Journal of Scientifi c & Industri al Research

Vol. 6 1, May 2002, pp 339-348

Alternatives to Salt Curing Techniques - A Review

J Kanagaraj * and N K Chandra Babu

Leather Processi ng Di vis ion, Central Leather Research Institute, Adyar, Chennai 600020, India

The raw hides and ski ns are fl ayed from the animal and processed further to manufacture leather. As the main constilUent of the raw skins and hides is protein , they are much susceptible for bacterial degradat ion. Thus, it is essenti al to preserve the protei n matri x and also to arrest mi crobial att ack temporaril y. Several chemi cal, biocidal and physical meth ods have been advocated and adopted, preservati on using salt remains the popular curing technique worldwide due to ease, cost­effecti veness and the quality of the fi ni shcd Icather produced. The skins/hides preserved with salt (40-50 per cent , green weight basis) generate huge amou nt of pollution in the form of total dissolved solids (TDS) and chlorides (en during leather processing. A great deal of research is being done world wide in search of a salt-free, alternati ve curing systems. An attempt has been made to review vari ous alternati ve curing techniques and their merits and demerits have been described.

Introduction

Leather Industry is an age-old industry and has been serving the soc iety as an important consumer industry . It provides a wide range of consumer goods such as shoes, garments, bags, etc. However the tanning industry has been categori sed as one of the hi ghly polluting industries and there is concern that leather making activity can have adverse impact on the environment. The globa l producti on of about 24 bi II ion m2 of leather by the year 2000 presents a considerable chall enge to the industry considering the harmful nature of so me of the chemicals used in leather process ing I . The tannery efflu ents are charac teri sed by hi gh contents of di ssolved and suspended organic and inorganic so lids giving ri se to high oxygen demand and potentiall y tox ic sulphide and chromium metal ion. Electrolytes, mainly neutral sa lts are al so coming under criti ca l scrutiny due to their contribution to total di sso lved so lids in the e ffluent. The discharge limit for TDS is qu ite stringent in India and hence, there is a tremendous pressure on the tanners to avo id the use of salts in process ing or resort to in-process control measures to avoid/reduce the discharge of sa lt bearing streams.

Sa lts in the effluent are mai nl y contributed by the conventi onall y empl oyed sa lt curin g methods , pickling and chroille tanning operat ions with the major proport ion coming from the common sa lt

x Aut hor for co rre~ rolldcllce; Tel : +tJ t - 44 4') 15730: Fax: +9! - 44491 1589: E-mail :JddrC'ss: jkraj()R @l hot lllail.com

used in the curing process. There are attempts made to bring about process in venti ons such as adopt ion of pickle-less chrome tann ing, recyc ling of pickle float, pickle-tan closed loop chrome tanning methods, etc to minimize sa lt di scharge from pickling and chrome tanning. But limited attention has been focused on the preservati on method wh ich employs huge amou nt of the common salt (40-50 per cent , green weight basis) for the preservation leading to huge amount of sa lt pollution. Continued use of sa lt cured stock without any pollution control measures for many years before the environmental consciousness could take roots has given ri se to degradation of land and pollution of the underground water due to hi gh salinity in and around many of the tanning c lusters in the country. The current strategy to deal with the hi ghl y sa line soak I iquor can be better termed a stop-gap arrangement and the permanent solution may re ly on sa lt -free curing methods . Hence, e fforts should be harnessed to search and adopt cost effec ti ve and environmentall y safer curing methods which wi ll not affect the quali ty of the result ant leathers adverse ly.

Inadequacy of the Current Approaches to Deal With Pollution Due to Salt Curing Methods

Sod iu m chl oride discharged in the soak liquor due to salt curing methods forms the larges t component of most tanning effluents in terms of co ntribution to disso lved so lids and chl orides in the efflu ent. Sod iul11 chloride is the 1110St diffi cult to treat and hence pose difficulties in the efflu ent treatment. They contribute in large mca~Ll rc (more than 40 per cent) to to tal

340 J SCIINDRES VOL 6 1 MAY 2002

di sso lved solid s (TDS) and to chl orides (about 55 per cent ) in the compos ite tannery e ffluent2

. Thi s e lec tro lyte rema ins a lmost at the same concentration leve ls even after treatment. There is sti II no techno logy ava il ab le for treating the effluent contai ning such hi gh concentration of neutral sa lts .

