressing at elevated temperatures, it was similar to the Finola variety. Based on this, it can be concluded that the effectiveness of the low-temperature stamping process also depends on the size of the raw material, which was also noticed by Sacilik et al. [33] The parameters of the seeds used and the pressing efficiency are presented in

Table 1. Selected parameters of the hemp seeds used in this study and their pressing efficiency.

		Hemp Variety
		Finola	Earlina 8FC	Secuieni Jubileu
Parameter	Oil content [%]	29.97	31.04	29.47
	Seed size [mm]	1.152	1.096	1.213
	Average moisture content [%]	8.389	8.319	8.312
	Average bulk density [kg/m3]	516.06	521.00	506.73
	CP efficiency [%]	66.26	69.26	62.66
	HP efficiency [%]	75.19	79.40	76.12

.

The hemp oils that were obtained from two pressing variants were subjected to preliminary analyses, the results of which are presented in

Table 2. Results of the preliminary analyses of the different varieties of hemp oils.

Oil	Parameters
	L*	a*	b*	Chlorophylls [μg/mL]	Carotenoids [μg/mL]
Finola CP	42.13	1.51	24.66	90.1	11.5
Finola HP	50.42	5.17	40.38	117.9	8.7
Earlina 8FC CP	37.89	0.99	20.14	81.2	17.3
Earlina 8FC HP	47.91	3.92	36.67	102.1	14.8
Secuieni Jubileu CP	45.01	2.12	26.81	106.7	12.9
Secuieni Jubileu HP	54.03	6.84	43.11	131.4	10.6

.

Two studies (Santoso et al. and Wroniak et al. [34,35]) have previously indicated that an increase in pressing temperature is associated with a deterioration in the organoleptic properties of the oils obtained (decrease in clarity, darker color) and a change in the content of natural dyes. The amount of chlorophylls increases, but the amount of valuable components (carotenoids, sterols) decreases, and unsaturated fatty acids are degraded. Our preliminary analysis confirmed these results. The oils obtained from low-temperature pressing were characterized by a lighter color and greater clarity than those obtained at elevated temperatures, as indicated by the results of tests using the CIE Lab method. Increasing the pressing temperature causes an increase in the L* parameter (photometric brightness), a* parameter (greenness for negative or redness for positive values), and b* parameter (blueness for negative or yellowness for positive, values) of the tested oils (i.e., ultimately, a darkening of the oils). Pressing at high temperatures causes a significant increase in the content of chlorophyll pigments (23–31%, depending on the variety). The Earlina 8FC variety was characterized by the smallest growth in chlorophyll content and the highest increase was observed in Finola variety. In the case of the range of carotenoid pigments, their decrease was observed from 13% for the Earlina 8FC variety to 32% for the Finola variety.

The results of changes in the tested parameters during bleaching using the bleaching processes described in Section 2.2 are discussed below. Bes obtained using the methodology described were averaged and converted into percentage changes for the preliminary test samples. The results are summarized for each oil sample in

Table 3. Percentage (%) changes in hemp oil parameters after bleaching processes.

