Nigella sativa oil: A promising prospective antifungal agent in the manufacture of low-salt soft cheese.

The current work studied the in-vivo antifungal activity of Nigella sativa oil (NSO) in ultrafiltered low-salt soft cheese as a proposed replacement for the synthetic preservatives which become unacceptable by consumers. Four different concentrations of NSO were examined during the manufacture of the cheese (0.3, 0.5, 1, and 3 % w/w). The effect of NSO supplementation was examined in 3 parallel lines; a ninepoint hedonic scale was used in the sensorial evaluation of soft cheese free of the fungal inoculum, the physicochemical properties of soft cheese were determined during storage as well as anti-fungal effects of different concentrations of NSO on inoculated cheese with different species of fungi: Candida albicans (104 cfu/ml) and Aspergillus parasiticus (102 cfu/ml) before coagulation. The Nigella sativa oil expressed an antifungal activity by using different levels of NSO which significantly reduced and inhibited the growth of the fungal counts (1.4 log cfu/g for Candida albicans and 2.30 log cfu/g for Aspergillus parasiticus) started from 0.5% concentration of NSO on the 14th day of the storage. In addition, it exhibited different physicochemical properties of soft cheese depending on the level of used NSO. However, the Sensory evaluation of cheese samples revealed the acceptance of soft cheese samples with 0.3% and 0.5% of NSO. ©Copyright: the Author(s).

Introduction

Low-salt soft cheese is considered one of the most highly marketed cheeses worldwide and frequently used by consumers due to its nutritional value and low salt content regarding health issues (Puvača et al., 2020). However, the higher moisture content and low salt making this type of cheese highly susceptible to rapid deterioration by spoilage microorganisms that shorten its shelf life as well as contamination with different pathogenic organisms to be harmful for human consumption (Oliveira et al., 2016). In manufacturing of soft cheese, milk is pasteurized to certain temperature with certain holding time to kill all pathogenic bacteria and then adjusted to a proper temperature to facilitate the manufacturing transactions. Its higher moisture content around 62-70%, pH (above 5) and water activity (above 0.940) are the key factor related to its microbiological safety and entire quality (Trmčić et al., 2017). Several mold species could cause numerous deteriorates in soft cheese leading to undesirable flavor, secretion of mycotoxins and affecting the safety of this type of cheese that may belong to Aspergillus, Cladosporium, Penicillium, Mucor, Fusarium, Monilia, and Alternaria beside, different yeast species include Candida spp., Kluyveromyces marxianus, Yarrowia lipolytica, Pichia spp. Geothricum candidum and Debaryomyces hansenii (Khorshidian et al., 2018). On the other hand, consumer awareness has increased towards eating safe food free from synthetic additives and preservatives, the matter made a limitation for its use in cheese manufacture (Khosravi et al., 2011). Natural additives have become the best choice for preserving and enhancing the quality of cheese products instead of chemical and synthetic ones. Essential oils might be a promising agent for protection against such infections besides their biological and therapeutic effects. Many essential volatile oils are being used as antiseptic and antibacterial agents, reduce inflammation, and might be active against fungal infections (Kostadinović et al., 2016). The hydrophobic nature of essential oils helping the easy penetration of bacterial membrane to interfere with the transportation mechanisms of macromolecules in bacterial cells that causing cell inactivation (Goni et al., 2009). However, essential oils exhibit inhibitory activity against Gram-positive bacteria mainly more than Gram-negative due to the lipopolysaccharides that present in the membrane of Gram-negative bacteria (Tehrani & Sadeghi, 2015). Essential oils could also be used as flavorings in foods besides their role in increasing the shelf life of food products due to many active constituents such as volatile components, monoterpenes, sesquiterpene hydrocarbons, aldehydes, alcohols, esters and other nonvolatile constitutes include hydrocarbons, fatty acids, sterols, carotenoids, waxes, cumarines and flavonoids (Khorshidian et al., 2018).

