Journal homepage: www.fia.usv.ro/fiajournal
Journal of Faculty of Food Engineering,

Ştefan cel Mare University of Suceava, Romania

Volume XVII, Issue 1 - 2018, pag. 48 - 52

WALNUT SHELLS BLEACHING USING OXIDIZING AND REDUCING AGENTS

* Eugenia BOAGHI, Vladislav RESITCA, Pavel TATAROV, Jorj CIUMAC

Faculty of Food Technology, Technical University of Moldova,

168 Stefan cel Mare blvd., MD 2004 Chisinau Republic of Moldova,

*eugenia.boaghi@toap.utm.md

Received 17 th October 2017, 19th March 2018

Abstract: Walnuts with dark-colored shells, with blackish brown spots that are more or less
punctuated, have poor consumer acceptability. To improve the appearance and the commercial
quality of shell nuts, certain bleaching processes are applied by means of several oxidizing and

reducing agents. Walnuts Juglans Regia L, variety Calarasi and Cogalniceanu, harvest 2015,
Exploitation Iargara, Moldova were analyzed. The determination of chromatic characteristics
according to CIELAB and effects of the reaction parameters on the color of the whitened shell by the
method of the response surfaces of the color variables L*, a* and b* was made. Walnuts bleaching
was performed with oxidative bleaching agents (hydrogen peroxide, calcium hypochlorite, sodium
perborate and Okorn 12) and reductants (sodium dithionite and sulphurous anhydride) at different
concentrations, pH values and temperatures. Based on the analysis of the effects of the independent
whitening parameters (the pH of the environment, the concentration of bleaching agents and the

temperature of the bleaching medium) and their interactive effects on the chromatic profile and the
whitening process of the nut shell it was found that the oxidative whitening agents are more effective.
Taking into account the whitening activityof the agents and their toxic emissions, the whitening of
walnuts can be done with sodium perborate(Cperbora t=5%, τ=60 min, t=60

0
C, pH=10), Okoron 12

(Cokoron12 =5%, τ=90 min, t=60
0
C, pH=10) and with hydrogen peroxide (CH2O2=10%, CNaOH =2.2 %,

t=60
0
C, 90 min).

Keywords: walnts, bleaching, chromatic parameters, oxidizing and reducing agents

1. Introduction

Walnuts are grown across the world [1].

Dark shell colored and stained walnuts

have poor consumer acceptability [2].

Moldova ranks sixth among world

exporters of walnuts, after the United

States, Mexic and China, being the third

among the European exporters of shelled

nuts, after France and Germany, with an

amount that worths $84.8 million dolars.

[3]. In recent years, due to the adoption of

the National Program for the walnut

culture Development until 2020, to the

more active involvement of state and non-

governmental institutions are increasingly

emphasized the paths of further walnut

culture development.

During the harvesting of the walnuts, the

wood peels are colored both by intact and

crushed green, which is rich in tannins.

Besides tannins, the green bark is also very

rich in hydrogeldone glucoside, which,

upon oxidation, releases juglone, causing

the endocarp to stain.

The quality requirements for exporting

walnuts require a clear, attractive product

that can be obtained by using various

oxidizing and reducing agents during the

controlled washing and drying process.

http://www.fia.usv.ro/fiajournal

Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava

Volume XVII, Issue 1 – 2018

Eugenia BOAGHI, Vladislav RESITCA, Pavel TATAROV, Jorj CIUMAC, Walnut shells bleaching using oxidizing and

reducing agents, Food and Environment Safety, Volume XVII, Issue 1 – 2018, pag. 48 – 52

Both oxidizing and reductive agents are

used for bleaching cellulosic products like

walnut shell. Data on walnuts bleaching

with oxidative agents was presented in

previous studies [4]. The bleaching activity

of the agents is due to the hydrogen

peroxide (HP)contained in their structure.

When bleaching with hydrogen peroxide,

direct action on whitening occurs due to

peroxide ion (HO2
-) resulting from the

hydrolytic dissociation of peroxide. These

ions are trained in reduction or oxidation

reactions of the chromogenic lignin

groups, thus contributing to the desired

color change [5, 6-11].

2. Materials and methods

Walnut fruits, variety Calarasi, were

collected from local walnut plantation,

Iargara, Moldova. All chemicals used for

experiments were at least analytical grade.

The walnut were cleaned to remove trash

and damaged, sunburned, and broken

walnuts.

For walnut bleaching were selected

oxidising agents: H2O2, Okoron12,

Na2B2O4(OH)4 and Ca(ClO)2
(concentrations in the neighborhood of 1 -

10% being found satisfactory). The process

was performed in the temperature range -

20-600C and at the pH range 3 to 10 (pH

adjusted with HCl or NaOH to the desired

value).