Oil Variety	Parameter	BE1 (2.5%)	BE1 (5.0%)	BE2 (2.5%)	BE2 (5.0%)	BE3 (2.5%)	BE3 (5.0%)	BE4 (2.5%)	BE4 (5.0%)	BE5 (2.5%)	BE5 (5.0%)	BE6 (2.5%)	BE6 (5.0%)	BE7 (2.5%)	BE7 (5.0%)
Finola CP	L*	11.37	20.08	9.78	18.32	17.42	25.44	12.18	23.51	6.18	17.67	9.14	16.04	11.03	21.19
	a*	−124.81	−202.26	−95.49	−172.93	−188.72	−210.53	−145.11	−215.04	−90.23	−209.02	−53.38	−189.47	−81.95	−222.56
	b*	55.03	31.26	53.56	41.50	106.03	23.22	82.45	110.24	34.73	105.30	26.33	90.86	46.80	110.97
	Chlorophylls	−65.41	−91.16	−60.93	−89.37	−92.31	−99.36	−85.65	−98.46	−47.87	−90.26	−41.46	−80.15	−65.03	−98.08
	Carotenoids	−26.30	−82.51	−33.75	−62.03	−34.12	−79.90	−27.79	−71.34	−75.31	−47.15	−32.26	−55.71	−0.62	−66.50
Finola HP	L*	7.11	17.79	5.38	15.03	8.01	22.65	8.57	18.39	1.29	12.40	3.10	12.14	3.23	14.99
	a*	−89.42	−184.13	−53.44	−137.04	−89.95	−187.83	−79.37	−147.62	−39.68	−110.58	−35.98	−111.64	−42.86	−157.67
	b*	131.10	179.60	104.35	180.60	158.19	234.78	142.14	269.90	66.22	179.93	62.21	187.63	67.22	231.77
	Chlorophylls	−52.70	−77.25	−53.74	−76.21	−66.88	−94.48	−58.60	−93.24	−36.90	−72.30	−32.14	−77.35	−41.47	−88.67
	Carotenoids	−21.31	−69.65	−15.13	−63.21	−32.32	−79.05	−5.19	−58.64	−50.94	−7.07	−26.23	−40.64	−14.95	−50.04
Earlina 8FC CP	L*	13.62	21.65	11.14	20.11	16.31	24.55	12.51	21.56	7.73	18.62	6.45	20.96	7.98	20.03
	a*	−188.11	−248.95	−146.85	−167.83	−206.29	−253.85	−179.02	−251.75	−103.50	−232.17	−100.00	−243.36	−122.38	−275.52
	b*	69.03	61.94	70.65	48.99	103.44	88.87	96.76	129.15	45.95	121.66	44.33	107.49	51.82	131.38
	Chlorophylls	−67.88	−87.17	−67.10	−87.40	−84.72	−95.43	−78.70	−94.87	−50.71	−88.85	−54.28	−90.30	−58.62	−93.64
	Carotenoids	−43.98	−78.63	−5.87	−65.06	−38.79	−74.59	−22.04	−69.68	−10.30	−50.34	−13.76	−60.73	−3.66	−67.37
Earlina 8FC HP	L*	15.64	22.26	14.84	21.53	15.52	24.97	15.77	22.09	11.91	19.54	8.73	18.44	8.18	20.43
	a*	−220.54	−264.29	−240.18	−226.79	−278.57	−280.36	−191.96	−276.79	−150.00	−264.29	−158.04	−303.57	−150.00	−321.43
	b*	96.66	52.12	110.02	55.46	134.97	85.52	124.50	122.05	86.41	162.81	88.86	140.09	94.43	144.54
	Chlorophylls	−76.25	−90.90	−81.45	−91.97	−85.11	−94.09	−84.52	−95.98	−67.15	−91.49	−59.94	−86.18	−67.03	−94.09
	Carotenoids	−65.77	−31.54	−59.73	−79.58	−57.72	−79.87	−40.75	−76.41	−15.15	−68.07	−18.60	−50.81	−36.05	−71.81
Secuieni Jubileu CP	L*	10.35	25.15	5.36	15.12	12.71	28.42	9.94	24.83	6.22	17.75	3.50	18.02	4.95	18.75
	a*	−80.39	−184.80	−29.90	−95.59	−108.82	−213.24	−74.02	−187.25	−42.65	−146.08	−31.86	−127.94	−37.75	−166.67
	b*	109.86	192.96	64.08	149.65	166.90	261.62	109.86	275.35	72.89	222.54	55.99	182.75	59.15	246.83
	Chlorophylls	−52.01	−81.69	−38.15	−73.93	−68.51	−94.27	−57.06	−92.39	−36.19	−81.54	−22.70	−76.87	−36.49	−84.40
	Carotenoids	−26.98	−66.72	−0.56	−53.55	−28.09	−75.34	−3.19	−54.27	−23.54	−30.09	−30.49	−41.58	−19.71	−44.45
Secuieni Jubileu HP	L*	9.33	17.56	7.88	20.01	13.08	26.66	8.47	21.68	4.93	14.89	4.75	16.66	6.93	19.83
	a*	−79.17	−171.88	−43.75	−119.27	−105.73	−208.85	−50.52	−183.85	−32.29	−132.29	−31.77	−106.25	−38.54	−142.19
	b*	91.28	126.17	67.60	136.14	138.01	212.15	56.70	238.63	52.96	179.75	42.68	125.55	34.89	167.91
	Chlorophylls	−69.49	−78.44	−67.15	−80.32	−67.74	−93.38	−44.79	−89.79	−54.33	−77.21	−29.02	−66.02	−32.04	−80.40
	Carotenoids	−52.34	−69.23	−42.38	−78.43	−56.56	−86.12	−26.24	−50.83	−25.94	−64.71	−19.61	−47.36	−16.89	−62.75