Nigella sativa oil is an oil of black seeds also known as black seed oil. It is being renowned for its antibacterial, antifungal, antioxidant, antidiabetic, gastroprotective, anti-inflammatory, anticancer, antihypertensive, therapeutic properties, and immune-enhancing effects of numerous pharmaceutical and active substances (Hassanien et al., 2014; Cakir et al., 2016; Çakır & Çakmakçı 2018; Georgescu et al., 2019; Puvača et al., 2020). Most of the active substances present in volatile essential oils are in higher percentages in black seeds. These substances are phenolic compounds that have antimicrobial and antifungal properties. In addition to, its aromatic nature making it usable as flavoring foodstuffs (Puvača, 2018). The main components of Nigella sativa are thymoquinone, p-cymene, carvacrol, t-anethole, 4-terpineol, and longifoline (Hosseinzadeh et al., 2007). The inhibitory activity of Nigella sativa is mostly due to the existence of Thymoqinone TQ (2-isopropyl-5-methylbenzoquinone), p-cymene and carvacrol which interfere with the protein synthesis in bacterial cell (Kahsai, 2002; Chebli & Hassani, 2014). It was also proven that Nigella sativa oil rich in essential fatty acids, linoleic and oleic acids, sterols, tocols, minerals, vitamins, and volatile compounds (Hassanien et al., 2014).

The antifungal activity of Nigella sativa oil has been studied extensively against a wide range of fungal strains in vitro. However, the available data regarding its efficacy in cheese is limited. Therefore, the aim of this work is to investigate the effect of Nigella sativa oil in low-salt soft cheese in conducting protection against spoilage fungi, as well as to detect their influence on consumer acceptability of cheese.

Materials and methods

Nigella sativa oil (NSO) was obtained from Alrehab Herbs Company, Fayoum, Egypt. Fungi strains of Candida albicans ATCC 10231 and Aspergillus parasiticus ATCC 28285 were kindly obtained from the Department of Microbiology, Faculty of Veterinary Medicine-Cairo University, Egypt. Ultrafiltered milk was supplied by the Dairy pilot plant in the Faculty of Agriculture, Fayoum University, Fayoum, Egypt. Powder rennet (CHY-MAX, 2280 IMCU/ml) from Chr-Hansen’s Laboratories (Denmark) and diluted with sterilized distilled water to a standard rennet solution before use. Commercial pure fine grade salt was bought from Emisal Company, Fayoum, Egypt. While Calcium chloride (Food quality grade) was from EL-Nasr Company, Cairo, Egypt.

Analysis of Nigella sativa oil

The Fatty acid and sterol composition of Nigella sativa oil was analyzed according to the method used by (Ramadan et al., 2010), while the total phenolic compounds were determined according to the method used by (Hassanien et al., 2014).

Preparation of inoculum

The cultures (Candida albicans & Aspergillus parasiticus) were activated with two successive passes in 9 ml of SDB (Sabouraud Dextrose chloramphenicol broth, Oxoid) and incubate aerobically at 25°C for 3-5 days to have a final concentration of approximately 104 cfu/ml for C. albicans and approximately 102 cfu/ml for Aspergillus parasiticus (determination of the count was done by plating on Sabouraud Dextrose chloramphenicol Agar, Oxoid).

Manufacture of ultrafiltered lowsalt soft cheese

Ultrafiltered milk was divided into two parts for conducting two independent experimental trials that were performed at separate times, the first was done to determine the sensory acceptability while the second one was carried out for determination of the viability of the fungal strains C. albicans & Aspergillus parasiticus with different Nigella sativa oil concentrations in low-salt white soft cheese.

In the first experiment, Ultrafiltered milk was divided into five equal groups, from which four groups supplemented with 0.3, 0.5, 1.00, and 3.00 % (w/w) of Nigella sativa oil while the 5th one left as the control group. For soft cheese manufacture, all milk treatments were pasteurized at 80°C for 30 min, cooled and adjusted to 40°C, then calcium chloride and sodium chloride were added at levels of 0.02%, and 2% (w/w), respectively, then NSO concentrations were added and renneted. Cheese samples were stored in plastic cups at 4°C for 21 days for physicochemical and sensory analysis at 0-, 7-, 14-, and 21-days intervals. The physicochemical properties of the prepared cheeses were analyzed by titratable acidity, fat, moisture and total protein contents which were determined as described in (AOAC, 2005). Curd tension was measured according to the method used by (Shahein et al., 2014). While the sensory evaluation was determined using a nine-point hedonic scale as the method used by (Amini et al., 2019). Cheese samples were prepared and stored at 4◦C. The samples were evaluated by 15 panelists for appearance, taste, texture, smell, and overall acceptability.

