Effect of Particle Size on Surface Properties of Apple Pomace

Abstract

Surface properties of dried apple pomace as a function of particle size were studied in order to increase it's acceptability and utilization as source of soluble dietary fiber in processed foods. For this cleaned, dried ground pomace was sieved through 30 mesh and 50 mesh sieves to obtain 30 mesh, 50 mesh pomace and unsieved pomace was used as control. Particle size had definite effect on surface properties such as hydration capacity, fat absorption, and emulsifying properties but not much effect on whipping properties and buffering capacity (BC).

Keywords:

• Particle size
• Surface properties
• Apple pomace
• Hydration capacity
• Fat absorption
• Whipping properties

Introduction

Apple (Malus domestica) is a temperate region fruit. Commercially apple is used mostly for extraction of apple juice. Apple pomace is the solid material that remains after extraction of juice from apple. A conventional process removes 75% of fresh weight of fruit as juice and 25% comes out as pomace.[1],[2] More than 500 fruit processing plants in USA produce a total of 1.3 million metric tons of pomace every year which involves annual disposal fees that exceed $10 millions. Most of apple pomace is not being utilized at present for human consumption but is dumped as such in fields which creates pollution problem because of uncontrolled fermentation and high COD (250–300 g/kg).

Utilization and composition of pomace have been evaluated by a number of research workers.3–8 Acceptability of apple pomace depends on its functional as well as nutritional properties. Nutritional and compositional properties have been studied earlier.[8] No information is available about functional properties of apple pomace. The present investigation was undertaken to determine the hydration and surface properties of apple pomace of different particle size (unsieved, 30 mesh and 50 mesh.)

Materials and Methods

Materials

Dried apple pomace was obtained from Apple Juice Concentrate Plant, Jammu and Kashmir (JK HPMC). Seeds and other extraneous materials were removed from pomace. It was ground in Hammer mill (Fitz) and passed through 30 mesh and 50 mesh sieves, a part of sample was kept unsieved. The material was stored in HDPE containers at refrigeration temperature (4–5°C) till used. The compositional constituents of apple pomace of different particles size used in present study have been given earlier.[9]

Methods

Hydration and Surface Properties

Water hydration capacity (WHC) was determined according to the method of Quin and Paton.[10] Weighed quantity (5 g) of sample was transferred to a centrifuge tube (50 mL) and weighed the tube with sample. Then distilled water was added in unmeasured increments till the pasty consistency was achieved. It was followed by centrifugation at 4000 rpm for 10 min. Supernatant if any was discarded and the tube was weighed again. Water hydration capacity was measured as mL of water absorbed by per g of pomace material.

Fat Binding Capacity

Fat binding capacity (FBC) was evaluated by the method of Lin et al.[11] To determine FBC, weighed quantity of sample (0.5 g) was thoroughly mixed with 3 g of soy oil in centrifuge tube (15 mL) and allowed to stand for 30 min followed by centrifugation at 1610 × g for 25 min. Then the volume of free oil was measured. Fat absorption was expressed as the amount of oil bound by 100 g of sample.

Emulsifying and Whipping Properties

Emulsifying and whipping properties were measured by method of Yasumatsu et al.[12]

(I) Emulsifying Properties

Emulsifying properties were evaluated in terms of emulsion activity (EA) and emulsion stability (ES).

i.

Emulsion Activity. For EA, a known quantity (7 g) of pomace preparation was suspended in 100 mL distilled water and added 100 mL of soybean oil followed by emulsification of mixture in a homogenizer at 10,000 rpm for 1 min. The emulsion obtained was centrifuged at 1300 × g for 5 min. The emulsion activity was calculated as the ratio of height of emulsion layer to the total emulsion height and expressed as %.

ii.

Emulsion Stability. Emulsion stability was measured as EA after heating emulsion at 80°C for 30 min. The ratio of height of remaining emulsion to the total height of emulsion was expressed as % ES.

(II) Whipping Properties

Whipping properties includes foam expansion and foam stability.

i.	Foam Expansion. Foam expansion was measured by shaking 50 mL aqueous suspension (1%) of apple pomace horizontally (6 cycle per second with 5 cm amplitude) for 1 mm in 100 mL stoppered measuring cylinder. The resulting foam volume was measured and expressed as foam expansion (%) or foam activity (FA).
ii.	Foam Stability. Foam stability (FS) was measured by measuring the foam volume after allowing the foam to stand for varying period of time.