. The results of the calculation of the total color change are presented in

Table 4. Total color change (ΔE) in hemp oils [%].

Oil Variety	ΔE
	BE1 (2.5%)	BE1 (5.0%)	BE2 (2.5%)	BE2 (5.0%)	BE3 (2.5%)	BE3 (5.0%)	BE4 (2.5%)	BE4 (5.0%)	BE5 (2.5%)	BE5 (5.0%)	BE6 (2.5%)	BE6 (5.0%)	BE7 (2.5%)	BE7 (5.0%)
Finola CP	136.88	205.64	109.92	178.78	217.17	213.33	167.34	242.79	96.88	234.72	60.22	210.74	95.02	249.59
Finola HP	158.85	257.83	117.36	227.21	182.15	301.52	163.02	308.18	77.21	211.56	71.93	218.66	79.79	280.72
Earlina 8FC CP	200.84	257.45	163.34	175.99	231.35	270.07	203.88	283.76	113.50	262.77	109.58	266.86	133.14	305.90
Earlina 8FC HP	241.30	270.29	264.60	234.46	309.93	294.17	229.34	303.31	173.52	311.02	181.52	334.84	177.44	353.02
Secuieni Jubileu CP	136.52	268.36	70.92	178.21	199.65	338.71	132.84	333.92	84.68	266.79	64.51	223.81	70.35	298.42
Secuieni Jubileu HP	121.19	213.93	80.91	182.10	174.34	298.90	76.41	302.02	62.22	223.68	53.42	165.31	52.45	220.92

.

Low-temperature bleaching was used in the conducted research because, as shown by Secilimis et al. [36], increasing the process temperature worsens the quality parameters of bleached oils. Based on the results obtained, it was found that the BEs used have different adsorption of natural dyes which, therefore, influences the color of the oils. Moreover, doubling the amount of added adsorbent causes disproportionate changes in the tested parameters. Therefore, when selecting bleaching earth, the type of oil and its primary and target parameters should be considered.

BE3 (5.0%) was the most effective in removing chlorophylls from Finola CP oil. It reduced their amount by over 99% but also removed almost 80% of carotenoids. Chlorophylls were least effectively removed by BE6 (2.5%), which reduced these pigments by 41%. BE7 (2.5%) was the least effective in reducing carotenoids from this oil (a decrease of only 0.62%). Moreover, the bleaching earth removed 65% of chlorophylls. As indicated by Dordevic et al. [37], to ensure the high quality of the oil, it is crucial to significantly reduce chlorophyll pigments with the lowest possible removal of carotenoids. Therefore, based on the conducted research, using chemically modified BE based on kerolite-hydrated magnesium silicate will be most advantageous.

BE3 (5.0%) was the best at removing chlorophylls from Finola HP oil (over 94%), but at the same time, it showed a significant reduction in carotenoids (79%), which is not a desirable phenomenon. For the HP variant, the lowest reduction in carotenoids occurred for BE4 (2.5%), with a simultaneous reduction in the share of chlorophylls by over 58%. However, for this oil variant, it will be more advantageous to use BE5 (2.5%), which adsorbed over 72% of chlorophylls and only 7% of carotenoids.