Currentl y adopted strategy to dea l with the spent soak liqu or invo lves segregati on of thi s secti onal stream from the res t of the effluent and taking it to so lar pans for evapo rati o n to recover the sa lt. At the outset, it may look an attractive opti on fo r a tropi ca l country like Indi a but in many tanner ies which have put up the ir so lar pans, litt le effort s are be ing made to recover the sa lt. In the absence of effecti ve sa lt recovery systems, the so lar pan concept may not rea ll y offe r a solut ion to the sa lt pollution. Moreover, in many tanning centres there are a lso constra ints regarding space availabl e for the construc ti on o f solar pans and in some others the e fficiency of so lar drying is poor due to prevai ling c limati c conditi on. Even in the few tanneries, which take pains to recover the sa lt from the pans , difficulties are often ex perienced with respect to di sposal of the recovered salt.

The improved desa lting methods prior to soak ing reduce the sa linity in the soak liquor. However, the di sposal of the recovered sa lt aga in poses problems in many tanneries. T he counte r cu rren t soak ing method which invo lves recyc ling of the spent soak I iqu or, though reduces soak I iquor pe r tonne raw materi a l cons iderabl y, may not prove to be an effect ive so luti on if the sa lt is not recovered by evaporation sUbsequentl y. Thus the lasting so luti on to the problem may rest on a lte rn ati ve curing meth ods based on saltl ess and less-salt prese rvati on. Hence the need for studying variou s curing methods ava il abl e and the factors responsibl e fo r hide/skin putrefacti on is imperati ve. This study would he lp in developing newe r curing sys tem in the near future.

Factors Responsible for Putrefaction of Hide/skin

The raw hides and skin s consist mainly of water and prote in , which make them vu lnerable to at tack by mi croorgani sms. The microorgani sms decompose the protein and it is technically called putrefaction of hide/sk in and eventua ll y make the hide/s kin unsuitabl e for the manufacture of good quality leather. The process or the treatment , which is followed in preventing attack by various kinds of

bacte ri a on raw hide and skin is termed as curing. The various factors to be considered in curing process are (a) Bac teriol ogica l aspects of curing, (b) Moisture content, (c) Temperature, (d) Humidity, (e) pH, and (D Pre-curing period

Bacteriological Aspects of Curing

The hide/skin once flayed is deprived of oxygen and nutrition al components. The remova l o f the metabo lites fro m the ce ll is stopped leading to the accumul ati on of toxic products, thi s in turn leads to breaking of the part o f the enzyme contro ll ed process sta rts. Process of self-di ges ting o r autolysi s of the ce ll starts. In thi s process inte rce llul a r enzyme catheps in is in vo lved ' . The va rious bac te ri a (aerobi c, anae robic and facultat ive) are in vo lved in the degradation of coll agen. But the most dangerous bac te ri a are anaerob ic type that dete ri orates protei ns into the stage of a mino ac ids. Various researchers have tri ed to find ou t the bacte ria responsible for co ll agen degradati on and inhibiti on of the same4

-R

. T he first mi c robi a l enzyme (co ll agenase) capab le of degrading co ll agen a t neutra l pH is iso lated from the spec ies Clostridium histolyticul11'). Thi s is shown to c leave the nati ve collagen molecule into two fragments in hi ghl y fashion at a tempe rature be low the denaturation te mperature o f the subst rate. It was shown that the c leavage of native co ll agen mo lecul e takes place at a specifi c s ite c loser to the c-terrninal end of the molecule , y ie lding segments of one quarter and three quarte rs of the le ngth of the nati ve coll agen mo lecule. The active bond c leaved in all species was Glycine-Leucine or Gl yc ine-Iso leucine lin k 'o.

Moisture

The moisture content o f the raw hi de/sk in is an important factor to be cons idered those cont ro ls the bacterial ac ti vity. Bacteria cannot grow unless there is a c riti ca l moi sture in the hide/skin . The princ ipl e of dehyd ration of wate r be low the level of minimum requirement was adopted in the curing process. The moisture content present in the raw hide/sk in is found to be 70 per cent. Hence, in the curi ng process the moisture content is brought be low the c ri t ical moisture content. The c riti ca l mo isture content was fo und out to be 50 pe r cent II. Above thi s leve l of moisture conte nt the hide/sk in is conduc ive for bac te rial attack and the contro l over moi sture content is an importa nt factor.

