Product sustainability looks at how products can provide economic benefits for the company while providing environmental and social benefits for society in general ^[1][24]. Product sustainability indicators are increasingly gaining recognition as a product sustainability assessment tool, which is always related to the company’s performance in aspects like energy, environment, resources, technical, and economic improvement ^[2][25]. More sustainable products provide opportunities to address consumption practices that increase waste. However, despite their best efforts to improve products, many companies lack a comprehensive strategy ^[2][25]. In manufacturing, developing new materials, better design methods, and society’s increasing demands on manufactured products, the design for manufacturing concept has paved the way for incorporating these into more sustainable product development ^[3][26].

The relation of product sustainability to WOM could be seen in a study on tourism, wherein a greater perception of foreign tourists in their sustainability assessment increased the WOM intention of foreign tourists ^[4][27]. Furthermore, positive and negative information related to sustainability on social media significantly influences consumers’ intention to purchase sustainable products ^[5][6][1,28]. As with product sustainability, previous studies on product attributes have shown different perspectives. They contribute to a literature review of sustainability labels that shows that consumers have positive attitudes toward olive oil that has a sustainability label, and they will pay more for products that carry those labels ^[7][29]. Other studies on sustainability messaging have investigated logos, certifications, and claims to show the different ways a product is advertised ^[8][30]. Messaging at Chinese shows had less of an emphasis on sustainability compared to those in Europe and the United States. Moreover, sustainability is also applied in other contexts, such as in methods that simultaneously evaluate environmental, economic, and social aspects, to project more sustainable designs of products and services ^[9][31].

Even though the case of sustainable dairy is sensitive, it is well known that milk and other dairy products are basic food products that are important in the development of healthy human beings ^[10][32]. Studies indicate that increased intake of milk and other dairy products to meet nutritional recommendations can protect against most common chronic diseases, and they have few reported side effects ^[11][33]. Dairy products are generally divided into seven categories, but consumers still mainly buy liquid milk ^[12][34]. One study recommended increasing dairy consumption by increasing consumer health awareness ^[12][34]. There is already enough evidence to proceed with a dietary change that involves switching from dairy products to plant-based alternatives ^[13][35]. However, plant-based milk alternatives are often lower nutritional substitutes than cow’s milk. The protein content of plant-based milk alternatives is an average of 48% of cow’s milk, and the levels of vitamins and minerals tend to be less consistent with plant-based milk alternatives ^[14][36]. In Indonesia, to promote dairy farm sustainability, their business sustainability factor is the standardization of a company’s management system ^[15][37].

The theory of planned behavior is a reliable model that focuses on several variables, such as consumer attitudes, subjective norms, and perceived behavioral control ^[16][38]. In a healthy workspace, the constructs of subjective norms, attitudes, and perceived control of behavior predict the safe behavior of supervisors ^[17][39], and extended TPB appears to be an efficient model with a focus on attitudes, knowledge, risk perception, and previous behavior ^[18][40]. Other contexts for product sustainability issues also confirm the TPB theory, such as pesticide handling ^[19][41], wellbeing food ^[20][42], green hotels ^[21][43], green pesticides ^[22][44], energy conservation ^[23][45], green restaurants ^[24][46], energy savers ^[25][47], and household waste sorting ^[26][48]. Studies that relate the TPB to WOM have shown significant relationships between attitudes and PBC and WOM intention ^[27][49], and that eWOM is related to TPB constructs ^[28][50].

Bachleda and Berrada-Fathi ^[29][51] found that N-eWOM plays an important role in service consumption decisions. Companies must actively manage N-eWOM because studies have shown that the effect of N-eWOM on consumer attitudes toward service providers and purchase intentions is far greater than the effect of positive eWOM ^[30][52]. N-eWOM is reported to have an impact on several important metrics, such as customer retention and loyalty ^[31][14], company profitability ^[6][28], and organizational reputation ^[5][32][1,15].

Bach Leda has defined attitudes as positive or negative feelings of individuals toward target behavior ^[33][53]. Comparing the impact of positive and negative reviews on hotel customer choices, Vermeulen and Seegers ^[34][54] emphasized that negative reviews resulted in negative attitudes. Conversely, positive reviews improved the attitudes of customers toward the hotel. Lee and Cranage ^[35][22] found that N-eWOM influenced attitudes toward restaurants more than positive eWOM.

