Plant Foods for Human Nutrition 47: 279-285, 1995. © 1995 Kluwer Academic Publishers. Printed in the Netherlands.

Effects of seed treatments and storage on the changes in lipids of pearl millet meal

R.V. KADLAG, J.K. CHAVAN & D.P. KACHARE Department of Biochemistry, Mahatma Phute Agricultural University, Rahuri 413 722, MS, India

Received 7 June 1993; accepted in revised form 23 February 1995

Key words: Lipids, Meal, Pearl millet, Seed treatments, Storage

Abstract. Lipids in pearl millet meal showed a rapid hydrolytic decomposition during storage. The magnitude of such degradation was influenced significantly by the nature of the storage container used, the temperature and heat treatments given to the seeds. The hydrolytic breakdown of lipids was significantly low in the meals stored in polyethylene bags, plastic boxes and under refrigerated (5 _+ 2 °C) conditions. Hot water blanching at 98 °C for 10 sec and dry heating of seeds at 100 °C for 120 min were found to be most effective in minimising the undesirable changes in lipids of the meal during storage.

Introduction

Pearl millet (Pennisetum americanum L. Leeke) is an important source of energy and nutrients for a large section of population in Asia and Africa [11. In India, almost the entire production is milled in a traditional stone mill and used to prepare unleavened pan bread (roti). A poor storage stability of the whole meal is a serious problem in the utilization of pearl millet.

The grains of pearl millet can be stored upto a year without substantial quality changes [2, 3]. However, the quality rapidly deteriorates when the grains are ground into meal [4]. Both hydrolytic and oxidative changes are reported in the lipids of the meal [4-61. Poor storage quality of the meal has attributed primarily to the hydrolytic changes associated with the action of lipase [2, 7, 8]. The lipid decomposition in the meal during storage also resulted in the loss of essential amino acids and biological value of meal proteins [9-111.

Earlier attempts to retard such deteriorative changes in lipids of the meal include development of dry milling processes that remove the major lipid- containing portions of the grain (germ and aleurone layers) from the endos- perm [12]. Applieation of dry heat, mixing of salt or antioxidants, solvent extraction of tipids, use of different containers for storage of the meal E91, and heating of the conditioned grains to inactivate lipid hydrotysing enzymes [8]. Dry milling results in a significant loss of edible dry matter, while the other methods appear to be impracticable at domestic level in the developing

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countries. In this investigation, simple heat treatments to dry seeds, adaptable at domestic level, were standardised to minimise the breakdown of lipids during storage to improve shelf-life of the pearl millet meal.

Materials and methods

Materials. The freshly harvested seeds of cultivar RHRBH-8609 were obtained from the Millet Breeder of the University and stored at 4 °C until used for experiments.

Storage containers. The seeds were ground in a laboratory stone mill to obtain about 60 mesh whole meal. The meals were stored in cotton bags, sealed polyethylene bags and tightly capped plastic boxes at ambient temperature (27 °C) and 70 to 80% relative humidity (RH) for 30 days.

Storage temperature. The fl'eshly prepared whole meals were stored in plastic boxes at ambient (27 °C) and refrigerated (5 ± 2 °C) temperature up to 30 days.

Dry heat treatments. The seeds were subjected to dry heat treatments in a hot-air oven at 100 _+ 2 °C for 60 to 120 min, rapidly cooled under a cool-air fan and milled to 60 mesh whole meal. The meals were stored in plastic boxes at ambient temperature (27 °C) and 70--80% RH up to 30 days.

Hot water blanchin9 treatment. The seeds, loosely tied in muslin cloth were dipped in boiling water at 98 °C for 10 or 20 sec, drained, dried at 40 °C to initial weight and milled to obtain 60 mesh whole meal. The meals were stored in plastic boxes at ambient conditions of temperature (27 °C) and 70-80% RH up to 30 days.

Chemical analyses. The changes in lipids in the meal were periodically monitored during storage. The fat acidity [13] was determined directly in meals while the acid value, % free fatty acids and peroxide value were estimated in petroleum either extracted tipids [14]. The changes in moisture content [13] of the meals during storage were also recorded at the initial (0 day) and final (30 days) stage of storage. Data obtained were analysed for least significant difference (LSD at 5%) using factorial completely randomised design [15].

Results and discussion

Effects of storage container. The lipids in the meal stored at ambient conditions were found to undergo a rapid hydrolytic breakdown during storage as evidenced by a significant increase in fat acidity of the meal, and acid value or

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% free fatty acids (% FFA) in the extracted oil (Table 1). The peroxide value of the extracted oil increased up to 5 days of storage and later decreased. The decrease in peroxide value can be attributed to a loss of volatiles during extended storage that are measured as peroxides by the procedure [2]. The storage containers exhibited a significant influence on the lipolytic and oxidative decomposition of lipids. Lipid degradation was the highest in the meal stored in cotton bags while such changes were found to be lower for the meal stored in plastic boxes or polyethylene bags under similar conditions. The initial moisture content of 10% was found to increase to 10.9% for the meal stored in cotton bags while it did not change in the meals stored in polyethy- lene bags or plastic boxes.