For Earlina 8FC CP oil, two adsorbents stand out: BE2 (2.5%), for removing 67% of chlorophylls and less than 6% of carotenoids, and BE7 (2.5%): for removing >58% chlorophylls and <4% carotenoids. This oil variant is susceptible to the action of adsorbents, which was confirmed by an almost two-fold increase in the adsorption efficiency of chlorophylls and even a sixteen-fold increase in carotenoids when doubling the dose of BE.

The BE5 adsorbent (2.5%) again showed the most favorable changes for the oil quality in terms of the content of natural dyes for Earlina 8FC HP oil. The use of BE2 (5.0%), BE3 (5.0%), BE4 (5.0%), and BE7 (5.0%) in the case of this oil will be unjustified because these adsorbents are characterized by the highest adsorption efficiency of natural dyes, so the oil obtained after bleaching may lose the desired features of niche oil.

The Secuieni Jubileu CP oil variant reacted the weakest with the adsorbents used. On average, only 64% of chlorophylls and 35% of carotenoids were adsorbed. Especially for BE variants (2.5%), 44% of chlorophylls and 19% of carotenoids were adsorbed. In the case of this oil, BE4 (2.5%) showed the most favorable properties. The adsorbent BE3 (5.0%) showed an above-average ability to adsorb natural dyes from this oil. On this basis, it can be concluded that using unmodified attapulgite clay for the Secuieni Jubileu variety is unjustified.

For Secuieni Jubileu HP oil, the most favorable ratio of removed chlorophylls to carotenoids was obtained with BE5 (2.5%). Adsorbents BE6 (2.5%) and BE7 (2.5%) had a negligible effect on the content of natural dyes compared to the others (up to 32% reduction in chlorophylls and 20% reduction in carotenoids). It follows that the abovementioned adsorbents in these doses are not suitable for use when bleaching this oil. On the other hand, BE3 (5.0%) showed over 93% adsorption of chlorophylls and over 86% adsorption of carotenoids, which is also unfavorable to the quality of oils.

The descriptive analysis showed that the most advantageous choice for CP oils would be using chemically modified BE based on kerolite-hydrated magnesium silicate in the amount of 2.5% of the adsorbent mass to the oil volume. In turn, unmodified magnesian bentonite should be used for bleaching HP oils in the same dose. To check and systematize the impact of the BEs interaction force on the tested oil variants and their adsorption efficiency, principal component analysis (PCA) was performed separately for the CIE-Lab color measurement method and the content of natural dyes.

The statistical analysis began with verifying color changes using the CIE-Lab method. Figure 1a,b shows the results of the analysis performed.

As shown by Suri et al. [38], the color of oils in the CIE-Lab method depends on the pre-treatment of the raw material. Thermal treatment of the raw material intensifies the oils’ color, making them more challenging to discolor. PCA analysis of the results of color measurement using the CIE-Lab method shows that all BEs have different strengths of effective influence on the color change of the tested oils (Figure 1a). Moreover, BE5, BE6, and BE7 bleaching earths can change the color of oils, especially to lighten them. Referring to the work of Rossi et al. [39], chemical modification significantly affects the ability to discolor oils, distinguishing BE6 and BE7 from other BEs used in the research. Based on this, it can be concluded that their use to change color will be most justified, but in strictly controlled doses. Figure 1b shows that the mass of added bleaching earth is crucial in determining the effectiveness of color change in the CIE-Lab method, which is further confirmed by the work of Marrakchi et al. [40] Doses of 2.5% by mass of BE5, BE6, and BE7 showed little ability to change the color of the tested oils (gray field in Figure 1b), but doses of 5.0% showed the greatest effectiveness in this matter (blue field in Figure 1b). Kong et al. [41] showed that slight changes in the oil color determined by the CIE-Lab method occurred during repeated oil filtration. Referring to the conducted research, it can be concluded that some BEs in small doses (i.e., BE7; 2.5%, Finola CP) do not show the proper adsorption capacity to carotenoid dyes. To change the color of the tested oils, it is best to use the BEs mentioned above in an amount within the range of 2.5% to 5.0%, which will make the oils appropriately clear, and their color will be characterized by a higher b* index, which, according to Spano et al. [42], is a value desired by consumers for whom the yellowness of the oil is an indicator of product quality.