Temperature

The hi gher tempe rature during curing always play negative role in the curing process. If the temperature

KANAGAR AJ & BABU: ALTER 1ATIVES TO SALT CUR l G TEC HNIQUES 34 1

is morc, the curing e ffi ciency is affected by bacterial damage or by the loss of hide substance. Hence, it is necessary that even the skin is being preserved should be stored below 30 "c. But the skin prese rved by ch illing (+2 to +5 DC) and freezing (- 10 (0 -20 "C) may injure the hides and skins by freezing the ice crystals around the material and cause rupture of the ti ssue. Hence keeping the raw materi al at optimum temperature is an important factor. Most of the bacteri a in li vi ng conditi ons showed optimum temperature between I 5 DC to 37 °C 12. 13. It has also been reported in I iterature th at better preservati on could be establi shed at temperature range between 10- 18 "c.

Humidity

It is very diffi cult to control the moisture content without control I i ng the atmospheric humidity. Elevated humidity, espec ially in the monsoon induces sodium chl oride (in the case of salt curing system) or curing agents to absorb more moisture and thereby he lping bac terial growth. Hence, elevated humidity is not suitabl e fo r the cunng process.

pH

pH is an important factor to be considered in any curing process of the hide/skin. The reason is that bacteri a or enzy me is active at the pH of neutral or sli ghtl y alkaline conditi ons. Hydrolys is of co llagen by proteo lyti c bacteri a shows a maximum at pH of 7.5. At the pH 9 the acti vity is weaker, while at pH 5.5 to 6.5 the ac tivity is practi ca ll y

I· 'bl 1-1 I'; B I ' I neg Igl e . ' . ut t le precautIOnary measure las to be taken in the case of alterati on of pH, once the pH goes be low 4, there is a chance of swelling of co ll agen and in turn would weaken the fibres which would make the hide/skin un suitable fo r leather making.

Pre-clIring Period

The pre-curing period is the time between the actual slaughtering of the animal and commencement of the curing process. It is an important factor as the hides and sk ins become putrefi ed to some ex tent due to delay in curing. If the cond iti on of the hide/skin is sufficientl y fresh and without the onset of bac teri al acti on, the aim of curing simpl y li es in quick dehydrati on of water from the materi al. It has been found that if the pre­curing peri od de layed for about 5 h, is sli ght

degenerati ve changes take place in the cell ' lying round the sweat glands . After II 11 , the remaining sk in structure al so gets affected leading to break down of the polypeptide into dipeptide level16

Alternatives to Salt Curing

Ma inl y, due to the pollution re lated problems assoc iated with the salt curi ng methods, many research groups have been act ively in vo lved in the development of alternati ve curing/preservat ion techniq ues. Some of the work carried out in the area by va ri ous research groups has already been rev iewed.

The popular sa lting methods are wet salting and dry salting. Here, in wet sa lting, sa lt at the leve l of 40 per cent (based on green weight) is applied to the fres h skin for preservati on. The applied salt reduces the moisture to 30 from 70 per cent. But in the case of dry salting, the salted hide/skin is further dri ed to reduce the moisture content of be low 20 per cent to enable to transport the goods eas il y, pose problem in wet ting back in the leather process ing. It has been esti mated that in India 20.3 per cent of the total hide production is cured by ordinary sun drying, 75.8 by wet salting and 3.9 per cent by dry sa lting. Of the total skin production in India, 47 .7 per cent is cured by wet salting, 49 .9 by dry salting and 2.4 per cent cured by fl ' d . 17 Int ryll1g .

Sun Drying

The simplest and cheapest meth od of curing hides and skin s without the use of salt is to dry them by evaporati on under the sun. It is one of the eco-friend ly curing process. Thi s meth od is however practicable onl y in countries with dry warm c limate. Thi s preservation method where ever practi ced is often poorl y controlled resulting in either over dryi ng leading to subsequent problem in further processing into leather or under drying lead ing to deteri oration in the skin quality. Generally, the sun-dri ed skin s produce inferi or quality leathers and these are di fficult ies in wetting back of the skins and hides whi le process ing into leather l 8 Since, the leathers produced with sun dri ed stock are of inferi or quality there are attempts made to improve the curing process by resorti ng to controll ed dryin g methods.

Controlled Drying

The fresh woo l skin can be dri ed in a contro ll ed environment within 24 h by using drying chamber l9

.

The chamber used in these ex periments consist of a 6

342 J SCI INO RES VOL 6 1 MAY 2002

meter refrigerated shipp ing co ntainer, the re frige rat ion unit of which is run in reverse cycle mode at 26 DC. In side the container is located a portab le refr igerated dehum id ifier, two large fan s (0.8 m diam), and a toggle drying frame capab le of holding 12 skins. Skins are toggled on the perrorated meta l frame, flesh s ide out, pri or to drying. Also located inside the contai ner is a DaUltaker 6000 datalogging dev ice which is used to monitor temperature and humidity at several po ints withi n the chambe r.

