Microbiology Project Topics

The Effects of Different Processing Techniques on the Organoleptic Quality of Soymilk Processing and Storage

The Effects of Different Processing Techniques on the Organoleptic Quality of Soymilk Processing and Storage

The Effects of Different Processing Techniques on the Organoleptic Quality of Soymilk Processing and Storage

Chapter One

Aims and objectives of the project    

The processing of soybeans into soymilk is aimed at gaining consumer acceptance of the legume by removal of the toxicants that contain and also improving organoleptic qualities of soymilk with special consideration to some adverse effect of these operation in soymilk  quality.

This project re views the effects of different methods used in the processing of soybeans into soymilk on the quality of the milk produced during processing and storage.



Soybean (Glycine max) belongs to the family of Leguminoseae. It is about the most utilized legume as well as the most well researched and health-promoting food material in the world today (William and Akiko, 2000, Akubor et al. 2002). This can be attributed to its high nutrient content and low cost. Soybean is processed into a variety of food items, feed and industrial products which include soymilk, soy flour, soy meal, soy oil, soy infant formula, soy cultured products (soy ice-cream, soy yoghurt, tempeh, soy cheese), soy-based meat substitute, textured protein, soy dog foods, soap, cosmetics, biodiesel (Liu, 1997; Endres, 2001; Giampiatro et al. 2004; Merritt and Jenks, 2004; Hoogenkamp, 2005; Riaz, 2006; Anonymous, 2008). Soymilk is a water extract of whole soybeans at a bean/water ratio of between 1:5 and 1:10. It is an off-white emulsion or suspension containing the water soluble proteins and most of the oils of the soybean. It doesn’t contain lactose and therefore suitable for lactose-intolerant individuals (Nelson et al, 1971; Osundahunsi et al. 2007; Sanful, 2009; Anonymous,

2012).Unlike some other beans, soybean offers a complete protein profile and polyunsaturated fatty acids (Henkel, 2000; Lindsey, 2012). The Protein Digestibility Corrected Amino Acid Score (PDCAAS) which is the standard for measuring protein quality rated soy protein to be nutritionally equivalent to meat, eggs and casein with soy protein isolate having a biological value of 74, whole soybeans 96, soymilk 91 and eggs 97 (FAO/WHO, 1989). For this reason, soy products are good alternatives for vegetarians and vegans (Osundahunsi et al, 2007; Sanful, 2009; Anonymous, 2012). Various processing techniques (Hauman, 1984; IITA, 1987; INTSOY, 1987) as well as associated numerous health benefits of soy products derived from its content of bioactive, functional and phytochemical ingredients such as natural phenols, phytic acid, polyunsaturated fatty acids (predominantly omega-6), glyceollins, and others have been reported (Nelson et al. 1971; Yoon et al. 1983; Anderson et al. 1995; Kriz-Silverstein et al. 2003; Vucenik and Shamsuddin, 2003; Sudheer et al. 2004; Symolon et al. 2004; Sacks et al. 2006; Hogervost et al. 2008; Jenkins et al. 2010; Santo et al. 2010; Anonymous, 2011). Soymilk, like other plant protein has poor consumer acceptability. However, the principal reasons for the poor acceptability of soy products are beany off-flavours and flatulenceinducing oligosaccharides namely starchyose and raffinose (Buono et al, 1990; Osundahunsi et al, 2007). This objectionable flavour is as a result of some ketones and aldehydes, particularly hexanals and heptanals, produced through endogenous lipoxydase-catalysed oxidation of soybean oil. These compounds are not contained in the whole soybean but are produced as soon the bean is wetted and ground. Research efforts have been deployed towards combating this off-flavour through both genetic and processing.