Buffering Capacity

Buffering capacity (BC) was estimated as described by Sathe and Salunkhe.[13] Sample was dispersed in 100 mL distilled water to form 0.5% pomace dispersion. Initial pH was recorded and the dispersion was titrated to a desired pH by using 0.1 HCl or 0.1 N NaOH.

Results and Discussion

Water Hydration Capacity and Fat Binding Capacity

The results presented in Table 1 revealed that (WHC) and (FBC) both decreased significantly (P ≤ 0.05) with the decrease in particle size. Unsieved pomace exhibited a value of 4.3 g/g and 355% for WHC and FBC, respectively. The corresponding values for 50 mesh were 3.9 g/g and 260%. All three samples differ significantly (P ≤ 0.05) with each other for WHC but for FBC and the significant decrease was observed between unsieved and 50 mesh size sample.

Table 1. Effect of particle size on WHC, FBC, EA, ES, FA, and FS

Particle size	WHC (g/g)	FBC (%)	EA (%)	ES (%)	FA (%)	FS (%)
Unseived	4.3	355	39.6	38.4	7.0	0
30 Mesh	4.0	327	43.8	40.2	8.5	0
50 Mesh	3.9	260	49.6	47.3	11.0	0
CD at 5%	0.25	31.5	7.6	6.4	1.43	—
SEM	0.05	7.07	1.72	1.4	0.32	—

Note: All the results are average of two replications.

Emulsifying Properties

The results showing the emulsifying properties are presented in Table 1. From results it can be revealed that EA as well as ES increased significantly (P≤ 0.05 ) with the decrease in particle size. The results indicated that during reduction in particle size exposure of some hydrophobic molecules increased and that of hydrophilic decreased as it is depicted by the results of WHC also. The value of EA and ES increased from 39.6 and 38.4 for control to 49.6 and 47.3 for 50 mesh, respectively. The unsieved and 30 mesh sample showed non‐significant (P ≥ 0.05) increased with respect to EA as well as ES but 50 mesh sample differed significantly (P ≤ 0.05).

Buffering Capacity

Results of BC are shown in Table 2. All samples showed higher BC below pH 3 and above pH 9. Original pH of apple pomace was 4.2. However, 30 mesh pomace showed minimum BC at pH 5 while 50 mesh and Unsieved had minimum BC at pH 4. All samples showed greater BC at pH above 10. The change in BC of all the samples up to pH 6 was non‐significant (P ≥ 0.05) but at pH above 7, 30 mesh sample exhibited significantly (P ≥ 0.05) higher BC than unsieved and 50 mesh sample.

Table 2. Effect of particle size on BC of apple pomace

	pH
Particle size (mesh no.)	2	3	4	5	6	7	8	9	10	11	12
Unsieved	30.80	3.07	0.13	0.29	1.08	2.02	3.80	6.34	8.82	17.87	181.58
30 Mesh	29.55	2.76	0.10	0.10	0.65	4.03	5.93	8.26	11.37	20.40	193.44
50 Mesh	31.67	3.37	0.09	0.22	0.72	2.35	3.65	5.57	9.98	18.77	185.64
CD at 5%	1.84	0.95	0.12	0.12	0.48	0.429	1.15	1.63	1.49	1.75	5.06
SEM	0.412	0.214	0.226	0.26	0.108	0.096	0.259	0.36	0.335	0.394	1.13

Note: All the results are average of two determinations.

Whipping Properties

The results of whipping properties of apple pomace are shown in Table 1. Whipping properties including FA and FS were affected to a lesser extent by particle size. Fifty mesh sample had maximum FA (11%) and unisieved had minimum FA (7%), however, difference was significant (P ≥ 0.05). Foam was unstable and disappeared within few minutes so FS was considered almost zero.

Conclusion

Particle size has distinct effect on surface properties of apple pomace. Fat binding capacity and emulsifying properties were affected to a greater extent by particle size than whipping and BC. Effect on hydration properties was intermediate. However, these properties of apple pomace are expected to vary with apple variety used, storage conditions, method of separation of pomace etc. These results will be useful in formulating products whenever, apple pomace is planned to be used as source of dietary fiber.