In thi s method, a much greater contro l over moisture content is possib le, thereby resu lt ing in a more cons istent product. Spo ilage due to putre facti on has been virtua ll y e liminated, and the rate or rehydration of the skin is s i mi lar to that or salted sk ins . Th is method prov ides long-term pres rvati on. However, thi s method requires high installati on and ru nning cost.

Cooling (Il1d Chilling

In many countries, indu stria l application of coo li ng sys tem for hides and sk ins is bei ng prac ti sed. A te mperature be tween +2 and +5 nC is emp loyed and the results are reported to be sati sfac tory. However, preservation time is limited and T abl e I summari zes the dependence of preservati on time on the storage temperature.

Qua lity of the preservati on depends on the te mperature during the transfer peri od . Coo led hides and skins should be kept in a we ll insulated store room and in piles to get the best results . In prac tice, one of the fo llowing methods is adopted to bring about refri geration of the skin s and hides

20.

(i) Cooled ai r treatment

(ii) Addition of ice

(iii) Carbonic dry ice addition

Table I- Preservati on time as a fun ction of temperature

Temperature, "e Time

0 3 wk

5 2 wk

10 I wk

IS 2 d

20 I d

25 Oh

Cooled (f ir treotment - This tec hn o logy is adap ted to large s laughterhouse whe re it is possible to automate and thererore coo l and store large quantities of hides without handli ng. By thi s technique it is poss ib le to process 300 hides/h on a continuous conveyor. H ides are coo led at 5 "c fo r about 45 min. It has been reported tha t in Australia , the ski ns preserved this way after sto rage for 5 dare tran , ported to a wet blue tannery about 700 km away. A Company in Germany is process ing 70 per cent of its produc ti on wit h coo led air. The h ides afte r f lay ing a re coo led at 3 nC and pil ed on pale ttes. In the case of hot weather, some ice is added be tween the hides . The use of conveyor during the coo ling process he lps to improve the therma l exchange on hanged hidesl' .

Addition 0/ Ice - It is poss ible to coo l hides and sk ins in a continuous way in a mixer by us ing some ice cubes, cakes or flakes, j ust after fl ay ing. It is also poss ible to pu t hides or skin s in a coo led tank and then to add some ice in the storage conta ine r. Wi thi n 2 h, hi des are coo led from 30 to 10 °C and can be stored for 24 h without further treatment. T his method is being fo ll owed on large-scale in Switzerland , Germany and Austria . The cost of an ice making mac hine is low compared to a co ld room in ves tment. T hi s techno logy has been further improved by B ri tish Leather Confederati on, UK by the use of preservati ve soluti on to produce ice. The limitation of the process is the draining of liquor containing high concentrati on of

. 22 preservati ve .

Use of Dry Ice - Compared to the normal ice, the hides and skins are cooled to - 35 nC and cooling is achieved rapidly in the who le area of sk in/hide. The method does not suffer from the problems with the use of ice, as given earlie r, such as re-wetting problem and brine draining from me lting of normal ice. It g ives a uni form cooling and preserves the hides or skins for a minimum of 48 h.

However, specia l care has to be taken because of the suffocation ri sks by the use of carbon di oxide. The cold conditions and the high pressure of carbon di ox ide storage must be taken into account. Dry ice is estimated to be applied @ 60 g/kg hide, corresponding to 15 L gaseous carbon di ox ide. Thi s type of process ing seems much more adopted fo r skins that keep the temperature longer than hides. It was poss ible to compare the ad vantages and di sad van tages of each technol ogy23.

. ,

KAN AGARAJ & BABU: ALTERNATIVES TO SALT CUR l, G TECHNIQ UES 343

The ad vantages and disadvantages of the above methods a re summari zed in Table 2.

Use of Chilling

The above mentioned coo ling systems provide only short term prese rvation for 24-48 h. When it is necessary to store hides or skin s for a longer pe riod o f time, it is possibl e to process it in mLlch co lder conditi ons up to - 10 or -20 DC. At - 10 DC , it is

possible to preserve hides fo r 3 mo without any prob lem 24 . Somet ime the ti ssue fibe r may be damaged , the refore , it does not appear to be re li ab le. It is a lso reported that at the end of the freezing pe ri od , bac te ri a l degradati on is faste r than with the coo led hides. Any hindrance in the freezing step may promote bacte ri a l growth .