Prepared walnut samples were immersed

in the solution of bleaching agent being

agitated to insure complete contact, after

which the walnuts were withdrawn from

the solution and allowed to dry. Walnuts

bleaching was performed at different

concentrations, pH values (3, 7, and 10)

and temperatures (20, 40, 60 ° C).

Color changes were judged after the total

color difference between the initial

(control) and bleached samples, calculated

using the formula:

E* = (L*2 + a*2 + b*2 )1/2

3. Results and Discussion

Previous research on walnut shell

whitening is limited and provides the

whitening only with solutions of hydrogen

peroxide or various combinations of it with

oxalic acid or sodium hydroxide and

chlorine compounds [12 - 14].

The first question when performing

experiences, was whether they are

effective or not for bleaching walnuts and

in how much time a 1 to 10%

concentration of bleaching agents will

manifest its effect. Below are the results

for walnuts bleached with sodium

perborate and Okoron 12 that have proven

to be the best oxidative bleaching agents.

In table 3 are presented the obtained results

for the walnuts bleached with sulfur

dioxide.

Table 1.

The effect of Sodium perborate concentration,

pH and temperature on chromatic parameters

of walnuts shell

Agent C, % t, min ΔE

Na2B2O4(OH)4

10 21.26

30 21.96

5%
10 22.49

30 23.19

Agent pH t, min ΔE

Na2B2O4(OH)4
(5%)

10 42.03

30 33.92

10 41.55

30 35.05

10 45.19

30 42.97

Agent t, 0C t, min ΔE

Na2B2O4(OH)4

(5%)

10 41.04

30 42.67

10 44.45

30 55.17

10 48.73

30 73.90

The results show that the brightness of the

walnut shell increases with the increase of

the agent concentration, the yellow shade

decreases (the value of the component b

https://www.hindawi.com/journals/ijd/2009/313845/#B2
https://www.hindawi.com/journals/ijd/2009/313845/#B3

Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava

Volume XVII, Issue 1 – 2018

Eugenia BOAGHI, Vladislav RESITCA, Pavel TATAROV, Jorj CIUMAC, Walnut shells bleaching using oxidizing and

reducing agents, Food and Environment Safety, Volume XVII, Issue 1 – 2018, pag. 48 – 52

increases). The maximum brightness value

is reached after 30 minutes of bleaching.

In all cases, for each bleacing agent,

bleaching rate increases significantly with

increasing the temperature of the mediu m

from 20 to 60 ° C. The sodium perborate

bleaching mechanism is similar to the

hydrogen peroxide bleaching mechanism,

but the perborate alkalinity is higher than

hydrogen peroxide and has more

significant effects on the delignification

and bleaching process at the same active

oxygen concentration.

Table 2.

The effect of Okoron 12 concentration, pH and

temperature on chromatic parameters of

walnuts shell

Agent C, % t, min ΔE

Okoron 12

10 5.64

30 6.32

5%
10 3.76

30 11.47

Agent pH t, min ΔE

Okoron 12
(5%)

10 7.27

30 9.38

10 9.44

30 10.91

10 15.52

30 19.54

Agent t, 0C t, min ΔE

Okoron 12

(5%)

10 22.71

30 25.61

10 25.03

30 34.19

10 31.39

30 34.63

The effectiveness of whitening, as can be

seen from the results presented in Table 4,

largely depends on the pH value of the

medium, which determines the

decomposition of the hydrosulfite, but also

the browning caused by the basic medium

too strong.

Table 3.

The effect of sulfur dioxide concentration, pH

and temperature on chromatic parameters of

walnuts shell

Agent C, % t, min ΔE

SO2

10 2.96

30 4.22

5%
10 3.14

30 9.51

Agent pH t, min ΔE

SO2(5%)

10 12.47

30 14.45

10 6.72

30 4.17

10 4.41

30 7.78

Agent t, 0C t, min ΔE

SO2 (5%)

10 3.09

30 7.12

10 12.63

30 20.77

10 24.30

30 30.22

Table 4.

The effect of Sodium dithionite concentration,

pH and temperature on chromatic parameters

of walnuts shell

Agent C, % t, min ΔE

Na2S2O4

10 4.64

30 11.33

5%
10 12.95

30 13.76

Agent pH t, min ΔE

Na2S2O4 (5%)

10 5.84

30 6.67

10 10.79

30 16.77

10 9.91

30 15.49

Agent t, 0C t, min ΔE

Na2S2O4 (5%)

10 12.95

30 13.76

10 18.72

30 29.48

10 27.32

30 35.88

Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava

Volume XVII, Issue 1 – 2018

Eugenia BOAGHI, Vladislav RESITCA, Pavel TATAROV, Jorj CIUMAC, Walnut shells bleaching using oxidizing and

reducing agents, Food and Environment Safety, Volume XVII, Issue 1 – 2018, pag. 48 – 52

Whitening of walnuts is relatively low at

pH≤5,0 and is caused by accelerated

decomposition of hydrosulfite. At pH

values greater than 6.0, the hydrosulphite

is stable and the bleaching process is more

pronounced. At the same time, with the

further increase of pH, the formation of

chromophores, which are little attacked by

hydrosulfite, is amplified.

