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Annals of Agri-BioResearch

l0(l) : 93-95,2005

Equilibrium Relative Humidity (ERrr) Studies of Guava Cheese SURABHI RAI, VIJAY SETHI AND IC S. JAYACHANDRAN Dlvision of Post-Harvest Technology, Indian Agrlcultural Research Instltute, Pusa, New Delhi-l10 012, India

ABSTRACT Measurement of water activity is essential to determine the optimum storage conditions of any product. ERH studies of guava cheese revealed that optimum RH for storage was 68.3%. The critical moisture content of the productwas23.5o/o having 79.6o/oRLl and beyond this point hte quality of the product deteriorated as the product became soft, darker and mould growth began. Key words : ERH, water activity, a*, fruit cheese

INTRODUCTION Water activity is a critical factor that detennines shelf life. While temperature, pH and several other factors can influence if and how fast organisms will grow in a product, water activity may be the most important factor in controlling spoilage. The water activity of a product can be determined from the relative humidity of the air surrounding the sample when the air and the sample are at equilibrium. Therefore, the sample must be in an enclosed space where this equilibrium can take place. Once this occurs, the water activity of the sample and the relative humidity of the air are equal. The measurement taken at equilibrium is called an equilibrium relative humidity or ERH. In the present study, the ERH studies of guava cheese were carried out to determine the optimum storage RH as well calculate the packaging material requirement.

MATERIALS AND METHODS Guava cheese was prepared according to the procedure given by Jain et al. ( 1954). Equilibrium relative humidity of guava cheese was determined by the weight equilibrium method (Wink, 1946). Two grams of cheese were weighed in small aluminium dishes and these dishes were exposed to different relative humidities, ranging from 0-100%at ambient temperature. The gain or loss in weight of the product under each humidity was determined at24h intervals. The observations were recorded for product appearance, discolouration and presence ofmold growth, etc. to find out the critical moisture content, danger point and ideal RH for storage. The selection of packaging material was done according to the required quantity of the material to be packed for a certain period of storage at a particular temperature and relative humidity as per the standard procedure (Ranganna, 1997).

RESULTS AND DISCUSSION The relationships between the equilibrium moisture content, number of days the product took to equilibriate at a particular relative humidity and the general condition ofthe product at different


Rai, Sethi and Jayachandran

relative humidities for the guava cheese are presented in Table l. The initial moisture content of guava cheese was 19.36%o. It can be seen from the moisture equilibrium curve @ig. l) that mould growth took place at relative humidity of 92%oand above.l spergillus spp. and Penicillium spp. were found to be the predominant moulds. The critical moisture content of the product Table


Relationship between the equilibrium moisture content and time of equilibration at different relative humidities forguava cheese at ambient temperature (l1.5 to 12.5"C) (Initial moisture content l%60$

Equilibrium Equilibrium Number of dals

relative humidity

moisture content


required to reach









8.76 t0.21

49 46






32 29




20.4 23.5 25.9




t4 t2

Product very hard, britrle, cracks all aroun4 fading ofcolour. Product very hard, brittlg cracks all arormd, fading ofcolour. Product very hard brittle caracks all aroun4 fading ofcolour. Product very har4 britle, cracts all arormd, fading of colour. Product had good te><hre but colour was frding Product had good colour and texture. Product had good texture but there was dartening ofcolour. Product became soft and darker. Prodcut became very soft and mould growth occurred after l0 days.


Product became semi-solid after three days and mould growth occurred after seven days.

a o

o o o o tr


MG : Mould Growth CP : Critical point


DP : Danger point


43.7 52

68.3 71.4


Equilibrium relative humidity (%) Fig. l. Humidity moisture equilibrium curve for guava cheese.

ERH of guava cheese


was23.5o/o having 79.6% RH and beyond this point the quality of the product deteriorated as

the product became soft, darker and mould growth began. Danger point was less by 5% relative humidity (20.4yo). The optimum relative humidity for the storage of guava cheese was found to be 68.3%. From the mathematical calculations presented in Table 2, the permeability limit (water vapour transmission) ofthe packaging material required for storage of guava cheese was 28.64 X l0{ cclcm2lsec/cm of Hg at 250C. The permeability limits indicated that guava cheese could be safely packed in a wide range of packaging materials like low density polyethylene,

high density polyethylene, polypropylene, etc. Table 2. Procedure for selection ofpackaging material for guava cheese

: : :

Initial moisture content of guava cheese Critical moisfure content Danger point Suppose it is required to pack the guava cheese


150 gram units to

65%RH. Permissible uptake of moisture by guava cheese by weight Permissible moisture uptake I50 grams guava cheese Thc water vapour pressure (assumed) outside the package The ERH atl9.36o/omoisture level The ERH at 23.5olo moisture level AverageERH Assuming average vapour pressure differential @) between and outside

ofthe package

19.360/o (690lo


(79.60% RH) 20.4o/o(79.60% RH) 23.5o/o

withsand 356 days of storage at 25oC and

: :

at25"C : = =

: : inside

23.5-19.36:4.t4vo (4.14x l50yl00-6.21g 650/o

saturation vapour pressure

79.60% 650/o

(79.60+65)D:72.3o/o 72.3-65:7 .25Yo of the saturation vapour pressure at25"C 2.378 cm (Rangannu 1997) 0.171216

The safuration vapour pressure at25"C = Vapour pressure differential is 7 -2o/o of 2.378 cm = The permeability (P) of film (gas or water vapour transmission of film) can be calculated by using the expression, P1/[At (P,-Pr)] Where,

q:quantity ofwater vapour or gas in cc t==time of storage in seconds P,-Pr:vapour pressure differential expressed in cm ofHg


area of package in cm



l50x l20x 24x60x60 x0.1735 Water vapour transmission of the package (PF28.64

x l0{

cc./cm2/sec/cm of Hg at 25oC

REFERENCES Jain, N. L., Das, D. P. and Lal, G. (1954). Preparation of guava cheese. Chem. Age of India Series 9 : 88. Rangann4 S. (i997). Handbook of Analysis Quality Control for Fruit and Vegetable Products, 2nd edn. Tata McGraw Hill Publishing Company Pvt. Limited, New Delhi. Winh W. A. (1946). Determining the moisture equilibrium curves of hygroscopic materials. Ind. Eng. Chem. Anal. 18:251.

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