Effects of storage temperature. Storage temperature exhibited a significant influence on both hydrolytic and oxidative stability of meal lipids. The lipolytic decomposition measured in terms of fat acidity (LSD 0.514), acid value (LSD 0.243) or % FFA (LSD 0.052) was found to be significantly lower in the meal stored at 5 °C as compared to the meal held at 27 °C (Table 2). Peroxide accumulation increased up to 20 days of storage in the meal stored at 55 °C, however the differences in mean peroxide values were nonsignificant.

Effect of dry heat treatment to seeds. Heating of grains before milling resulted in decrease in the fat acidity while it increased the peroxide value of oil from fresh meals (Table 3). During storage of meals at 27 °C however, the fat acidity, acid value and % FFA were found to increase significantly in both unheated

Table 1. Effects of containers on changes in the lipids of pearl millet meal during storage

Container Storage period (days) LSD at 5%

0 5 10 15 20 30

Fatty acidity (mg KOH/100 g meal) Cottong bag 13.2 85.2 109.3 159.4 237.6 234.4 Container 0.704 Polyethylene bag 13.2 61.2 96.6 143.5 187.4 230.9 Period 1.000 Plastic box 13.2 66.9 89.5 123.6 143.3 209.4 Interaction 1.730

Acid value (mg KOH/g oil) Cotton bag 1.6 8.7 13.1 20.1 24.4 39.2 Container 0.215 Polyethylene bag 1.6 7.4 11.8 16.3 20.1 28.7 Period 0.305 Plastic box 1.6 6.4 10.4 14,7 18.3 25.6 Interaction 0.528

Free fatty acids (%) Cotton bag 0.9 4.3 6.6 10.0 12.0 19.7 Container 0.075 Polyethylene bag 0.9 3.7 5.9 8.2 10.1 14.5 Period 0.t06 Plastic box 0.9 2.9 5.2 7.4 9.4 12.6 Interaction 0.183

Peroxide value (meq/kg oil) Cotton 1.5 5.3 4.5 3.2 2.9 2.5 Container 0.112 Polyethylene bag 1.5 13.8 6.2 5.7 4.2 3.8 Period 0.158 Plastic box 1.5 7.7 5.7 4.5 3.3 3.0 Interaction 0.275

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Table 2. Effects of temperatures on changes in the lipids of pearl millet meal during storage

Storage Storage period (days) temperature (in °C) 0 5 10 15 20 30

LSD at 5%

Fat acidity (rag KOH/100 g meal) 27 _+ 2 13.2 53.5 89.5

5 + 2 t3.2 22.4 24.6

Acid value (mg K O H / g oil) 27 4- 2 1.6 8 2 t0.4

5 ± 2 1.6 3.4 4.1

Free fatty acids (%) 27 + 2 0.9 3.9 5.2

5 + 2 0.9 1.5 2.1

Peroxide value (meqNg oil) 27 _ 2 1.5 7.7 5.7

5 _+ 2 1.5 3.2 4.1

123.6 143.3 209.4 Temperature 0.514 Period 0.890

29.7 34.9 48.4 Interaction 1.250

14.8 18.3 25.6 Temperature 0.243 Period 0.420

4.5 5.0 5.9 Interaction 0.594

7.4 9.4 12.6 Temperature 0.052 Period 0.089

2.7 3.2 4.4 Interaction 0.126

4.5 3.3 3.0 Temperature 0.13 Period 0.225

5.4 7.6 4.6 Interaction 0.319

or heated samples. The rate of increase in fat acidity, acid value or % FFA in the meal obtained from heated grains was, however, 3 to 4 fold lower than the meal from unheated grains. Heating of grains for 120 min was found to be most effective for maximum retardation of the lipolytic decomposition of tipids during storage. Although the peroxide level was higher in fresh meals of dry heated grains, it was found to decrease subsequently during storage in all the meals. Dry heating of grains decreased the moisture content from 10 to 6.5% which do not change subsequently during storage.

Effect of boiling water blanching treatment to seeds. The dry grains subjected to blanching treatment for 10 or 20 sec before milling produced results similar to that of dry heating of grains at 100 °C for 120 min. The values of fat acidity, acid value or % FFA for meals obtained from boiling water treated grains were 3 to 4 fold lower than the meal from untreated grains (Table 4). Boiling water treatment to grains resulted in uptake of 5% additional moisture which was reduced to the initial level by drying of blanched grains at 40 °C for 2 hours. These results clearly suggest that the undesirable lipid changes in the pearl millet meal can be effectively minimised by a simple boiling water treatment of the grains at 98 °C for 10 sec before milling.