Results with a similar trend have been

obtained with other whitening agents:

Hidrogen peroxide and calcium

hypochlorite.

It has been found that oxidative bleaching

agents are more effective and that the

bleaching process depends on the

concentration of the agents, the

temperature and the pH of the medium and

the duration of retention of the walnuts in

the bleaching medium. After the whitening

process of the walnut shell, the agents

studied form the following series:

Na2B2O4(OH)4 >Okoron 12>H2O2 > Ca

(ClO)2> Na2S2O4 > SO2.

The obtained results are correlating with

others researcher’s data that mention that

the effectiveness of the whitening process

depends on the concentration, temperature,

time, environment pH. All these factors

must keep in balance to achieve the desired

degree of bleaching [16, 17].

Taking into account the bleaching activity

of the agents and the toxic emissions

produced by them, whitening of the walnut

can be done with sodium perborate

(%Na2(H4B2O8)=5%, 60 min, t=60
0C,

pH=10), Okoron 12 (%Okoron 12 =5%, 90

min, t=600C, pH=10) and hydrogen

peroxide (%H2O2=10%, % NaOH =2,2 %,

t=600C, 90 min).

5. Conclusions

Colour is one of the most important

quality attributes influencing consumer

food choices, perceptions and purchase

behaviour. Colour measurement and

analysis is therefore important in

postharvest handling and biopro-cessing to

optimize the quality and value of

food. There were quantified chromatic

characteristics of walnuts in shells by

CIELAB system and conducted

experimental studies on theyr bleaching

with oxidizing and reductive agents.

Taking into account the whitening

activityof the agents and the toxic

emissions produced by them, whitening of

the walnuts can be done with sodium

perborate (Cperborat=5%, τ=60 min, t=60
0C,

pH=10), Okoron 12 (Cokoron =5%, τ=90

min, t=600C, pH=10) and with hydrogen

peroxide (CH2O2=10%, CNaOH =2,2 %,

t=600C, 90 min).

6. References

[1]. LABUCKAS D.O., MAESTRI D.M., PERELLO
M., MARTINEZ M.L., LAMARQUE A.L., Phenolics
from walnut (Juglans regia L.) kernels: antioxidant

activity and interactions with proteins, Food

Chemistry, 107, (2008). pp. 607-612.

[2]. RAMOS D.E., Walnut Production Manual,
University of California, Division of Agriculture and

Natural Resources, Oakland, California, Publication

3373, (1998).

[3]. Foreign Agricultural Service, Tree Nuts: World
Markets and Trade, (2017).

[4]. BOAGHI E., Impact of treatment with oxidative
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chromatic parameters. Ukrainian Food Journal

Volume 5, Issue 4 2016, ISSN 2304–974X, (2016),

p.644.

[5]. YARBOROUGH D.K., The safety and efficacy
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1990, Compendium of Continuing Education in
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[6]. DAHL J.E., PALLESEN U., Tooth bleaching—a
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[7]. ROBERT I., LAZAU R., Dyes and pigments,
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Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava

Volume XVII, Issue 1 – 2018

Eugenia BOAGHI, Vladislav RESITCA, Pavel TATAROV, Jorj CIUMAC, Walnut shells bleaching using oxidizing and

reducing agents, Food and Environment Safety, Volume XVII, Issue 1 – 2018, pag. 48 – 52

[10]. MOSS J.R., OTTEN L., A relationship between
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roasting of peanut, Canadian Institute of Food Science

and Technology Journal, 22, (1989), pp. 34–39.

[11]. SALVADOR A., SANZ T., FISZMAN S.M.,
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[12]. KRISTIANSSON L., Chemical bleaching of
wood and its aging: An investigation of mahogany,

walnut, rosewood, padauk and purpleheart. Bachelor
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[13]. SUESS H.U. (2010), Pulp Bleaching Today,
Walter de Gruyter, Berlin.

[14]. MILOVIDOVA L.A., KOMAROVA G.V.,
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[15]. U.S. Forest Service research note FPL, V. 165,
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[16]. UYSAL B, ATAR M and ÖZÇIFÇI A., The
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[17]. SINGH RP., Principles of pulp bleaching. The
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(1979), p. 17.

* Eugenia BOAGHI, Vladislav RESITCA, Pavel TATAROV, Jorj CIUMAC
*eugenia.boaghi@toap.utm.md
Keywords: walnts, bleaching, chromatic parameters, oxidizing and reducing agents
1. Introduction