In TPB, subjective norms are defined as perceived social pressure to perform or not perform behavior by individuals ^[36][55]. Jalilvand and Samiei ^[37][56] studied the impact of eWOM on the selection of tourist destinations and the influence of past travel using eWOM and TPB construction. They found that positive eWOM had a significant impact on attitudes to visit Isfahan, subjective norms, PBC, and the intention to travel. Tourism experiences have a significant impact on the use of eWOM and the TPB construct. Researchers have found that negative WOM had a positive effect on subjective norms, and it led to brand switching by consumers. In simple terms, it can be concluded that N-eWOM makes subjective norms smaller, meaning that the orientation of other people’s views is small and their attitude toward the brand is also reduced, which results in moving to another brand.

Perceived behavioral control (PBC) is a measure of the extent to which individuals believe that displaying certain behaviors will be easy or difficult ^[38][57]. PBC is an individual’s perception of the ease or difficulty of behavior and the control the person has to implement that behavior ^[39][58]. Therefore, if someone has the opportunity and the ability to act according to information they believe, then they will be motivated to act. For example, after receiving negative information that a product contains hazardous substances, they will avoid buying and consuming the product. Thus, eWOM negatively influences decision-making and hinders purchasing ^[40][41][11,59]. Jalilvand and Samiei ^[42][60] found that eWOM affected PBC. An individual’s attention to negative news and the credibility of the news received by consumers determines the perception of risk for the product being reported. Thus, PBC consumers are affected by negative reviews. Purchasing behavior in the TPB framework has a mediator’s attitudes, subjective norms, and PBC. If eWOM is negative with mediator attitudes, subjective norms, and PBC, and it results in negative behavior, such as not buying a product, then the three mediators following the N-eWOM become negative.

The eWOM phenomenon has changed people’s behavior and decisions, such that they rely more on the opinions of and information from other users. They even make offline decisions based on information obtained online ^[43][61]. Besides influencing results on sales figures at the corporate level, for example, eWOM also influences individual end-users in terms of their attitudes, trust, or purchase intentions ^[44][45][62,63]. Wu et al. ^[46][64] conducted a study in Taiwan on the effects of eWOM on the purchase of notebooks, a product with high levels of involvement, and shampoos, a daily consumption product with low involvement. They found that eWOM positively influenced the purchase intentions of the two products. Bachleda and Berrada-Fathi ^[29][51] found that N-eWOM, as well as negative WOM, played an important role in the choice of services that had not been used before. Thus, it can be hypothesized that:

Product claims are a way for manufacturers to use products’ intrinsic cues to be clearly visible to consumers. Manufacturers communicate attributes of products or services that are considered persuasive ^[47][65] so consumers will be interested in buying them. Claims, illustrations, and symbols convey important information about what can be expected of the product ^[48][66]. When understood by consumers, such product claims can improve marketing communication ^[49][67]. If a product is exposed to negative reviews online, the product claim can represent the company as the information provider. Generally, N-eWOM is overcome by clarifying explanations by the company’s public relations office to address negative issues. However, even if the company does not rebut negative reviews, the product claims are already there to do that, or at least to provide authentic information about the product. Consumers who want correct information quickly can at least get it through product claims attached to product packaging or advertised. Product claims are primarily, or even exclusively, a type of direct information for consumers. When faced with situations where information is uncertain, consumers can use product claims as primary information. Chen ^[50][68] found that when there is negative information about a product, the three constructs of TPB—attitudes, subjective norms, and PBC—become negative, resulting in cautious attitudes in consumers not to buy the product. Product claims as signals of product quality are expected to reduce these negative impacts.

Product claims are a way for producers to use intrinsic product cues to make them clearly visible to consumers ^[51][69]. Product claims are one way of communicating product or service attributes that are considered persuasive, therefore consumers will be interested in buying them ^[47][65]. There has been a positive increase in sales of wheat, high fiber cereals, folate-fortified breakfast foods, and cooking oil following claims or media coverage of the health benefits of these products ^[48][66].

Congruity theory examines how conformity or non-conformity with expectations affects individual responses, including information processing and evaluation ^[52][70]. When people find new information that matches their previous knowledge, they easily accept the new information. If the new information is not appropriate, it will challenge previous knowledge. Stayman et al. ^[53][71] examined how conformity affects satisfaction, and they found that when trials did not match the schema’s expectations, participants’ evaluations of the product were more negative. Congruent schemes are preferable and easier to process ^[54][72]. New information that complements existing knowledge is preferred.