In the second experiment, the ultrafiltered milk was divided into six equal groups instead of five in the first one, the first four groups were supplemented by Nigella sativa oil (0.3, 0.5, 1.00 and 3.00 % w/w) beside control positive and control negative groups. The manufacturing process was done typically as the first experiment then each group subdivided into 2 subgroups to inoculate one of them with C. albicans and the other one was inoculated with Aspergillus parasiticus before the Rennet was added. Cheese samples were stored in plastic cups at 4°C for 21 days for microbiological analysis at 0, 3, 7, 14 and 21st days intervals. The microbiological analysis of samples was done by stomaching ten grams of the inoculated prepared soft cheese samples with 90 ml of 0.1% peptone water serially dilution for plating on duplicate plates of Sabouraud Dextrose chloramphenicol Agar (Oxoid). The plates were incubated aerobically at 25°C for 3-5 days for determining the count of the inoculated fungal strains.

Statistical analysis

Data were statistically analyzed using ANOVA variance analysis through the general linear model (GLM) procedure of the statistical analysis system software (SAS version 9.1, SAS Institute, Inc., 2003). The model included treatment, storage time, and their interaction as fixed effects. Differences between effects were assessed by the Duncan test (P≤0.05).

Results and discussion

Compositional analysis of NSO

The fatty acid profile of the used NSO was shown in Table 1, the level of unsaturated fatty acids was 84.33%, while the level of saturated fatty acids was 15.67 %. The profile analysis of NSO by GC-MS indicated that the essential omega-6 (Linoleic acid C18:2) was detected in a higher percentage 55.69 %, while oleic acid, linolenic acid, and palmitoleic acid were detected in levels, 27.92, 0.53, and 0.19 % respectively. On the other hand, the saturated fatty acids were dominated by palmitic acid (C16:0) 12.19 %. NSO contained also the phytosterol, ß-sitosterol 48.7 %, stigmasterol 16.9 %, campesterol 12.6 %, Δ5-avenasterol 12.1 %, Δ7-avenasterol 2 %, cholesterol 0.8 % and Δ7-stigmasterol 0.7 %. In addition, NSO had a higher level of polyphenols which was 3.8 g/kg.

Table 1.

Fatty acids, phytosterols and total phenolics of NSO.

Component	Percentage
Fatty acids
Myristic acid (C14:0)	0.17
Palmitic acid (C16:0)	12.19
Palmitoleic acid (C16:1)	0.19
Stearic acid (C18:0)	3.19
Oleic acid (C18:1)	27.92
Linoleic acid (C18:2)	55.69
Linolenic acid (C18:3)	0.53
Arachidic acid (C20:0)	0.12
Σ Saturated fatty acids	15.67
Σ Unsaturated fatty acids	84.33
Phytosterols
Cholesterol	0.8
Campesterol	12.6
Stigmasterol	16.9
ß-Sitosterol	48.7
Δ5-Avenasterol	12.1
Δ7-Stigmasterol	0.7
Δ7-Avenasterol	2.00
Total phenolics	3.8 g/kg

The higher level of unsaturated fatty acids, high content of phenolics and phytosterols in NSO make it a functional ingredient with various health benefits, biological activities and enhancing the keeping quality of food applications (Hassanien et al., 2014). It was reported also that NSO has higher antioxidant activities due to its valuable content of polyphenols with redox potential (Bettaieb et al., 2010).

Physicochemical properties of lowsalt soft cheese supplemented with NSO

The effect of adding Nigella sativa oil on the properties of low-salt soft cheese is presented in Table 2. The highest moisture content (69.70 %) was recorded in control cheese while the lowest moisture content (69.34 %) in cheese samples with 3 % NSO at zero time of storage. A slight decrease in moisture content between cheeses treatments was detected but it was significant at p≤0.05 which could be attributed to the effect of NSO on the dry matter content of cheese and replacement some of the cheese’s moisture. A gradual decrease in moisture content was found during storage periods of all cheese treatments. The detectable loss of moisture in cheese samples was due to the evaporation and curd shrinkage because of the increase of acidity and the loss of whey from the cheese matrix. These results were in line with Cakir et al., (2016) who found that with the addition of NSO to Erzincan Tulum cheese the compositional analysis affected, and the moisture of cheese content decreased by the end of the storage period.

Table 2.

Physicochemical properties of UF low-salt soft cheese during storage.