Advantages o f thi s chilling system are the good gra in quality of the resultant leathers, fewer environmenta l proble ms because of no use of sa lt , safe r handling of the cooled and chill ed hides. The disad vantages a re the hi gh in vestment and operation costs and hi gh powe r consumpti on, especiall y in warm c limates and hi gher slipperiness of the pe lt during fles hing.

Curin g by Potassium Chloride

The preserva ti o n of animal hides and skins w ith potass ium chloride25 in pl ace o f common salt has been carried out with s teer hide by brine curing method . The cured stock cannot be di stingui shed fro m that cured with common sa lt except that the potass ium chloride hides are cons iderab ly dri e r on the surface.

The advantages of this method are:

• The preventi on of red heat and

• Potassium is a macro-nutri en t to the plan t25

.

The disadvantages are:

• The hi gh cost of potass ium chloride and

• The dependence of solubility of potassium chlor ide on the te mperature; the so lubili ty of potassium c hl oride decreases if the

LiriclIre

1 ) temperature drops -. .

Atte mpts for hide and skin preservat ion using powder bioc ide compositions have been made26

. The powder preservat ives are app lied to fres hl y flayed sheep skin s for effecting curing. Minimum effect ive dosages and re lati ve acti viti es of vari ous preservatives have been worked out. In thi s method, a mixture of anti septics with medium coarse sawdust (p ine) is app lied uniforml y to cover the fl esh surface. The powder preservati ves used are ch lor i nated pheno l (PCMC) and EDTA (eth ylene di-amine te tra aceti c ac id). Fini shed leathe rs are manufactured from the sheep skins and cattl e hides after the storage period of 12 mo . The phys ica l properties a re reported to be comparable w ith sod ium chl oride cured system. A seri ous di sadvantage is th at EDT A conte nt in Liricure powder w hich may cause difficulties in precip itating chromium compou nds in efflu ent treatemnts.

Preservation by Irradiation

Radiation process ing ( irradi a ti on) is a sa fe method in vo lving systemati c exposure of materials to 10lll Slllg energy to effect spec ific c hem ical or bi o logica l changes. An a lte rnati ve method to

Table 2-Advantage and di sadvantages of various technologies used

Techno logy

Cooled air

Icc add it ion

Dry ice

Advantages

Autom~llion or handling

No increase in weight

Adopted to sorting

Low in ves tment cost

Low running cost

Adopted to low quantities

Moderate investment cost

Reduced increase in weight

Di sadvant ages

High in ves tment

Need in trucks equipments

Constant feeding with hides

Increase in weight

Brine 10 recover

Limi ted protecti on in time

Risk s with carbo n di-oxide

Limited protect ion in time

344 J SC I INO RES VOL 6 1 MAY 2(J02

conventi onal the rmal methods for Illitiating and driv ing che mi ca l reac ti o ll s, irrad iati on is a co ld process , which minim ises energy expendit ure and dramatica ll y reduces the need fo r cos tl y or tox ic chem ical ca talysts. Whi le suffi c ientl y energe ti c to drive c hem ical rec!ct ions, the radi ati on does not induce radi oac ti vi ty. Conseq uent ly ex posed

d d· · I '7_Je) pro uc ts can not pose any ra latl on ( ange r- - .

The two principle radia ti ons used by industry are gamma rays ~pho t o n emiss ions from radi oactive materials) and e lec tron beams. For hide processing, for reasons of e ffi cacy, safe ty, versatilit y. speed and cost, elec tron beams are superi or to gamma rays. In e lectron beam p rocess ing, ioni si ng energy is produced without the use of an y radioactive mate rial s. A n acce le rator gene rates an e lectrica l fi e ld through whic h e lec trons are brou ght up to near the speed of li g ht s imilar to a te lev is ion tube. As thi s strea m o f e lec trons ex it the accele rator, it is magnetica ll y scanned or shaped to form a fa n shaped beam. As produc t ~ are passed through thi s b am, they are exposed.

A mi croprocessor contro ls the e lectron beam dose paramete rs o f the ex posure, monitoring them everyone hund red m illi seconds and documenting them continuously. The "dose" or measure of energy absorbed by the treated product is reproduc ible to w ith in a frac ti on of a pe r cent. Once the syste m parameters a re establi shed fo r a g iven produc t, the compute ri sed contro l sys te ms ensure that the process can be repeated without dev iati onJo.

H igh-speed e lectrons are used to ste rili ze the hides. If the hides are s te ril e and the enzymes in them are inacti vated and they are not a ll owed to be re-infected , the hides wi ll reta in the properties of fresh green hides . Ad vantages of thi s system are the decreased sa linity of effluents and there is no tox ic effec t. The di sad vantages are ( i) need for very expensive equipment , ( ii ) need for full protecti on of the workers operating the equipment , ( iii ) poss ible reducti on in tens ile strength of the leather, (iv) need for covering eac h hide with plasti c bags to prevent recontamin ati on with microorgani sm during storage and (v) hi gh cos t of treatment J 1

.