Legumes generally, including soybean contain potentially toxic and anti-nutritional factors such as saponins, phytohaemaglutinins, protease and amylase inhibitors, etc., which must be inactivated or destroyed by heat before usage (Giami and Bakebain, 1992; Soetan and Oyewole, 2009; Akande et al., 2010). Considering its unique nutritional and health benefits, soy products, e.g. soymilk, have a utility role to play in addressing malnutrition in poor sub-regions of the world as an inexpensive substitute for animal products (INTSOY, 1987; Nsofor and Maduako, 1992; Nsofor and Osuji, 1997; Baghei et al, 2008; Akubor, 2003; Hoogenkamp, 2005).

The objective of this research, therefore, was to evaluate the extent to which various processing technique qes would affect the nutrient content, sensory qualities and andorganoleptic acceptability of soymilk.





Soybean was purchased from mile 1 market and kept at ambient temperature prior to usage. They were analyzed within a day of purchase.


Soymilk was prepared using two methods modified from Illinois method.

Method 1: soybean was sorted to remove stones and damaged, deformed seeds. The soybean was washed and soaked in water (500g in 1 Liter) for 12 hours. It was rinsed and blanched in 1.25% NaHCO3 for 30 minutes. The soybean was washed, manually dehulled and rinsed. The soybean seeds were ground in blender (kenwood) and expressed in the ratio of 3:1 to remove the okra. The resultant slurry was formulated by adding 0.1% of sodium benzoate and 0.1% potassium sorbate, 2% sucrose and propy gallate and Ascorbic pamitate at this ratios: 100ppm Ascorbic palmitate and 100ppm propyl gallate, 200ppm Ascorbic palmitate, 200ppm propyl gallate and Control (without preservative and antioxidant). The milk was heated at 71oC for 15 seconds and subsequently bottled and stored at ambient and refrigeration temperature.



In this study, sample A was NaHCO3 Soymilk treated with Propyl Gallate, sample B was NaHCO3 Soymilk treated with Ascorbyl Palmitate, sample C was NaHCO3 Soymilk treated with both Propyl Gallate and Ascorbyl Palmitate, sample D was NaHCO3 Control, sample E was Na2CO3 Soymilk treated with Propyl Gallate, sample F was Na2CO3 Soymilk treated with Ascorbyl Palmitate, sample G was Na2CO3 Soymilk treated with both Propyl Gallate and Ascorbyl Palmitate while sample H was Na2CO3 Control.




Soymilk is an idea medium for bacteria growth and hence and hence a thermal treatment is necessary to extend its shelf life. Heat processes are involved at several stages during soymilk preparation, including the pre-treatment of beans and extraction to produce the soymilk, followed by either pasteurization or sterilization to increase its shelf life. By controlling the microbiology of the product and packaging it in appropriate containers, the shelf life of soymilk can be greatly extended and the product can be distributed over a wider area.

Proximate analysis of the “soy milk” from each variety of soybeans shows that averagely it has a water content of 91.24, Protein of 2.74, fat content of 2.08, Ash content of 0.32 and Carbohydrate of 2.26 this agrees with the findings of Nelson et al. (1978) and Gesinde et al. (2008) with the exception of protein and carbohydrate content which is lower in their own report. This may be due to the variety of soybean used, the method of extraction and other equipments used (Gesinde et al., 2008). It has been reported that soybean varieties greatly affect the protein content and colour of soymilk (Min et al., 2005, Gesinde et al., 2008). Soy milk prepared from beans pre-soaked in NaHCO3 contained more protein and a higher viscosity than milk prepared from beans pre-soaked in Na2CO3. Soaking with NaHCO3 as well as blanching gave a higher protein content of soymilk because soaking gives a tender product which results in finer slurry and thus more filtrate will pass thru the filter cloth thereby increasing yield and subsequently the protein content of soymilk (Bourne, 1976). The low protein value for Na2CO3 method can also be because Na2CO3 reacts with protein and forms a complex, which reduces the protein availability of produced soymilk (Tunde-Akintunde and Souley, 2009).