Another analyzed factor was the impact of BEs on the content of natural dyes in the tested oils. Figure 2a,b shows the results of the analysis performed.

According to the work of Mansouri et al. [43] and Tura et al. [44] the initial content of natural dyes depends mainly on the hemp variety and pressing temperature. Moreover, increasing the processing temperature improves the solubility of chlorophylls and carotenoids in oils, which makes their removal more difficult. BE7 had the most effective impact on the tested samples; the impacts of BE1 and BE2 were slightly lower, and BE5 and BE6 also had a considerable impact (Figure 2a). The remaining BEs did not significantly impact the content of natural dyes in the tested oils. In the case of BE3, this is due to there being too large of a disproportion between the parameters of the oils obtained in the CP and HP variants, especially considering the ratio of removed chlorophylls to carotenoids. Referring to the obtained results of the work of Lo Turco et al. [45], where it was found that the reduction in natural dyes occurs at the stage of oil filtration, it can be concluded that BE4 (2.5%) has poor adsorption properties regarding carotenoid dyes. BE1, BE6, and BE7 at doses of 5.0% were the most effective at removing natural dyes (blue field in Figure 2b). BE3, BE6, and BE7 at amounts of 2.5% (a gray area in Figure 2b), in turn, showed the worst ratio of removed chlorophylls to carotenoids. The work by Chew and Ali [46] confirms the conclusions drawn in this study; the authors indicated that the weight of the BE used was a critical factor in determining the bleaching efficiency, temperature, and process duration. The inefficiency of the adsorption of natural dyes by some of the Bes used may be because these Bes gain adsorption capacity at high temperatures, as indicated by the work of Chew et al. [47]. The occurrence of BE6 and BE7 in two groups is because some adsorbents show a sharp increase in adsorption efficiency after exceeding the limit dose, as indicated by Liang et al. [48]. For these BEs, the dose limit should be between 3.0% and 4.0%. There is no universal model for selecting the dose of bleaching earth in the literature, and the work of Ramli et al. [49] indicates that dose selection should be done experimentally.

4. Conclusions

This research tested the adsorption properties of seven bleaching earths, differing in chemical composition, during a low-temperature bleaching process of hemp oils. The selected BEs were applied in two variants of 2.5% and 5% m/m, and the process was carried out on oils pressed through CP and HP from three varieties of hempseed (Finola, Earlina 8FC, and Secuieni Jubileu). Color was measured calorimetrically using the CIE Lab method, and natural dyes were determined using a spectrophotometer.

After verifying the hypothesis based on the results obtained and relating them to the literature data, the following conclusions were made:

• Two doses of selected adsorbents were used during bleaching, and it was found that, in most cases, the use of more BE is associated with an increased reduction in dyes. This is not a favorable phenomenon for carotenoids, which are desirable compounds that have a beneficial effect on human health.
• Taking into account hemp varieties and pressing temperature, it was noted that for Finola CP and Earlina 8FC CP oils, the adsorbent with the most favorable adsorption ratio of chlorophylls to carotenoids was modified BEs 2.5%, especially kerolite-hydrated magnesium silicate. They remove sufficient chlorophylls while causing slight changes in carotenoid content. When bleaching HP oils from these varieties, using unmodified BEs 2.5% based on magnesian bentonite is recommended.
• Oil obtained from the Secuieni Jubileu CP and HP hemp variety should be bleached with unmodified magnesian bentonite at 2.5%. Unmodified attapulgite clay is not recommended for this variety, as it strongly adsorbs carotenoids from the oil.
• Tests conducted showed that an increase in the L* parameter (i.e., brightness) is caused by BEs based on modified magnesian bentonite and kerolite-hydrated magnesium silicate, although they do not show major changes in oil color (i.e., a* and b*), especially at doses below 5%.

In conclusion, selecting the right bleaching earth and its quantity is key in determining the most suitable bleaching process. It is important to consider the chemical composition of the adsorbent and whether it is modified so that bleaching can be carried out in the most favorable way.