Parameters	Storage periods	Treatments
	(days)	Control	T1	T2	T3	T4
			(0.3% NSO)	(0.5% NSO)	(1.0% NSO)	(3.0% NSO)
Acidity%	0	0.26m±0.01	0.30kl±0.01	0.31jk±0.02	0.40fg±0.01	0.51d±0.01
	7	0.29l±0.02	0.31jk±0.03	0.34i±0.03	0.46e±0.02	0.59c±0.02
	14	0.31jk±0.01	0.33ij±0.02	0.36h±0.05	0.49d±0.04	0.63b±0.04
	21	0.37h±0.05	0.39g±0.03	0.41f±0.02	0.62b±0.02	0.76a±0.01
Moisture%	0	69.70a±0.05	69.64b±0.05	69.51c±0.08	69.48c±0.03	69.34d±0.03
	7	68.47e±0.06	68.41f±0.03	68.31g±0.03	67.98h±0.02	67.74i±0.02
	14	67.17j±0.03	67.05k±0.04	66.71l±0.02	66.28m±0.04	65.89o±0.05
	21	65.97n±0.02	65.75p±0.03	65.38q±0.02	64.62r±0.02	63.99s±0.02
Fat %	0	14.70m±0.20	14.97l±0.06	15.13k±0.06	15.67h±0.06	17.63d±0.06
	7	14.88l±0.10	15.17jk±0.06	15.33i±0.06	16.07f±0.07	17.94c±0.06
	14	15.09k±0.05	15.36i±0.04	15.56h±0.05	16.47e±0.03	18.33b±0.03
	21	15.26ij±0.06	15.57h±0.07	15.78g±0.06	16.57e±0.05	18.62a±0.04
Protein%	0	10.50n±0.02	10.53n±0.02	10.54mn±0.02	10.58lm±0.06	10.63jk±0.04
	7	10.61kl±0.03	10.64jk±0.03	10.67j±0.03	10.75i±0.02	10.82g±0.04
	14	10.77hi±0.01	10.81gh±0.02	10.83g±0.01	10.94ef±0.02	11.02c±0.03
	21	10.92f±0.02	10.97de±0.01	11.01cd±0.02	11.12b±0.03	11.23a±0.02
Curd tension mg/100 mg	0	33.23i±0.15	33.12j±0.11	32.93k±0.12	32.44m±0.11	31.97o±0.16
	7	34.31c±0.09	33.43g±0.08	33.14j±0.07	33.12j±0.06	32.20n±0.05
	14	34.64b±0.07	33.60e±0.08	33.34h±0.07	33.25i±0.06	32.48m±0.05
	21	34.87a±0.10	33.83d±0.11	33.52f±0.12	33.38gh±0.11	32.85l±0.12

*Data expressed as (mean±SD) of three replicates. Means in the same column/row showing the same letters are not significantly different (p≤0.05).

Titratable acidity of NSO treated cheese was higher than control samples and the acidity increased in all cheese samples during storage. Significant differences (P≤0.05) in acidity values were found between control and treated cheese. The acidity values were 0.37, 0.39, 0.41, 0.62 and 0.76 % for control, T1, T2, T3 and T4 respectively at 21 days of storage. The development of acidity might be due to the fermentation of residual lactose and the acidity effect of Nigella sativa oil due to its fatty acid composition which interfere with acidity results. The results of fat% illustrated significant differences (P≤0.05) between all cheese samples. Fat increased gradually in all cheese samples during storage as a result of moisture loss in relation to the dry matter of cheese curd. At the end of storage period, higher fat values 15.57, 15.78, 16.47, and 18.62 % were recorded with T1, T2, T3, and T4, respectively than control cheese. A positive correlation was found between oil concentration and fat percentages of examined cheese. There were no significant differences in protein content among cheese samples at the first week of storage, with a continuous increase in all cheese samples during storage due to the loss of water/total solids. These results were in accordance with Hassanien et al., (2014) ; Abd Elmontaleb et al., (2020). The curd tension and firmness of soft cheese were significantly (P≤0.05) decreased with the addition of NSO to cheese which might be due to oil interference in the cheese matrix and affecting casein diffusion.