The higher ash content of NaHCO3 could be due to more mineral being extracted in soymilk due to the action of the acid. These values are comparable to Onuorah et al. (2007) findings. The higher moisture content of Na2CO3 could be as a result of coagulation of protein and hereby restricting more water expulsions from the cake (Bourne, 1976). The carbohydrate content of NaHCO3 milk was higher and similar to that reported by Wikens et al. (1967). The fat content of NaHCO3 was higher and it correlated with the report obtained by Adetunji et al. (2006). Farinde et al. (2008) suggested that the total solid of soymilk could be improved by adding soybean flour to the soymilk. Increasing the total solid increases the nutritive value of the product thereby improving the keeping quality.

The sensory properties showed that sample stored at refrigeration temperature maintained good quality up to 16 days storage while samples stored at room temperature were of poor quality by the 4th day. Preservation of soymilk by refrigeration has been shown to be relatively effective in retarding microbial growth. The shelf life of pasteurized milk products subjected to ultra high temperature (UHT) is usually extended especially if adequately stored (Saidu, 2005). This can be done through the use of chemical preservatives to minimize food spoilage. In economically under developed countries, lack of functional storage facilities and the inadequacy of transportation and communications may increase the necessity of using certain food additives for purpose of preservation. In tropical regions, high temperature and humidity favour microbial attack and increase the rate of development of oxidative rancidity. Food additives might be used to supplement the effectiveness of traditional methods of food preservation rather than to replace these methods (Alais and Linden, 1999 cited in Egbo, 2012).

The sensory attributes of perceived color and flavor are the most important characteristics in soymilk because they are readily assessed by consumers. Soymilk when subjected to severe heating acquires a brown color and cooked flavor (Kwok et al., 2000 cited in Egbo, 2012). Kwok and Niranjan (1995 cited in Egbo, 2012) have demonstrated the effects of thermal processing on the quality of soymilk and concluded that the main chemical reaction that gives rise to heat-induced color and flavor changes is the maillard reaction. Most work done on soybean products made reference to future research to be done to improve colour, taste and aroma of soybean products either through flavour additives and heat treatments (Farinde et al., 2008; Ikpeme et al, 2009). It has been reported that the use of preservatives also reduced Staphylococcus aureus population in soymilk to less than hazardous level at the end of ambient storage and lag-phase periods were extended by the preservatives resulting in longer shelf life (Nwanebu, 1994 cited in Egbo, 2012).

The sensory evaluation shows that sample produced with Na2CO3 has a higher preference for smell and taste because the methods reduced the beany flavor as reported by Liu (1997 cited in Egbo, 2012). However these samples have the lowest preference in terms of colour. Tunde-Akintunde and Souley (2009) reported that from their results, they noticed that the sensory properties of soymilk increased with decrease in nutritional quality indicating that methods which increase sensory properties of soymilk by reducing its beany flavor have lower nutritional qualities. Wikens et al. (1967) found that the off-flavours of soymilk were present in the dry soybean but were formed during the processing and that blanching the beans in hot water prevented the formation of the strong beany flavours. They attributed this result to the rapid heat inactivation of the lipoxidase in the soybean precluding its attacking the unsaturated fatty acid chains in the soybean oil to form a number of lower molecular weight compounds that have objectionable flavor impact. Lipoxygenase catalyze the hydroperoxidation of polyunsaturated lipids in the presence of molecular oxygen and the primary products are hydroperoxide. The volatile carbonyl compounds including aldehydes, ketones and alcohols are partly responsible for the objectionable odor and flavor in soymilk. During the preparation of soymilk, soybean is ground with water and the LOX activity is greatly enhanced when the soybean is damaged or crushed. Therefore the inactivation is carried out at a higher temperature of 80-100oC during the preparation of soymilk. Inversely at these temperature, protein molecules are denatured (Prabhakaran, 2005), therefore other methods (antioxidants) are sort to complement the use of high temperature.

