An Overview of Biotransformation for the Sustainability of Sweet Tasting Proteins as Natural Sugar Replacers

Abstract

According to WHO, sugar intake rates should be reduced due to the connection between sugar and diseases. However, reducing sugar in foods is a challenge both for food manufacturers and consumers. Therefore, sweet-tasting proteins may solve this problem with a sweet taste, health benefits, and without caloric contents. Thus far, known natural sweet-tasting proteins are brazzein, curculin, thaumatin, monellin, miraculin, and mabinlin. Nevertheless, natural sources of sweet proteins might be extinct in the future due to overconsumption. Thus, biotransformation studies of sweet proteins are promising as they produce high yield rates, quality, fewer by-products, and more sustainable solutions.

Full text

Citation: Yusuf, E.H. An Overview of
Biotransformation for the
Sustainability of Sweet-Tasting
Proteins as Natural Sugar Replacers.
Chem. Proc. 2022,8, 85.
https://doi.org/10.3390/
ecsoc-25-11640
Academic Editor: Julio A. Seijas
Published: 12 November 2021
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Proceeding Paper
An Overview of Biotransformation for the Sustainability of
Sweet-Tasting Proteins as Natural Sugar Replacers †
Emel Hasan Yusuf
Department of Fruit, Vegetable and Nutraceutical Plant Technology, The Wroclaw University of Environmental
and Life Sciences, 51-630 Wroclaw, Poland; emel.hasan.yusuf@upwr.edu.pl
† Presented at the 25th International Electronic Conference on Synthetic Organic Chemistry, 15–30 November
2021; Available online: https://ecsoc-25.sciforum.net/.
Abstract:
According to WHO, sugar intake rates should be reduced due to the connection between
sugar and diseases. However, reducing sugar in foods is a challenge both for food manufacturers
and consumers. Therefore, sweet-tasting proteins may solve this problem with a sweet taste, health
benefits, and without caloric contents. Thus far, known natural sweet-tasting proteins are brazzein,
curculin, thaumatin, monellin, miraculin, and mabinlin. Nevertheless, natural sources of sweet
proteins might be extinct in the future due to overconsumption. Thus, biotransformation studies of
sweet proteins are promising as they produce high yield rates, quality, fewer by-products, and more
sustainable solutions.
Keywords: sugar overconsumption; natural sugar substitutes; sugar replacement
1. Introduction
Sugar is a crucial compound for food processing with characteristics of texture, stability,
mouthfeel, flavour, colour, and preservation features [
1
]. Moreover, sugar is an energy
source for our body, but excessive sugar consumption is an issue that causes obesity [
2
].
According to the World Health Organization [
3
], less than 10% of total energy should
be intaken from free sugars for adults. However, nowadays, sugar overconsumption
is a challenging issue because of caused disorders in the body such as weakening of
immunity [4], diabetes, cardiovascular diseases, and cancer [5].
On the other hand, consuming sweet foods is a genetically evolutionary survival mech-
anism for human-beings because of psychological necessity [
6
]. Nevertheless, sweetness
causes addiction with tooth decay, weight gain, obesity, type 2 diabetes mellitus, high blood
cholesterol, depression, and cancer [
7
–
9
]. Thus, due to the side effects of sugar overcon-
sumption, it has been suggested to remove sugar from the GRAS (generally regarded as
safe) list of the FDA (U.S. Food and Drug Administration) by Lustig et al. [10].
In conclusion, this review aims to discuss natural, sweet-tasting proteins in combi-
nation with health-promoting activities and sustainability features. Thu far, the known
natural sweet proteins are brazzein, curculin, thaumatin, monellin, miraculin, and mabinlin.
Thus, the scope includes identifying sweet-tasting proteins as natural food ingredients.
2. Sweet-Tasting Proteins
2.1. Brazzein
Brazzein is a derivative of Pentadiplandra brazzeana Baillon, which is found in African
tropical forests naturally [
11
]. Brazzein is the smallest sweet-tasting protein with a
54 amino
acid structure [
12
]. The sweetness of brazzein is 2000 times higher than a 5% sucrose
solution [
13
], and the stability of brazzein maintains up to 80
◦
C [
14
], which is an important
feature for food manufacturing.
Because of original plant location and limited brazzein production, the alternative
method of bioconversion is the best way to manufacture brazzein close to natural. The
Chem. Proc. 2022,8, 85. https://doi.org/10.3390/ecsoc-25- 11640 https://www.mdpi.com/journal/chemproc