BCII -;.a lkonillm Chlo ride as {/ Preservati ve Jar Hides/Skills

Benza lkonium chl oride (BAC) is tested for the short ti me preservati on o f hides and skins and as an adj unct fo r use with sa lt in brine-curing and green

sa lt ing . The per centage of (BAC) u . .;ed for til e preservat ion o f fres h calf skin ranges from 0. 1-0.4 per cent. There is no difficu lty in process in g hides treated wi th this mate ri a l and no adve rse e ffec t o n the leather is found

J2 BAC is a wide ly used househ o ld and

industrial anti septic . There are no tox icit y prob lems at the recommended concentrati o n but BAC may be unacceptab le in some by-p roducts.

Presen 'otioll rlRmt" Hides olld SkillS 'vVi th Boric Acid

Te mporary preservation o f hides us ing a sa turated aqueous so lution of bori c acid (approx imate ly 4.5 pe r cent , v/v) both a lo ne and in conjunc ti on w ith saturated sodium chl oride so luti on has been in vesti gated . Hides soaked in saturated ac id so luti on a lone had storage life o f only 5 d3J

Bori c ac id is stab le to heat, li g ht and air and it is not vo la tile so that it wi ll re main effec tive on the hides dur ing suspe ns ion drying . It is nontox ic to humans (LD:;o va lue = 3000 mg. kg-I

) and it is un like ly to cause e fflu ent prob l e l1l\~. But the use o f boric ac id for curing has recentl y come unde r e fflu e nt restri cti on in Austra li a .

T he additi on of boric acid and naphtha lene (0 .5 pe r cent) became standard practice in New Zealand in the ea rl y 1960s. These additi ves are fou nd to improve the she lf life of the sa lt cured stock apprec iabl y and to be instrumenta l in e liminating bad odours, f lies and other pests from hides s tored .

Curing by Silica Gel

In pl ace of the sa lt whi ch functi ons mainl y because of its ability to dehydrate the hide/skin be low criti cal moisture content from its bacte ri osati c property, a dehydrant silica ge l is used to preserve the raw hide/skinJ5

-J7

. Sil ica ge l at the level of 15 per cent a lone and the level o f 10 per cent w ith biocide Para Chl oro M eta C resol (PCMC) could bring about effec ti ve preservati on of the hide/skin. S imila rl y silica ge l at the level of 5 pe r cent w ith minimum amount of sa lt 5 per cent wi th or wit~out PCMC a lso establi shed a preservati on effec t in the raw hide/ski n. The effi cacy of the sys tems are analyzed over conduc ting various tests in the preservati on pe ri od such as moisture content , tota l extractable nitrogen co ntent , NM R imaging for moisture content , bacte ria l count and polluti on load generated during leather processing. T he results showed that the lea the r obta ined is comparable in properties with a potenti a l to reduce polluti on load

.., .

+

KANAGARAJ & BABU : ALTERNATIVES TO SALT CU Rl G TECHNIQ ES 145

in terms of TDS by 70-75 per cent and chl orides 80-85 per cent over sa lt curing system.

Curing bv Arvl A lcohol. Hypo

A short term preservation with aryl alcohol has been reported JR-J9 . It was found that aryl al cohol a tota l dosage of around 2-3 per cent (on green weight ) gave satisfactory preservati on fo r about two months . It was abo found th at hypo at the level of 5 per cent was able , 0 preserve the buffal o hide and goat sk in for 10 d. The zinc su lphate at the leve l of 5 per cent and a mi xture of benza ldehyde (0. 5 per cent ) with B-napthol (0 .5 per cent) could also preserve the stock for 3 d and I wk, respectively.

Other Reported Curing Methods

It has been found that the wide range of materials, both organic and inorganic, can be used for the preservation of skin/hi de without employing common sa lt40. They include sodium chl orite, sodiu m carbonate, prop ionic acid and peracetic acid as well as organi c anti septi c reagents such as tebori t and hyamine.