Sensory evaluation

The sensory evaluation of the treated cheese with NSO was illustrated in Table 3. There were no significant differences (P≤0.05) in appearance between all cheese treatments. Nevertheless, during storage, little differences were noticed in the appearance of cheese samples. While the texture results were significantly higher (P≤0.05) in treated cheese samples than control by addition of NSO with a gradual improvement in texture of all cheese samples was observed until 14 days of storage. These results could be attributed to the softening effect of oil on the cheese texture and the effect of storage on the development of cheese texture by proteolysis, lipolysis, and hydrolysis of cheese matrix during storage. However, the texture values of T3 and T4 decreased especially at the end of the storage period which might be due to the over-concentration of NSO and more lipolysis occurred that changing of the cheese texture. Generally, there were clear differences between control and treated cheese samples in smell values. Higher smell values were reported in T1 and T2 supplemented with low concentrations of NSO than T3 and T4 of high oil concentrations. The smell of cheese samples with high-level content of NSO (1%, 3%) wasn’t accepted by panelists throughout the storage period. The overall acceptability of cheese went to T1 (0.3 % NSO) and T2 (0.5 % NSO) with values 8.20 and 8.67, respectively, at the end of storage time. Nearly similar results were detected by Halamova et al. (2010) and Hamid (2014) who observed improvement in the flavor of the examined cheese with addition of 0.1% and 0.3% of NSO respectively, while higher concentration levels could cause undesirable changes in color and flavor. likewise, Georgescu et al. (2018) found that the best ranges of Nigella sativa oil to improve the sensory characteristics of soft cheese were 0.05 to 0.2% w/w. Higher results were reported by Abd Elmontaleb et al. (2020) for the overall acceptability of Edam cheese with an additional higher level of NSO (0.6%).

Table 3.

Sensory properties of UF low-salt soft cheese during storage.

Parameters	Storage periods	Treatments
	(days)	Control	T1	T2	T3	T4
			(0.3% NSO)	(0.5% NSO)	(1.0% NSO)	(3.0% NSO)
Appearance	0	7.80abc±0.41	7.87ab±0.35	7.87ab±0.35	7.60abcd±0.51	7.33d±0.49
	7	7.67abcd±0.49	7.80abc±0.41	7.60abcd±0.51	7.40cd±0.51	7.33d±0.49
	14	7.47bcd±0.52	7.73abcd±0.46	7.60abcd±0.51	7.47bcd±0.52	7.40cd±0.51
	21	7.40cd±0.51	7.80abc±0.41	7.87a±0.35	7.53abcd±0.52	7.40cd±0.51
Texture	0	6.47h±0.52	7.33cde±0.49	7.53bcde±0.52	7.40bcde±0.51	7.27def±0.46
	7	6.67gh±0.49	7.73bc±0.46	7.80b±0.41	7.40bcde±0.51	7.20ef±0.68
	14	7.47bcde±0.52	8.53a±0.52	8.53a±0.52	7.67bcd±0.49	7.13ef±0.52
	21	7.53bcde±0.52	8.40a±0.63	7.47bcde±0.64	7.20ef±0.56	6.87fg±0.74
Smell	0	5.73h±0.46	6.67cde±0.49	6.60cde±0.51	6.47def±0.52	5.87gh±0.64
	7	5.93gh±0.46	7.07bc±0.59	7.33b±0.72	6.27defg±0.46	5.80gh±0.41
	14	6.27defg±0.46	8.07a±0.88	8.33a±0.62	6.73cd±0.59	6.00fgh±0.53
	21	6.20efgh±0.56	8.21a±0.80	8.13a±0.74	7.07bc±0.59	6.27defg±0.70
Overall acceptability	0	6.40cd±0.51	7.07b±0.46	7.33b±0.49	6.53c±0.52	6.13cde±0.64
	7	6.33cd±0.49	8.20a±0.77	8.47a±0.64	6.20cd±0.68	5.93de±0.70
	14	5.67ef±0.46	8.40a±0.74	8.47a±0.64	7.27b±0.70	5.93de±0.59
	21	5.27fg±0.46	8.20a±0.68	8.67a±0.62	6.13cde±0.52	5.07g±0.80

Data expressed as (mean±SD) of three replicates. Means in the same column/row showing the same letters are not significantly different (p≤0.05).

Microbiological analysis

The antifungal effect of Nigella sativa oil on the survival of C. Albicans in different soft cheese treatments was represented in Table 4 that revealed, there were no significant reductions in the counts of C. Albicans in the first three days of storage within all cheese NSO treatments. However, the significant reduction in the count started at 7th day of storage in (T2) 0.5% of NSO (3.95±0.43 log10 cfu/g), (T3) 1% of NSO (3.89±0.29 log10 cfu/g) and (T4) 3% of NSO (2.90±0.27 log10 cfu/g) treatments compared to control (5.11±0.44 log10 cfu/g). These reductions continued until the growth of C. Albicans couldn’t be detected by end of the examination period 21st day. There was a significant decrease in the count of C. Albicans in all cheese treatment groups at the 14th and 21st days of the storage period.