Lipase located in the germ and surface layers of the grain gets mixed throughout the meal during milling and decomposes the meal lipids into free fatty acids during storage [4]. This is evidenced by a rapid increase in fact acidity of the stored meal, and acid value or % FFA of the meal oil [2, 10, 11]. The role of peroxidation in spoilage of pearl millet meal during storage has been debated [2]. The values of fat acidity, acid value or peroxide value of

Table 3. storage

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Effects of dry heat treatments to the seeds on changes in lipids of pearl millet during

Heat treatments Storage period (days) LSD at 5%

0 5 10 t5 20 30

Fat acidity (mg KOH/100 g meal) Unheated (control) 13,7 60.6 108.0 147,4 201.3 267.6 Heat treatment 0.725 Heated at 100 °C Storage period 0.795

60 rain 9.5 20.7 35.3 46.6 60.4 82.4 Interaction 1.770 90 rain 6.1 18.5 28.7 33.6 42.5 61.4

120 rain 8.4 14.5 21.5 26,9 32.7 46.0

Acid value (mg KOH/g oil) Unheated (control) 1,6 8.2 11.5 18.3 23.0 31,9 Heat treatment 0.264 Heated at 100 °C Storage period 0.289

60 rain 1.9 3.4 5.4 7.2 9.6 10.4 Interaction 0.048 90 min 1.9 2.6 4.1 4.8 6.3 8.4

120 min 2.0 2.3 3.1 4.6 5.9 7.1

Free fatty acids (%) Unheated (control) 0,82 4.5 6.5 8,7 11.7 16.9 Heat treatment 0.095 Heated at 100 °C Storage period 0.105

60 rain 1.0 1.7 2.7 3.1 4.8 5.6 Interaction 0.234 90 rain 1.0 1.3 2.1 2.4 3.5 4.2

120 rain 1.0 1.2 1.6 2.3 3.0 3.7

Peroxide value (meq/kg oil) Unheated (control) 1.7 6.4 6.0 3,7 2.7 2.1 Heat treatment 0.144 Heated at 100 °C Storage period 0.158

60min 10.4 10.1 6.8 5.6 4.7 3.5 Interaction 0.353 90 rain 10.8 9.4 8.6 6.4 4.3 3.7

120 min 10.2 10.1 9.4 7.6 5.2 4.2

fresh, and s tored con t ro l meal samples ob ta ined in the present s tudy concur wi th the l i te ra ture [2, 10, 113.

Since the l ipase act ivi ty is a ma jo r cause of spoi lage of pear l millet meal, its inac t iva t ion before mil l ing is essential. A m o n g the conta iners , c lo th bags a l low free access to a tmospher i c mois tu re resul t ing in a r ap id hydro ly t i c l ipid

decomposition [23. Both sealed polye thylene bags and plast ic boxes offer par t ia l p ro tec t ion f rom such agents. The use of plast ic conta iners m a y be more convenien t and economica l for bo th small or large-scale s torage of the meal. The app l i ca t ion of d ry hea t to the meal effectively re ta rded the l ipase act ivi ty and minimised l ip id decompos i t i on dur ing s torage [9]. I t is more convenient to hea t seeds than meal. Such a hea t t r ea tment needs to be mi ld to avo id de t r imen ta l effects on o ther nut r ients in the seeds. Hea t ing of mois t grains of p roso millet at 97 °C for 12min, p roduced a meal with improved shelf-life wi thou t adverse effects on the funct ional p roper t ies [8]. The use of boi l ing water t r ea tmen t reduces the hea t ing t ime to 10 sec to ob ta in s imilar results for pear l millet. The boi l ing water t r ea tmen t to dry seeds is simple, economica l and a d a p t a b l e for bo th domes t ic as well as large-scale processing.

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Table 4. storage

Effects of hot water blanching of the seeds on changes in the lipids of pearl millet during

Storage period (days) LSD at 5%

0 5 10 15 20 30

Fat acidity (mg KOH/100 g meal) Unblanched (control) 13.7 60.6 108.0 147.4 201.3 267.6 Blanching 0.772 Blanched at 98 ± 2 °C Storage period 1.020

10 sec 8.7 9.5 31.3 32.5 41.4 71.8 Interaction 1.770 20 sec 10.6 11.7 30.3 33.2 45.6 80.2

Acid value (rag KOH/g oil) Unblanched (control) 1.7 8.2 11.5 18.3 23.0 32.0 Blanching 0.337 Blanched at 98 _+ 2 °C Storage period 0.477

10 sec 1.2 4.1 4.7 5.2 7.4 8.5 Interaction 0.827 20 sec 1.4 4.4 4.3 5.9 8.4 9.1

Free fatty acids (%) Unblanched (control) 0.8 4.9 6.5 8.7 11.7 16.8 Blanching 0.146 Blanched at 98 i 2 °C Storage period 0.207

10 sec 0.6 2.1 2.1 2.7 3.1 4.1 Interaction 0.358 20 sec 0.7 3.1 2.2 2.4 3.6 4.6

Peroxidase value (meq/kg oil) Unblanched (control) 1.7 0.4 6.0 3.7 2.7 2.1 Blanching 0.172 Blanched at 98 _+ 2 °C Storage period 0.243

10sec 7.1 3.0 4.9 4.8 3.1 2.3 Interaction 0.422 20 sec 5.9 4.9 3.8 3.6 2.8 2.6

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