Preserva ti on of skin by usi ng sul phites, bisulphites and meta bisulphites used in conjunction with an acetic ac id are also present 41-45 . It is possible to effec t short term preservati on employing a mi xture consisting of 2 per cent sodium sulphate and 4 per cent sodium bisulphite which is applied on the flesh side of raw hides. The authors claimed that the hides are kept free of bacterial contaminati on by treatment for at least four weeks and that the leather produced from the treated hides is of equal standard with that produced fro m regular wet salted stock. An incidental disadvantage encountered by Nathan using the process, however is that the Swedi sh acceptable limit for atmospheric sulphur di ox ide (2 ppm) is exceeded.

Screening and evaluati on of preservati ves for skin preservation46 is carried out at CLRI. This is carried out by studying the inhibitory acti on of the preservati ves against certain strains of bacteri a. Preservati ves are added to nutrient broth media in di fferent proportions (0.0 I, 0.02 and 0.04 per cent, w/v) , the medium is di stributed in tubes ( 10 mLitube) and sterili zed. The tubes are then cultured with a strain of Bac illus species and Pseudomonas aeruginosa spec ies. The culture tubes are then incubated at 3 ODC fo r 24 h and growth is estimated by Klett-S ummerson photoelectric colorimeter using

660 I11fl filter. Un- inocul ated tubes are taken as control. Results obtained are ex pressed as optica l density . Opt ica l density va lues of 5 or below 5 are considered insignificant.

It has been reported that sodium sulphite can be used to avoid pollution associated with the use of sodium chl oride. The effec ti ve preserving agent is the sodium su lphite. Its li se is c laimed to meet the requirement s for a low cost, non-po lluting, easy to app ly curing system that does not have any adverse effect on the leather qual ity produced47

.

It is also reported th at enhanced antisepti c ac ti vity is obtained when some commercial ant iseptic materi als, including merpin TKE, Nercolan GLO and vantocil CL are applied to washed fl eshed hides at leve ls of 0.5- 1.0 per cent in assoc iation with a reduced leve l of sodi um chl oride 10- 15 per cent , all treatments bei ng based on green hide weight. The leather quality is comparable with conventional salt curing method47

.

It is poss ible to hold woo l sheep skins at 25 °C for 20 d in good condition after treatment with a combination of sod ium chl orite, sodiu m sili co flu oride and boric ac id . Sodium silico-f1 uoride (SSF) is mai nl y used as a sa lt additi ve but it was recommended that hides and skins should be dipped fo r 30 mjn in a saturated solution of SSF prior to shade drying. Controversy arose on leather making quality of treated hides, mainl y the sheep skins. Although SSF has LDso of 125 mg/kg, it has been considered safe. The chemical is stable to heat, light and air and is nonvolatile. As flu oride is precipitated by calc ium, effluent treatment in the tannery will precipitate out most of the fluoride used48

.

Chlorites and hypochl orites are effective bactericides but they may cause adverse effects on leather making qualities. More grain blemishes are present than are normall y seen on leathers produced from wet salted skins49

.

It has been found that thc preservat ion of skin/hide is possible by the application of IS per cent salt in conjunction with other preserving chemicals at 1-3 per cent. Hides thus treated were stored for 3 mo at 20-25 DC, 85 per cent RH. At the end of storage, the hides were in a sati sfactory conditi on without any adverse odour with bacterial counts within limits49

.

It has been found that a further alternative to the traditional salt cure, advocating the use of fo rmaldehyde at a concentrati on of 025 per cent on hide weight. They added the reagent th rough the

346 J SCI IND RES YOL(>1 MAY 2002

holl ow ax le of a rowtory drum containing the flayed hides, wit h no fl oat50 With increasing concern about the forma ldehyde in the leather du e to its tox ic ity the use of fo rmaldehyde for preserva ti on is ruled out at thi s stage.

The deve lopment of sa ltl ess preservation by the LI se of neem oi l with alcohol has also been reported"' . The neem ex tracts were app li ed to both flesh ane! hair sides at a rate of about I per cent on the green weight. After the treatment the experimental sk ins are all owed to dry in the shade. The skin s, by this meth od can be preserved for more than 6 months but the resultant leathers were of inferi or qua lity.

In India, immersion of hides and skiins for 4-8 h in a mixed solu tion of zinc chl oride and sodium penta chiaro phenate (PCP), at 0. 15 and 0. 16 per cent respecti ve ly, prior to sa lt curing is found to preserve hides and skins for at least 7 d. Sut due to po llution concern the use of PCP is banned52

.

Use of anti biot ics to control green hide biodegradati on has been also reported)' , where effects of aureomyc in , terramycin , chloromycetin , tetracyc line, and streptomyc ine have been examined. The auth or has deve loped microbiological bioassays to determine the antibacteria l activ ity of se lected ~ - I actams tetracyc l ines and <lminoglycos ides aga inst Vibrio (l ligil1o/yticlIs , used as co ll agenolyti c test bacterium Results indicated that the tetracyc line type ant ibioti cs are most effective at per cent , w/v, with ~ - I actam to a lesser extent .