Chem. Proc. 2022,8, 85 2 of 5
first brazzein biotransformation study was made via Escherichia coli in 2000. However,
following brazzein biotransformation studies in E. coli, it exhibited a lower sweet taste than
the product of the original plant. Later, sweet brazzein manufacturing was achieved with
Pichia pastoris.Pichia cells released about 120 mg/L of brazzein in 6 days. Nevertheless,
Kluyveromyces lactis produced about 104 mg/L of brazzein into the cultured medium in a
short period, and recombinant brazzein’s sensory characteristics were similar to the original
plant product [
11
]. Recently, Bacillus licheniformis was applied for brazzein extraction, due to
its fast growth, high secretion, and low cost [
12
]. Thus, the rbrazzein genes were expressed
and 57 mg/L of brazzein was produced at 36 h. Ebrazzein and Bbrazzein demonstrated
400 and 266 times more sweetness characteristics than sucrose, respectively.
For plant biotransformation studies of brazzein, the most applied mediums are maize,
corn, rice, and lettuce [
15
,
16
]. Moreover, brazzein was achieved when produced in about
400
µ
g/g of corn seeds, and corn brazzein allows for industrial production, which can
solve issues related to the sustainability of the original brazzein in the future [
11
]. Thus, the
sweet taste of brazzein is felt slower than sucrose and may replace sugar in food processing
with novel food applications.
2.2. Curculin
Curculin is extracted from Molineria latifolia (Dryand. ex W.T.Aiton) Herb. ex Kurz,
which is native to Malaysia [
11
]. Dried fruits of Molineria are used by local people against
the bitter taste of black tea and sour foods [
17
]. Therefore, the compound is a promising
ingredient for future food production as a novel material.
Indeed, curculin demonstrates 550 times more sweetness than sucrose on a weight
basis [
18
]. Moreover, water solutions of the curculin exhibit a strong sweet taste at low
pH [17]. Thus, the feature might be applied for innovative food productions.
Thus far, gene expression studies of curculin have been made via E. coli, but homod-
imeric forms of the compound have not exhibited any sweet taste; heterodimeric forms of
curculin have demonstrated characteristics of sweet taste [
19
]. Furthermore, the natural
source of curculin is unsustainable, and biotransformation studies of curculin have exhib-
ited valuable results for flavour enhancement and sweet-taste features [
20
]. Therefore, the
characteristics of curculin are attractive and promising.
2.3. Mabinlin
Mabinlin is found in the seeds of Capparis masaikai Levl., which comes from the Yunnan
region in China [
11
]. Mabinlin possesses four isoforms, which are mabinlin I-1, mabinlin II,
mabinlin III, and mabinlin IV [
21
]. Mabinlin II is the only compound that is heat stable and
the sweetness is maintained following 48 h of incubation at 80
◦
C. Therefore, the sweetness
of mabinlin is 400 times higher than sucrose on a molar basis [22].
Mabinlin II is difficult to extract from the Capparis plant, but biotransformation studies
via E. coli and Lactococcus lactis provide availabilities to produce mabinlin in wide spectrums
for food applications [
23
]. Moreover, biotransformation studies of mabinlin in plants
researched in potato, where mabinlin II had an astringent sweet taste and the amount was
1 mg/mL [
19
]. Therefore, the sweetness characteristics of mabinlin provide possibilities to
apply to vegan foods to mask the bitterness of plant-based ingredients.
2.4. Miraculin
Miraculin is found in Richardella (Synsepalum) dulcifica (Schumach. & Thonn.) Baehni,
and demonstrates an unsweet feature but can transform a sour taste into a sweet feeling.
Miraculin consists of 191 amino acids and N-linked oligosaccharide [
24
], which is solely
extracted from the Richardella fruit after 6 weeks of pollination, following the fruit colour
change from green to dark red [
25
]. The miraculin provides abilities to use for the taste en-
hancement of acids [
26
]. Thus, miraculin solutions may enhance the flavour characteristics
of acids in food products for more than 1 h [27].