Although soymilk is a potential substitute for cow milk and could be used for solving malnutrition problems in developing countries, its utilization is hampered by a number of factors. However, acceptability of soybean products has been enhanced by modification of processing methods. Some of the modified soymilk extraction methods include application of heat, soaking of soybean in ethanol or alkali and acid grinding (Iwe, 2003). Kolapo and Oladimeji (2008) reported on the use of natural flavourants to improve soymilk acceptability. It has been reported that pasteurization of vegetable milk extract at 121oC for 15 minutes effects maximum destruction of microorganisms and made the products microbiologically safe (Onweluzo and Nwakalor, 2009). The effect of pre-soaking soybean in solutions of various chemicals on the reduction of beany flavor in soymilk was investigated. Na2CO3 had a significant effect on the reduction of beany flavor in soymilk. Na2CO3 soaking at 1.25% for 12 hours was significantly better than NaHCO3 pre- soaking treatment. Beans pre-soaked in carbonate were easier to process than NaHCO3 (Khaleque et al, 1970).

The shelf life of the soy milk produced at room temperature was up to 4 days, this deviated from the average shelf life reported generally for most milk and milk- based products. This is the reason why several workers are exploring the use of chemical preservatives for prolonging the shelf life of milk products (Sumati and Shalini, 1986; Uriah and Iwagbe, 1990; Gesinde et al., 2008). Statistical analysis of the data on the organoleptic assay showed that there was no significant different among the variety for sweetness of their soymilk. There were no significant difference (P>0.05) in the color, odor and texture of the soy milks treated with NaHCO3 and Na2CO3. Benzoic and sorbic acid are among the most commonly used as preservatives in their salt forms (Sodium or potassium) (Wibbertmann et al., 2005). Potassium sorbate has been reported to be more effective against moulds involved in spoilage of foods at pH of 4.0 to 6.0 than against the bacterial flora especially lactic acid. Sodium benzoate has been reported to be less effective than potassium sorbate against moulds though the two preservatives are capable of inhibiting aerobic and catalase positive bacteria such as Staphylococcus aureus, coliforms and psychotropic spoilage bacteria (Nwanebu, 1994 cited in Egbo, 2012; Wibbertmann et al., 2005).


Soybean can be processed using various techniques into different products. Generally, processing treatment significantly affected the quality of the products. The milk products were highly acceptable. Fermentation increased the protein content of the product. Variations in chemical values of the samples were a function of processing treatment. The products were microbiologically stable during storage. the samples had no growth of microorganisms throughout storage. Based on these results, an acceptable standard procedure can be developed for processing soybean into various products with an effective process control programme.


The effect of certain preservatives at various concentrations within their maximum permissible levels along with pasteurization and refrigeration storage on the microbial keeping quality of soymilk used showed that soymilk samples blanched in NaHCO3, pasteurized at 75oC for 15 seconds and then formulated with 0.1%potassium sorbate and sodium benzoate in addition with either 200ppm propyl gallate or 100ppm propyl gallate and ascorbyl palmitate, gave soymilk of high microbial quality and shelf life stability. Study on the suspension stability of soymilk should be considered as the soymilk separated during the period of study unlike the most market soymilk which is relatively stable. Also other method of preserving soymilk, so as to extend its shelf life should be studied (carbonation).

In line with other studies, it was demonstrated that processing method, storage temperature and storage duration have significant combined effects on the proximate chemical composition and sensory attributes of soymilk. According to most authors, soymilk produced from flour produces better nutritional profile and more desirable sensory properties than milk produced from wet blanched beans. However, liquid soymilk produced from the traditional wet methods, are most stable in sensory attributes when stored at very low temperatures. Conclusively, the quest for cheap source of protein has enhanced small scale production of vegetable protein products of which soymilk is an example. Soymilk consumption has encouraged small scale production of the product under household condition with little or no regard to quality control measures. Soymilk therefore has the potential to substitute dairy milk.


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