A short term preservati on techn ique for the cattl e hides using a comb inati on of sodium chl oride and hydro lysed starch-poly acrylo nitril e graft co­polymers after washing with 4 per cent aceti c acid has been reported. The approach made is based on the pri nciple that by regulating water ac ti vity to ma intain micro bio-stati c conditi ons to produce a quality hide with minimum handling process ing and storage. This method of short term preserva ti on depe~ds on cont rol of gram-pos iti ve mi crococc i and bac illi and there was no sign of bac teri al or mo ld

k f· 54 growth after I I w ' o · storage· .

The preservati on of hi des and skins by tax idermi sts approach )) where the I i vi ng shape of an animal is re-created in a rela ti ve ly permanent cond it ion, often illustrative of an attitude characte ri sti c of the li ving animal. Thi s is done by

Llslng vari ous chemicals inc lud ing mercury, bichromate, alum, borax, boric ac id and phenol derivat ives , but toxi city of many of th ese chemi ca ls is ques ti onable.

For the short term preservat ion of hides , a Zimbabwean meth od based on chilling. and treatment with bacter i c ide~ such as benzalkonium chl oride, bori c acid, vantoc il (IS ), Susan 30 and Susan 52 have been reported to yield better results in comparison with conventi onal salt cured method·16

Vantoc il lB , a polymeri c biguani dine hyd roc hl ori de is wide ly used as food bacteri cide. This has been tried as a preservat ive on hides in admixture with another ventoci I product CL. A short term preservati on for 8 d at 25 "c has been possib le with thi s curing method. However, storage at lower

. f d'i6 temperatures IS pre erre . .

A short term preservati ons by using Susan 30, 2(­Thiocyano methyl thi o-benzothi azo le) (TCMTB) are used in conjunction with boric ac id . The recommended treatment is 2 h drum application of busan 30 (0.3 per cent) plus boric ac id (0.9 per cent) in 10 per cent float, based on green hide weight. Cost and quality are comparable with conventi onal salt curing process .

Sy using Susan 52, a product based on a mi xture of sodium 2-mercapto benzo thi azole and potassium N­hydroxy-N-Methyl dithiocarbamate, the short term preservati on of hi de has been carried OLl t. Good result is reported when thi s product is used in conjunct ion with boric acid , naphthalene and a wettin g agent56

.

Another short term preservation of catt le hi de using 20 per cent, w/v, soda as h has also been reported57 The raw cattl e hides, thoroughiy washed in co ld water, are immersed in the solut ion for 4, 6 or 8 h and then stacked fl es h side up after dr ip draining fo r about 45 min and then cover ing samples with wrapping paper and preserved at ambient temperature of 25 °C. The treatment may be used for preservin g raw catt le hide for 8 d.

Fo r the short term preservati on of hides by the use of zinc chloride or ca lcium hypo chl ori te as alternative to sod ium chl oride have bee n reported 58

Three option for carryi ng out the preservation have been suggested-zinc chl ori de (0.3-0 .5 per cent) with 0.05 per cent biocide, or 0.05-0. 1 per cent phenol with 20-25 per cent fl oat-for drumming about 2 h will preserve hides for a max imum of 5 or 6 d at 25 "c. The second option is I per cent applicati on of a 30 per cent sodium chl orite e g, Imprapell CO, Hoechst (equivalent

''1/ •

KANAGARAJ & BAF3U : ALTERNATIVES TO SALT CU RING TECHNIQ UES 347

to 0.375 per cent so lid sodium chl oride, 80 per cent act ive) with 0.05 bioc ide or 0.05-0.1 per cent alcoho l with float 20- 30 per cent drumming for 2 h ga ve better resu It .

Conclusion

It is evident that curin g agents could preserve the raw hide and skin by proper control of parameters I ike moisture content , temperature, humidity, pH and concentrati on of curing agents app lied, pertaining the raw hide/skin is immediately preserved. But any curing agent app li ed should inhibit the enzyme responsible for hide/skin putrefaction . This cou ld be eas il y achieved in the biocidal type of curing technique but control over moisture is not possible in thi s type whi ch may later lead to putrefaction . But in the case of chemical type preservation where control over moisture is achieved could not inhibit the specific enzy me. Hence, combination of eco-friendl y chemi ca l and bioc ide curing system could yield better results.

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