Chem. Proc. 2022,8, 85 3 of 5
The first biotransformation study of miraculin was made via E. coli [
28
] without sweet-
taste characteristics after recombinant miraculin was produced in transgenic lettuce, and
the amount of miraculin was between 33.7 and 43.5
µ
g/g of fresh weight with sweet-taste
feeling characteristics [
29
]. Following this, miraculin was produced in a transgenic tomato
and strawberry as well [
30
]. Thus, biotransformation of miraculin causes low cost, and it is
genetically stable [25].
2.5. Monellin
Monellin contains 44 amino acids in one chain and 50 amino acids in another chain as
polypeptides [19]. Monellin is a sweet-tasting protein of Dioscoreophyllum cumminsii Diels,
and the plant grows naturally in African forests [
11
]. The sweetness of monellin is 4000
times higher than sucrose on a weight basis [31].
Cultivation studies of Dioscoreophyllum have not been achieved except in natural
habitats to obtain stabile monellin [
32
]. For this reason, biotransformation studies have been
implemented, and a specific form of monellin provides flexibility for biotransformation. For
instance, the transformation of monellin via E. coli supports a sweet flavour during heating,
with pH stability better than the original compound [
19
]. Moreover, biotransformation of
monellin via S. cerevisiae yielded about 54 g of purified monellin [33].
Transgenic plant studies of monellin were made in transgenic tomato and lettuce [
34
].
Therefore, an ethylene-applied transgenic tomato provided about 23.9
µ
g/g of fresh weight
of monellin with high heat stability and elevated sweet taste [35].
To conclude, biotransformation studies of monellin will be carried on to find sustain-
able solutions for broad applications of the component in food manufacturing. As monellin
possesses a zero glycemic index, it can be applied to the diets of diabetic people [
36
]. Addi-
tionally, any adverse effects of monellin have not been reported for food applications thus
far [
37
]. Thus, the compound may find varied applications in food processing forthcoming.
2.6. Thaumatin
The arils of the African species Thaumatococcus daniellii Bennett include the sweet-
tasting thaumatin proteins; the amount of thaumatin in a ripe fruit is about 30–55 mg/g
of fresh weight [
38
]. The sweetness level of thaumatin is 3000 times more than sucrose
without caloric values [39].
The sweetness characteristics of thaumatin attract researchers to find alternative
production ways due to sustainability issues. Therefore, biotransformation studies of
thaumatin exhibit promising results for future implementations. Thus far, thaumatin
gene expressions were made in rice [
40
], strawberry, barley, tomatoes, potatoes [
41
], cu-
cumber, and pear to enhance the taste of fruit and vegetables [
19
]. Hence, plant gene
expression studies of thaumatin demonstrate advantages such as low toxicity and a rise in
economical incomes.
On the other hand, biotransformation of thaumatin by bacteria and fungi provides
much faster growth, control of the pathway, and a high yield of thaumatin [
42
]. For
instance, E. coli is the most used bacteria for protein expression due to well-understood
genomics. However, the production of thaumatin via E. coli has supplied low amounts of
total thaumatin [
43
]. Faus et al. [
44
] applied synthetic genes of E. coli to express thaumatin
proteins, and the study provided a similar molecular weight with original thaumatin.
Following those studies, in 2000 Daniell et al. [
45
] achieved producing about 40 mg of pure
thaumatin with similar sweetness characteristics of the original compound. Nevertheless,
the disadvantage of E. coli is that it is toxic with by-products [
41
]. For this reason, Lactococcus
lactis, which has been recommended for the gene expression of thaumatin, has been
approved as GRAS [46].
Moreover, thaumatin was produced also by yeast, where the yield was about
100 mg/L [
19
], and Pichia pastoris is a good example for commercial thaumatin production
without toxins [
41
]. Hence, thaumatin might be utilized for varied food products with
forthcoming sustainability features due to biotransformation studies.

Chem. Proc. 2022,8, 85 4 of 5
3. Conclusions
Natural sugar substitutes promote activities against obesity, type 2 diabetes, and
cardiovascular diseases. Forthcoming, we may see many more applications of natural
sugar replacers with wide utilities. However, huge interest in natural sugar replacers may
create extinctions of the sources of sugar substitutes. Therefore, biotransformation studies
may bring solutions for issues related to sources of natural sugar substitutes, and with
extra advantages, such as how biotransformation creates fewer environmental issues and
is more sustainable for production [47].
Supplementary Materials:
The following supporting information can be downloaded at: https:
//www.mdpi.com/article/10.3390/ecsoc-25-11640/s1.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The author declares no conflict of interest.
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