Table 4.

Survival of candida albicans in different UF low-salt soft cheese treatments: (log10 Mean ±SE).

Storage days	0 time	3 days	7 days	14 days	21 days
T1 (0.3% of NSO)	4.15±0.95a	4.07±0.47 a	4.28±0.41ad	3.19±0.33a	2.48±0.11 a
T2 (0.5% of NSO)	4.7±0.25 a	4.30±0.30 a	3.95±0.43 abc	3.28±0.30 a	2.90±0.28 a
T3 (1% of NSO)	4.09±0.23 a	4.17±0.62 a	3.89±0.29 abc	3.04±0.24 a	No growth b
T4 (3% of NSO)	4.04±0.12 a	4.31±0.29 a	2.90±0.27 bc	3.18±0.10 a	No growth b
Control C +	4.00±0.35 a	4.79±0.28 a	5.11±0.44 ad	3.40±0.16 b	4.78±0.40 c

Data expressed as (mean ± SD) of three replicates. Means in the same column showing the same letters are not significantly different (p≤0.05). Control C +: positive control of candida albicans ; Control A +: positive control of Aspergillus parasiticus.

On the other hand, although the count of Aspergillus parasiticus showed the same pattern as C. albicans and didn’t show a significant reduction in the counts in the first three days of examination in all cheese NSO treatments, the count reduced significantly at p≤0.05 in the 7th day of examination in cheese treatments with 1% and 3% NSO compared to control and cheese treatments with 0.3% and 0.5% NSO. While the Aspergillus parasiticus couldn’t grow on the 14th and 21st day of examination in all cheese treatments of NSO (Table 5).

Table 5.

Survival of Aspergillus parasiticus in different UF low-salt soft cheese treatments: (log10 Mean ±SE).

Storage days	0 time	3 days	7 days	14 days	21 days
T1 (0.3% of NSO)	2.48±0.21 a	2.60±0.25 a	2.30±0.24a	No growth a	No growth a
T2 (0.5% of NSO)	2.30±0.13 a	2.47±0.31 a	2.07±0.10a	No growth a	No growth a
T3 (1% of NSO)	2.30±0.23 a	2.00±0.35 a	1.09±0.04b	No growth a	No growth a
T4 (3% of NSO)	2.04±0.12 a	2.10±0.13 a	1.00±0.17b	No growth a	No growth a
Control A +	2.48±0.35 a	2.60±0.15 a	2.30±0.20ac	1.300±0.16 b	1.86±0.40 b

Data expressed as (mean ± SD) of three replicates. Means in the same column showing the same letters are not significantly different (p≤0.05). Control C +: positive control of candida albicans, Control A +: positive control of Aspergillus parasiticus

It was observed that, although there are many co-factors helping in decreasing the count of Aspergillus parasiticus as refrigeration temperature and storage in strictly closed cups under anaerobic conditions. The combination of these factors with Nigella sativa oil accelerates reduction as well as prevention the growth of Aspergillus parasiticus starting from the low concentration oil (0.3%) at day 14th of the storage period which indicates that the addition of oil improves the quality of white soft cheese and increase its shelf life. In general, the antifungal activities of back seed oil against different types of pathogenic fungi were attributed in several studies to the presence of β-sitosterol, stigmastero, oleic acid, and long-chain fatty acids (Gupta et al., 2012; Asdadi et al., 2014). Nigella sativa oil has a moderate efficacy against Candida species (Halamova et al., 2010;Taha et al., 2010; Shokri, 2016). other results reported by Khosravi et al., (2011) suggested using Nigella sativa oil as natural inhibitors in foods at low levels to protect food from fungal and toxin contaminations by Aspergillus parasiticus. However, Maraqa et al., (2007) and El-Nagerabi et al., (2012) reported that a higher concentration of NSO (3%) was the most effective level to cause complete inhibition of aflatoxin B1 produced by Aspergillus parasiticus.

Conclusions

Nigella sativa oil could be used in food as a promising natural preservative enhancing the shelf life of the cheeses. Supplementation of low salt soft cheese with NSO (0.5% w/w) is the most effective oil concentration in this study that kept considerable physicochemical and sensorial properties of cheese. In addition, it significantly reduced the counts of the examined fungal strains (C. albicans and Aspergillus parasiticus) by 1.4 and 2.30 logs cfu/g, respectively after 14 days of storage.